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VP8 Data Format and Decoding Guide
RFC 6386

Independent Submission                                       J. Bankoski
Request for Comments: 6386                                   J. Koleszar
Category: Informational                                       L. Quillio
ISSN: 2070-1721                                               J. Salonen
                                                              P. Wilkins
                                                                   Y. Xu
                                                             Google Inc.
                                                           November 2011

                   VP8 Data Format and Decoding Guide

Abstract

   This document describes the VP8 compressed video data format,
   together with a discussion of the decoding procedure for the format.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6386.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

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Table of Contents

   1. Introduction ....................................................4
   2. Format Overview .................................................5
   3. Compressed Frame Types ..........................................7
   4. Overview of Compressed Data Format ..............................8
   5. Overview of the Decoding Process ................................9
   6. Description of Algorithms ......................................14
   7. Boolean Entropy Decoder ........................................16
      7.1. Underlying Theory of Coding ...............................17
      7.2. Practical Algorithm Description ...........................18
      7.3. Actual Implementation .....................................20
   8. Compressed Data Components .....................................25
      8.1. Tree Coding Implementation ................................27
      8.2. Tree Coding Example .......................................28
   9. Frame Header ...................................................30
      9.1. Uncompressed Data Chunk ...................................30
      9.2. Color Space and Pixel Type (Key Frames Only) ..............33
      9.3. Segment-Based Adjustments .................................34
      9.4. Loop Filter Type and Levels ...............................35
      9.5. Token Partition and Partition Data Offsets ................36
      9.6. Dequantization Indices ....................................37
      9.7. Refresh Golden Frame and Altref Frame .....................38
      9.8. Refresh Last Frame Buffer .................................39
      9.9. DCT Coefficient Probability Update ........................39
      9.10. Remaining Frame Header Data (Non-Key Frame) ..............40
      9.11. Remaining Frame Header Data (Key Frame) ..................41
   10. Segment-Based Feature Adjustments .............................41
   11. Key Frame Macroblock Prediction Records .......................42
      11.1. mb_skip_coeff ............................................42
      11.2. Luma Modes ...............................................42
      11.3. Subblock Mode Contexts ...................................45
      11.4. Chroma Modes .............................................46
      11.5. Subblock Mode Probability Table ..........................47
   12. Intraframe Prediction .........................................50
      12.1. mb_skip_coeff ............................................51
      12.2. Chroma Prediction ........................................51
      12.3. Luma Prediction ..........................................54
   13. DCT Coefficient Decoding ......................................60
      13.1. Macroblock without Non-Zero Coefficient Values ...........61
      13.2. Coding of Individual Coefficient Values ..................61
      13.3. Token Probabilities ......................................63
      13.4. Token Probability Updates ................................68
      13.5. Default Token Probability Table ..........................73

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   14. DCT and WHT Inversion and Macroblock Reconstruction ...........76
      14.1. Dequantization ...........................................76
      14.2. Inverse Transforms .......................................78
      14.3. Implementation of the WHT Inversion ......................78
      14.4. Implementation of the DCT Inversion ......................81
      14.5. Summation of Predictor and Residue .......................83
   15. Loop Filter ...................................................84
      15.1. Filter Geometry and Overall Procedure ....................85
      15.2. Simple Filter ............................................87
      15.3. Normal Filter ............................................91
      15.4. Calculation of Control Parameters ........................95
   16. Interframe Macroblock Prediction Records ......................97
      16.1. Intra-Predicted Macroblocks ..............................97
      16.2. Inter-Predicted Macroblocks ..............................98
      16.3. Mode and Motion Vector Contexts ..........................99
      16.4. Split Prediction ........................................105
   17. Motion Vector Decoding .......................................108
      17.1. Coding of Each Component ................................108
      17.2. Probability Updates .....................................110
   18. Interframe Prediction ........................................113
      18.1. Bounds on, and Adjustment of, Motion Vectors ............113
      18.2. Prediction Subblocks ....................................115
      18.3. Sub-Pixel Interpolation .................................115
      18.4. Filter Properties .......................................118
   19. Annex A: Bitstream Syntax ....................................120
      19.1. Uncompressed Data Chunk .................................121
      19.2. Frame Header ............................................122
      19.3. Macroblock Data .........................................130
   20. Attachment One: Reference Decoder Source Code ................133
      20.1. bit_ops.h ...............................................133
      20.2. bool_decoder.h ..........................................133
      20.3. dequant_data.h ..........................................137
      20.4. dixie.c .................................................138
      20.5. dixie.h .................................................151
      20.6. dixie_loopfilter.c ......................................158
      20.7. dixie_loopfilter.h ......................................170
      20.8. idct_add.c ..............................................171
      20.9. idct_add.h ..............................................174
      20.10. mem.h ..................................................175
      20.11. modemv.c ...............................................176
      20.12. modemv.h ...............................................192
      20.13. modemv_data.h ..........................................193
      20.14. predict.c ..............................................198
      20.15. predict.h ..............................................231
      20.16. tokens.c ...............................................232
      20.17. tokens.h ...............................................242
      20.18. vp8_prob_data.h ........................................243
      20.19. vpx_codec_internal.h ...................................252

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      20.20. vpx_decoder.h ..........................................263
      20.21. vpx_decoder_compat.h ...................................271
      20.22. vpx_image.c ............................................285
      20.23. vpx_image.h ............................................291
      20.24. vpx_integer.h ..........................................298
      20.25. AUTHORS File ...........................................299
      20.26. LICENSE ................................................301
      20.27. PATENTS ................................................302
   21. Security Considerations ......................................302
   22. References ...................................................303
      22.1. Normative Reference .....................................303
      22.2. Informative References ..................................303

1.  Introduction

   This document describes the VP8 compressed video data format,
   together with a discussion of the decoding procedure for the format.
   It is intended to be used in conjunction with, and as a guide to, the
   reference decoder source code provided in Attachment One
   (Section 20).  If there are any conflicts between this narrative and
   the reference source code, the reference source code should be
   considered correct.  The bitstream is defined by the reference source
   code and not this narrative.

   Like many modern video compression schemes, VP8 is based on
   decomposition of frames into square subblocks of pixels, prediction
   of such subblocks using previously constructed blocks, and adjustment
   of such predictions (as well as synthesis of unpredicted blocks)
   using a discrete cosine transform (hereafter abbreviated as DCT).  In
   one special case, however, VP8 uses a Walsh-Hadamard transform
   (hereafter abbreviated as WHT) instead of a DCT.

   Roughly speaking, such systems reduce datarate by exploiting the
   temporal and spatial coherence of most video signals.  It is more
   efficient to specify the location of a visually similar portion of a
   prior frame than it is to specify pixel values.  The frequency
   segregation provided by the DCT and WHT facilitates the exploitation
   of both spatial coherence in the original signal and the tolerance of
   the human visual system to moderate losses of fidelity in the
   reconstituted signal.

   VP8 augments these basic concepts with, among other things,
   sophisticated usage of contextual probabilities.  The result is a
   significant reduction in datarate at a given quality.

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   Unlike some similar schemes (the older MPEG formats, for example),
   VP8 specifies exact values for reconstructed pixels.  Specifically,
   the specification for the DCT and WHT portions of the reconstruction
   does not allow for any "drift" caused by truncation of fractions.
   Rather, the algorithm is specified using fixed-precision integer
   operations exclusively.  This greatly facilitates the verification of
   the correctness of a decoder implementation and also avoids
   difficult-to-predict visual incongruities between such
   implementations.

   It should be remarked that, in a complete video playback system, the
   displayed frames may or may not be identical to the reconstructed
   frames.  Many systems apply a final level of filtering (commonly
   referred to as postprocessing) to the reconstructed frames prior to
   viewing.  Such postprocessing has no effect on the decoding and
   reconstruction of subsequent frames (which are predicted using the
   completely specified reconstructed frames) and is beyond the scope of
   this document.  In practice, the nature and extent of this sort of
   postprocessing is dependent on both the taste of the user and on the
   computational facilities of the playback environment.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Format Overview

   VP8 works exclusively with an 8-bit YUV 4:2:0 image format.  In this
   format, each 8-bit pixel in the two chroma planes (U and V)
   corresponds positionally to a 2x2 block of 8-bit luma pixels in the
   Y plane; coordinates of the upper left corner of the Y block are of
   course exactly twice the coordinates of the corresponding chroma
   pixels.  When we refer to pixels or pixel distances without
   specifying a plane, we are implicitly referring to the Y plane or to
   the complete image, both of which have the same (full) resolution.

   As is usually the case, the pixels are simply a large array of bytes
   stored in rows from top to bottom, each row being stored from left to
   right.  This "left to right" then "top to bottom" raster-scan order
   is reflected in the layout of the compressed data as well.

   Provision has been made in the VP8 bitstream header for the support
   of a secondary YUV color format, in the form of a reserved bit.

   Occasionally, at very low datarates, a compression system may decide
   to reduce the resolution of the input signal to facilitate efficient
   compression.  The VP8 data format supports this via optional
   upscaling of its internal reconstruction buffer prior to output (this

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   is completely distinct from the optional postprocessing discussed
   earlier, which has nothing to do with decoding per se).  This
   upsampling restores the video frames to their original resolution.
   In other words, the compression/decompression system can be viewed as
   a "black box", where the input and output are always at a given
   resolution.  The compressor might decide to "cheat" and process the
   signal at a lower resolution.  In that case, the decompressor needs
   the ability to restore the signal to its original resolution.

   Internally, VP8 decomposes each output frame into an array of
   macroblocks.  A macroblock is a square array of pixels whose Y
   dimensions are 16x16 and whose U and V dimensions are 8x8.
   Macroblock-level data in a compressed frame occurs (and must be
   processed) in a raster order similar to that of the pixels comprising
   the frame.

   Macroblocks are further decomposed into 4x4 subblocks.  Every
   macroblock has 16 Y subblocks, 4 U subblocks, and 4 V subblocks.  Any
   subblock-level data (and processing of such data) again occurs in
   raster order, this time in raster order within the containing
   macroblock.

   As discussed in further detail below, data can be specified at the
   levels of both macroblocks and their subblocks.

   Pixels are always treated, at a minimum, at the level of subblocks,
   which may be thought of as the "atoms" of the VP8 algorithm.  In
   particular, the 2x2 chroma blocks corresponding to 4x4 Y subblocks
   are never treated explicitly in the data format or in the algorithm
   specification.

   The DCT and WHT always operate at a 4x4 resolution.  The DCT is used
   for the 16Y, 4U, and 4V subblocks.  The WHT is used (with some but
   not all prediction modes) to encode a 4x4 array comprising the
   average intensities of the 16 Y subblocks of a macroblock.  These
   average intensities are, up to a constant normalization factor,
   nothing more than the 0th DCT coefficients of the Y subblocks.  This
   "higher-level" WHT is a substitute for the explicit specification of
   those coefficients, in exactly the same way as the DCT of a subblock
   substitutes for the specification of the pixel values comprising the
   subblock.  We consider this 4x4 array as a second-order subblock
   called Y2, and think of a macroblock as containing 24 "real"
   subblocks and, sometimes, a 25th "virtual" subblock.  This is dealt
   with further in Section 13.

   The frame layout used by the reference decoder may be found in the
   file vpx_image.h (Section 20.23).

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3.  Compressed Frame Types

   There are only two types of frames in VP8.

   Intraframes (also called key frames and, in MPEG terminology,
   I-frames) are decoded without reference to any other frame in a
   sequence; that is, the decompressor reconstructs such frames
   beginning from its "default" state.  Key frames provide random access
   (or seeking) points in a video stream.

   Interframes (also called prediction frames and, in MPEG terminology,
   P-frames) are encoded with reference to prior frames, specifically
   all prior frames up to and including the most recent key frame.
   Generally speaking, the correct decoding of an interframe depends on
   the correct decoding of the most recent key frame and all ensuing
   frames.  Consequently, the decoding algorithm is not tolerant of
   dropped frames: In an environment in which frames may be dropped or
   corrupted, correct decoding will not be possible until a key frame is
   correctly received.

   In contrast to MPEG, there is no use of bidirectional prediction.  No
   frame is predicted using frames temporally subsequent to it; there is
   no analog to an MPEG B-frame.

   Secondly, VP8 augments these notions with that of alternate
   prediction frames, called golden frames and altref frames
   (alternative reference frames).  Blocks in an interframe may be
   predicted using blocks in the immediately previous frame as well as
   the most recent golden frame or altref frame.  Every key frame is
   automatically golden and altref, and any interframe may optionally
   replace the most recent golden or altref frame.

   Golden frames and altref frames may also be used to partially
   overcome the intolerance to dropped frames discussed above: If a
   compressor is configured to code golden frames only with reference to
   the prior golden frame (and key frame), then the "substream" of key
   and golden frames may be decoded regardless of loss of other
   interframes.  Roughly speaking, the implementation requires (on the
   compressor side) that golden frames subsume and recode any context
   updates effected by the intervening interframes.  A typical
   application of this approach is video conferencing, in which
   retransmission of a prior golden frame and/or a delay in playback
   until receipt of the next golden frame is preferable to a larger
   retransmit and/or delay until the next key frame.

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4.  Overview of Compressed Data Format

   The input to a VP8 decoder is a sequence of compressed frames whose
   order matches their order in time.  Issues such as the duration of
   frames, the corresponding audio, and synchronization are generally
   provided by the playback environment and are irrelevant to the
   decoding process itself; however, to aid in fast seeking, a start
   code is included in the header of each key frame.

   The decoder is simply presented with a sequence of compressed frames
   and produces a sequence of decompressed (reconstructed) YUV frames
   corresponding to the input sequence.  As stated in the Introduction,
   the exact pixel values in the reconstructed frame are part of VP8's
   specification.  This document specifies the layout of the compressed
   frames and gives unambiguous algorithms for the correct production of
   reconstructed frames.

   The first frame presented to the decompressor is of course a key
   frame.  This may be followed by any number of interframes; the
   correct reconstruction of each frame depends on all prior frames up
   to the key frame.  The next key frame restarts this process: The
   decompressor resets to its default initial condition upon reception
   of a key frame, and the decoding of a key frame (and its ensuing
   interframes) is completely independent of any prior decoding.

   At the highest level, every compressed frame has three or more
   pieces.  It begins with an uncompressed data chunk comprising
   10 bytes in the case of key frames and 3 bytes for interframes.  This
   is followed by two or more blocks of compressed data (called
   partitions).  These compressed data partitions begin and end on byte
   boundaries.

   The first compressed partition has two subsections:

   1.  Header information that applies to the frame as a whole.

   2.  Per-macroblock information specifying how each macroblock is
       predicted from the already-reconstructed data that is available
       to the decompressor.

   As stated above, the macroblock-level information occurs in raster-
   scan order.

   The rest of the partitions contain, for each block, the DCT/WHT
   coefficients (quantized and logically compressed) of the residue
   signal to be added to the predicted block values.  It typically
   accounts for roughly 70% of the overall datarate.  VP8 supports
   packing the compressed DCT/WHT coefficients' data from macroblock

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   rows into separate partitions.  If there is more than one partition
   for these coefficients, the sizes of the partitions -- except the
   last partition -- in bytes are also present in the bitstream right
   after the above first partition.  Each of the sizes is a 3-byte data
   item written in little endian format.  These sizes provide the
   decoder direct access to all DCT/WHT coefficient partitions, which
   enables parallel processing of the coefficients in a decoder.

   The separate partitioning of the prediction data and coefficient data
   also allows flexibility in the implementation of a decompressor: An
   implementation may decode and store the prediction information for
   the whole frame and then decode, transform, and add the residue
   signal to the entire frame, or it may simultaneously decode both
   partitions, calculating prediction information and adding in the
   residue signal for each block in order.  The length field in the
   frame tag, which allows decoding of the second partition to begin
   before the first partition has been completely decoded, is necessary
   for the second "block-at-a-time" decoder implementation.

   All partitions are decoded using separate instances of the boolean
   entropy decoder described in Section 7.  Although some of the data
   represented within the partitions is conceptually "flat" (a bit is
   just a bit with no probabilistic expectation one way or the other),
   because of the way such coders work, there is never a direct
   correspondence between a "conceptual bit" and an actual physical bit
   in the compressed data partitions.  Only in the 3- or 10-byte
   uncompressed chunk described above is there such a physical
   correspondence.

   A related matter is that seeking within a partition is not supported.
   The data must be decompressed and processed (or at least stored) in
   the order in which it occurs in the partition.

   While this document specifies the ordering of the partition data
   correctly, the details and semantics of this data are discussed in a
   more logical fashion to facilitate comprehension.  For example, the
   frame header contains updates to many probability tables used in
   decoding per-macroblock data.  The per-macroblock data is often
   described before the layouts of the probabilities and their updates,
   even though this is the opposite of their order in the bitstream.

5.  Overview of the Decoding Process

   A VP8 decoder needs to maintain four YUV frame buffers whose
   resolutions are at least equal to that of the encoded image.  These
   buffers hold the current frame being reconstructed, the immediately
   previous reconstructed frame, the most recent golden frame, and the
   most recent altref frame.

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   Most implementations will wish to "pad" these buffers with
   "invisible" pixels that extend a moderate number of pixels beyond all
   four edges of the visible image.  This simplifies interframe
   prediction by allowing all (or most) prediction blocks -- which are
   not guaranteed to lie within the visible area of a prior frame -- to
   address usable image data.

   Regardless of the amount of padding chosen, the invisible rows above
   (or below) the image are filled with copies of the top (or bottom)
   row of the image; the invisible columns to the left (or right) of the
   image are filled with copies of the leftmost (or rightmost) visible
   row; and the four invisible corners are filled with copies of the
   corresponding visible corner pixels.  The use of these prediction
   buffers (and suggested sizes for the halo) will be elaborated on in
   the discussion of motion vectors, interframe prediction, and
   sub-pixel interpolation later in this document.

   As will be seen in the description of the frame header, the image
   dimensions are specified (and can change) with every key frame.
   These buffers (and any other data structures whose size depends on
   the size of the image) should be allocated (or re-allocated)
   immediately after the dimensions are decoded.

   Leaving most of the details for later elaboration, the following is
   an outline of the decoding process.

   First, the frame header (the beginning of the first data partition)
   is decoded.  Altering or augmenting the maintained state of the
   decoder, this provides the context in which the per-macroblock data
   can be interpreted.

   The macroblock data occurs (and must be processed) in raster-scan
   order.  This data comes in two or more parts.  The first (prediction
   or mode) part comes in the remainder of the first data partition.
   The other parts comprise the data partition(s) for the DCT/WHT
   coefficients of the residue signal.  For each macroblock, the
   prediction data must be processed before the residue.

   Each macroblock is predicted using one (and only one) of four
   possible frames.  All macroblocks in a key frame, and all intra-coded
   macroblocks in an interframe, are predicted using the already-decoded
   macroblocks in the current frame.  Macroblocks in an interframe may
   also be predicted using the previous frame, the golden frame, or the
   altref frame.  Such macroblocks are said to be inter-coded.

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   The purpose of prediction is to use already-constructed image data to
   approximate the portion of the original image being reconstructed.
   The effect of any of the prediction modes is then to write a
   macroblock-sized prediction buffer containing this approximation.

   Regardless of the prediction method, the residue DCT signal is
   decoded, dequantized, reverse-transformed, and added to the
   prediction buffer to produce the (almost final) reconstruction value
   of the macroblock, which is stored in the correct position of the
   current frame buffer.

   The residue signal consists of 24 (sixteen Y, four U, and four V) 4x4
   quantized and losslessly compressed DCT transforms approximating the
   difference between the original macroblock in the uncompressed source
   and the prediction buffer.  For most prediction modes, the 0th
   coefficients of the sixteen Y subblocks are expressed via a 25th WHT
   of the second-order virtual Y2 subblock discussed above.

   Intra-prediction exploits the spatial coherence of frames.  The 16x16
   luma (Y) and 8x8 chroma (UV) components are predicted independently
   of each other using one of four simple means of pixel propagation,
   starting from the already-reconstructed (16-pixel-long luma, 8-pixel-
   long chroma) row above, and column to the left of, the current
   macroblock.  The four methods are:

   1.  Copying the row from above throughout the prediction buffer.

   2.  Copying the column from the left throughout the prediction
       buffer.

   3.  Copying the average value of the row and column throughout the
       prediction buffer.

   4.  Extrapolation from the row and column using the (fixed) second
       difference (horizontal and vertical) from the upper left corner.

   Additionally, the sixteen Y subblocks may be predicted independently
   of each other using one of ten different modes, four of which are 4x4
   analogs of those described above, augmented with six "diagonal"
   prediction methods.  There are two types of predictions, one intra
   and one prediction (among all the modes), for which the residue
   signal does not use the Y2 block to encode the DC portion of the
   sixteen 4x4 Y subblock DCTs.  This "independent Y subblock" mode has
   no effect on the 8x8 chroma prediction.

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   Inter-prediction exploits the temporal coherence between nearby
   frames.  Except for the choice of the prediction frame itself, there
   is no difference between inter-prediction based on the previous frame
   and that based on the golden frame or altref frame.

   Inter-prediction is conceptually very simple.  While, for reasons of
   efficiency, there are several methods of encoding the relationship
   between the current macroblock and corresponding sections of the
   prediction frame, ultimately each of the sixteen Y subblocks is
   related to a 4x4 subblock of the prediction frame, whose position in
   that frame differs from the current subblock position by a (usually
   small) displacement.  These two-dimensional displacements are called
   motion vectors.

   The motion vectors used by VP8 have quarter-pixel precision.
   Prediction of a subblock using a motion vector that happens to have
   integer (whole number) components is very easy: The 4x4 block of
   pixels from the displaced block in the previous, golden, or altref
   frame is simply copied into the correct position of the current
   macroblock's prediction buffer.

   Fractional displacements are conceptually and implementationally more
   complex.  They require the inference (or synthesis) of sample values
   that, strictly speaking, do not exist.  This is one of the most basic
   problems in signal processing, and readers conversant with that
   subject will see that the approach taken by VP8 provides a good
   balance of robustness, accuracy, and efficiency.

   Leaving the details for the implementation discussion below, the
   pixel interpolation is calculated by applying a kernel filter (using
   reasonable-precision integer math) three pixels on either side, both
   horizontally and vertically, of the pixel to be synthesized.  The
   resulting 4x4 block of synthetic pixels is then copied into position
   exactly as in the case of integer displacements.

   Each of the eight chroma subblocks is handled similarly.  Their
   motion vectors are never specified explicitly; instead, the motion
   vector for each chroma subblock is calculated by averaging the
   vectors of the four Y subblocks that occupy the same area of the
   frame.  Since chroma pixels have twice the diameter (and four times
   the area) of luma pixels, the calculated chroma motion vectors have
   1/8-pixel resolution, but the procedure for copying or generating
   pixels for each subblock is essentially identical to that done in the
   luma plane.

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   After all the macroblocks have been generated (predicted and
   corrected with the DCT/WHT residue), a filtering step (the loop
   filter) is applied to the entire frame.  The purpose of the loop
   filter is to reduce blocking artifacts at the boundaries between
   macroblocks and between subblocks of the macroblocks.  The term "loop
   filter" is used because this filter is part of the "coding loop";
   that is, it affects the reconstructed frame buffers that are used to
   predict ensuing frames.  This is distinguished from the
   postprocessing filters discussed earlier, which affect only the
   viewed video and do not "feed into" subsequent frames.

   Next, if signaled in the data, the current frame may replace the
   golden frame prediction buffer and/or the altref frame buffer.

   The halos of the frame buffers are next filled as specified above.
   Finally, at least as far as decoding is concerned, the (references
   to) the "current" and "last" frame buffers should be exchanged in
   preparation for the next frame.

   Various processes may be required (or desired) before viewing the
   generated frame.  As discussed in the frame dimension information
   below, truncation and/or upscaling of the frame may be required.
   Some playback systems may require a different frame format (RGB,
   YUY2, etc.).  Finally, as mentioned in the Introduction, further
   postprocessing or filtering of the image prior to viewing may be
   desired.  Since the primary purpose of this document is a decoding
   specification, the postprocessing is not specified in this document.

   While the basic ideas of prediction and correction used by VP8 are
   straightforward, many of the details are quite complex.  The
   management of probabilities is particularly elaborate.  Not only do
   the various modes of intra-prediction and motion vector specification
   have associated probabilities, but they, together with the coding of
   DCT coefficients and motion vectors, often base these probabilities
   on a variety of contextual information (calculated from what has been
   decoded so far), as well as on explicit modification via the frame
   header.

   The "top-level" of decoding and frame reconstruction is implemented
   in the reference decoder file dixie.c (Section 20.4).

   This concludes our summary of decoding and reconstruction; we
   continue by discussing the individual aspects in more depth.

   A reasonable "divide and conquer" approach to implementation of a
   decoder is to begin by decoding streams composed exclusively of key
   frames.  After that works reliably, interframe handling can be added
   more easily than if complete functionality were attempted

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   immediately.  In accordance with this, we first discuss components
   needed to decode key frames (most of which are also used in the
   decoding of interframes) and conclude with topics exclusive to
   interframes.

6.  Description of Algorithms

   As the intent of this document, together with the reference decoder
   source code, is to specify a platform-independent procedure for the
   decoding and reconstruction of a VP8 video stream, many (small)
   algorithms must be described exactly.

   Due to its near-universality, terseness, ability to easily describe
   calculation at specific precisions, and the fact that On2's reference
   VP8 decoder is written in C, these algorithm fragments are written
   using the C programming language, augmented with a few simple
   definitions below.

   The standard (and best) reference for C is [Kernighan].

   Many code fragments will be presented in this document.  Some will be
   nearly identical to corresponding sections of the reference decoder;
   others will differ.  Roughly speaking, there are three reasons for
   such differences:

   1.  For reasons of efficiency, the reference decoder version may be
       less obvious.

   2.  The reference decoder often uses large data structures to
       maintain context that need not be described or used here.

   3.  The authors of this document felt that a different expression of
       the same algorithm might facilitate exposition.

   Regardless of the chosen presentation, the calculation effected by
   any of the algorithms described here is identical to that effected by
   the corresponding portion of the reference decoder.

   All VP8 decoding algorithms use integer math.  To facilitate
   specification of arithmetic precision, we define the following types.

   ---- Begin code block --------------------------------------

   typedef   signed char  int8; /* signed int exactly 8 bits wide */
   typedef unsigned char uint8; /* unsigned "" */

   typedef short int16;         /* signed int exactly 16 bits wide */
   typedef unsigned int16 uint16; /* unsigned "" */

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   /* int32 is a signed integer type at least 32 bits wide */

   typedef long int32; /* guaranteed to work on all systems */
   typedef int  int32; /* will be more efficient on some systems */

   typedef unsigned int32 uint32;

   /* unsigned integer type, at least 16 bits wide, whose exact size
      is most convenient to whatever processor we are using */

   typedef unsigned int uint;

   /* While pixels themselves are 8-bit unsigned integers,
      pixel arithmetic often occurs at 16- or 32-bit precision and
      the results need to be "saturated" or clamped to an 8-bit
      range. */

   typedef uint8 Pixel;

   Pixel clamp255(int32 v) { return v < 0? 0 : (v < 255? v : 255);}

   /*  As is elaborated in the discussion of the bool_decoder below,
       VP8 represents probabilities as unsigned 8-bit numbers. */

   typedef uint8 Prob;

   ---- End code block ----------------------------------------

   We occasionally need to discuss mathematical functions involving
   honest-to-goodness "infinite precision" real numbers.  The DCT is
   first described via the cosine function cos; the ratio of the lengths
   of the circumference and diameter of a circle is denoted pi; at one
   point, we take a (base 1/2) logarithm, denoted log; and pow(x, y)
   denotes x raised to the power y.  If x = 2 and y is a small
   non-negative integer, pow(2, y) may be expressed in C as 1 << y.

   Finally, we sometimes need to divide signed integers by powers of
   two; that is, we occasionally right-shift signed numbers.  The
   behavior of such shifts (i.e., the propagation of the sign bit) is,
   perhaps surprisingly, not defined by the C language itself and is
   left up to individual compilers.  Because of the utility of this
   frequently needed operation, it is at least arguable that it should
   be defined by the language (to naturally propagate the sign bit) and,
   at a minimum, should be correctly implemented by any reasonable
   compiler.  In the interest of strict portability, we attempt to call
   attention to these shifts when they arise.

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7.  Boolean Entropy Decoder

   As discussed in the overview above, essentially the entire VP8 data
   stream is encoded using a boolean entropy coder.

   An understanding of the bool_decoder is critical to the
   implementation of a VP8 decompressor, so we discuss the bool_decoder
   in detail.  It is easier to comprehend the bool_decoder in
   conjunction with the bool_encoder used by the compressor to write the
   compressed data partitions.

   The bool_encoder encodes (and the bool_decoder decodes) one bool
   (zero-or-one boolean value) at a time.  Its purpose is to losslessly
   compress a sequence of bools for which the probability of their being
   zero or one can be well-estimated (via constant or previously coded
   information) at the time they are written, using identical
   corresponding probabilities at the time they are read.

   As the reader is probably aware, if a bool is much more likely to be
   zero than one (for instance), it can, on average, be faithfully
   encoded using much less than one bit per value.  The bool_encoder
   exploits this.

   In the 1940s, [Shannon] proved that there is a lower bound for the
   average datarate of a faithful encoding of a sequence of bools (whose
   probability distributions are known and are independent of each
   other) and also that there are encoding algorithms that approximate
   this lower bound as closely as one wishes.

   If we encode a sequence of bools whose probability of being zero is p
   (and whose probability of being 1 is 1-p), the lowest possible
   datarate per value is

   plog(p) + (1-p)log(1-p);

   taking the logarithms to the base 1/2 expresses the datarate in bits/
   value.

   We give two simple examples.  At one extreme, if p = 1/2, then log(p)
   = log(1-p) = 1, and the lowest possible datarate per bool is 1/2 +
   1/2 = 1; that is, we cannot do any better than simply literally
   writing out bits.  At another extreme, if p is very small, say p =
   1/1024, then log(p)=10, log(1-p) is roughly .0014, and the lowest
   possible datarate is approximately 10/1024 + .0014, roughly 1/100 of
   a bit per bool.

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   Because most of the bools in the VP8 datastream have zero-
   probabilities nowhere near 1/2, the compression provided by the
   bool_encoder is critical to the performance of VP8.

   The boolean coder used by VP8 is a variant of an arithmetic coder.
   An excellent discussion of arithmetic coding (and other lossless
   compression techniques) can be found in [Bell].

7.1.  Underlying Theory of Coding

   The basic idea used by the boolean coder is to consider the entire
   data stream (either of the partitions in our case) as the binary
   expansion of a single number x with 0 <= x < 1.  The bits (or bytes)
   in x are of course written from high to low order, and if b[j] (B[j])
   is the j^(th) bit (byte) in the partition, the value x is simply the
   sum (starting with j = 1) of pow(2, -j) * b[j] or pow(256, -j) *
   B[j].

   Before the first bool is coded, all values of x are possible.

   The coding of each bool restricts the possible values of x in
   proportion to the probability of what is coded.  If p1 is the
   probability of the first bool being zero and a zero is coded, the
   range of possible values of x is restricted to 0 <= x < p1.  If a one
   is coded, the range becomes p1 <= x < 1.

   The coding continues by repeating the same idea.  At every stage,
   there is an interval a <= x < b of possible values of x.  If p is the
   probability of a zero being coded at this stage and a zero is coded,
   the interval becomes a <= x < a + (p(b-a)).  If a one is coded, the
   possible values of x are restricted to a + (p(b-a)) <= x < b.

   Assuming that only finitely many values are to be coded, after the
   encoder has received the last bool, it can write as its output any
   value x that lies in the final interval.  VP8 simply writes the left
   endpoint of the final interval.  Consequently, the output it would
   make if encoding were to stop at any time either increases or stays
   the same as each bool is encoded.

   Decoding parallels encoding.  The decoder is presented with the
   number x, which has only the initial restriction 0 <= x < 1.  To
   decode the first bool, the decoder is given the first probability p1.
   If x < p1, a zero is decoded; if x >= p1, a one is decoded.  In
   either case, the new restriction on x -- that is, the interval of
   possible values of x -- is remembered.

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   Decoding continues in exactly the same way: If a <= x < b is the
   current interval and we are to decode a bool with zero-probability p,
   we return a zero if a <= x < a + (p(b-a)) and a one if a + (p(b-a))
   <= x < b.  In either case, the new restriction is remembered in
   preparation for decoding the next bool.

   The process outlined above uses real numbers of infinite precision to
   express the probabilities and ranges.  It is true that, if one could
   actualize this process and coded a large number of bools whose
   supplied probabilities matched their value distributions, the
   datarate achieved would approach the theoretical minimum as the
   number of bools encoded increased.

   Unfortunately, computers operate at finite precision, and an
   approximation to the theoretically perfect process described above is
   necessary.  Such approximation increases the datarate but, at quite
   moderate precision and for a wide variety of data sets, this increase
   is negligible.

   The only conceptual limitations are, first, that coder probabilities
   must be expressed at finite precision and, second, that the decoder
   be able to detect each individual modification to the value interval
   via examination of a fixed amount of input.  As a practical matter,
   many of the implementation details stem from the fact that the coder
   can function using only a small "window" to incrementally read or
   write the arbitrarily precise number x.

7.2.  Practical Algorithm Description

   VP8's boolean coder works with 8-bit probabilities p.  The range of
   such p is 0 <= p <= 255; the actual probability represented by p is
   p/256.  Also, the coder is designed so that decoding of a bool
   requires no more than an 8-bit comparison, and so that the state of
   both the encoder and decoder can be easily represented using a small
   number of unsigned 16-bit integers.

   The details are most easily understood if we first describe the
   algorithm using bit-at-a-time input and output.  Aside from the
   ability to maintain a position in this bitstream and write/read bits,
   the encoder also needs the ability to add 1 to the bits already
   output; after writing n bits, adding 1 to the existing output is the
   same thing as adding pow(2, -n) to x.

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   Together with the bit position, the encoder must maintain two
   unsigned 8-bit numbers, which we call "bottom" and "range".  Writing
   w for the n bits already written and S = pow(2, - n - 8) for the
   scale of the current bit position one byte out, we have the following
   constraint on all future values v of w (including the final value
   v = x):

   w + ( S * bottom ) <= v < w + ( S * ( bottom + range ) )

   Thus, appending bottom to the already-written bits w gives the left
   endpoint of the interval of possible values, appending bottom + range
   gives the right endpoint, and range itself (scaled to the current
   output position) is the length of the interval.

   So that our probabilistic encodings are reasonably accurate, we do
   not let range vary by more than a factor of two: It stays within the
   bounds 128 <= range <= 255.

   The process for encoding a boolean value val whose probability of
   being zero is prob / 256 -- and whose probability of being one is
   ( 256 - prob ) / 256 -- with 1 <= prob <= 255 is as follows.

   Using an unsigned 16-bit multiply followed by an unsigned right
   shift, we calculate an unsigned 8-bit split value:

   split = 1 + (((range - 1) * probability)]] >> 8)

   split is approximately ( prob / 256 ) * range and lies within the
   bounds 1 <= split <= range - 1.  These bounds ensure the correctness
   of the decoding procedure described below.

   If the incoming boolean val to be encoded is false, we leave the left
   interval endpoint bottom alone and reduce range, replacing it by
   split.  If the incoming val is true, we move up the left endpoint to
   bottom + split, propagating any carry to the already-written value w
   (this is where we need the ability to add 1 to w), and reduce range
   to range - split.

   Regardless of the value encoded, range has been reduced and now has
   the bounds 1 <= range <= 254.  If range < 128, the encoder doubles it
   and shifts the high-order bit out of bottom to the output as it also
   doubles bottom, repeating this process one bit at a time until 128 <=
   range <= 255.  Once this is completed, the encoder is ready to accept
   another bool, maintaining the constraints described above.

   After encoding the last bool, the partition may be completed by
   appending bottom to the bitstream.

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   The decoder mimics the state of the encoder.  It maintains, together
   with an input bit position, two unsigned 8-bit numbers, a range
   identical to that maintained by the encoder and a value.  Decoding
   one bool at a time, the decoder (in effect) tracks the same left
   interval endpoint as does the encoder and subtracts it from the
   remaining input.  Appending the unread portion of the bitstream to
   the 8-bit value gives the difference between the actual value encoded
   and the known left endpoint.

   The decoder is initialized by setting range = 255 and reading the
   first 16 input bits into value.  The decoder maintains range and
   calculates split in exactly the same way as does the encoder.

   To decode a bool, it compares value to split; if value < split, the
   bool is zero, and range is replaced with split.  If value >= split,
   the bool is one, range is replaced with range - split, and value is
   replaced with value - split.

   Again, range is doubled one bit at a time until it is at least 128.
   The value is doubled in parallel, shifting a new input bit into the
   bottom each time.

   Writing Value for value together with the unread input bits and Range
   for range extended indefinitely on the right by zeros, the condition
   Value < Range is maintained at all times by the decoder.  In
   particular, the bits shifted out of value as it is doubled are always
   zero.

7.3.  Actual Implementation

   The C code below gives complete implementations of the encoder and
   decoder described above.  While they are logically identical to the
   "bit-at-a-time" versions, they internally buffer a couple of extra
   bytes of the bitstream.  This allows I/O to be done (more
   practically) a byte at a time and drastically reduces the number of
   carries the encoder has to propagate into the already-written data.

   Another (logically equivalent) implementation may be found in the
   reference decoder file bool_decoder.h (Section 20.2).

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   ---- Begin code block --------------------------------------

   /* Encoder first */

   typedef struct {
     uint8 *output;  /* ptr to next byte to be written */
     uint32 range;   /* 128 <= range <= 255 */
     uint32 bottom;  /* minimum value of remaining output */
     int bit_count;  /* # of shifts before an output byte
                        is available */
   } bool_encoder;

   /* Must set initial state of encoder before writing any bools. */

   void init_bool_encoder(bool_encoder *e, uint8 *start_partition)
   {
     e->output = start_partition;
     e->range = 255;
     e->bottom = 0;
     e->bit_count = 24;
   }

   /* Encoding very rarely produces a carry that must be propagated
      to the already-written output.  The arithmetic guarantees that
      the propagation will never go beyond the beginning of the
      output.  Put another way, the encoded value x is always less
      than one. */

   void add_one_to_output(uint8 *q)
   {
     while (*--q == 255)
       *q = 0;
     ++*q;
   }

   /* Main function writes a bool_value whose probability of being
      zero is (expected to be) prob/256. */

   void write_bool(bool_encoder *e, Prob prob, int bool_value)
   {
     /* split is approximately (range * prob) / 256 and,
        crucially, is strictly bigger than zero and strictly
        smaller than range */

     uint32 split = 1 + (((e->range - 1) * prob) >> 8);

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     if (bool_value) {
       e->bottom += split; /* move up bottom of interval */
       e->range -= split;  /* with corresponding decrease in range */
     } else
       e->range = split;   /* decrease range, leaving bottom alone */

     while (e->range < 128)
     {
       e->range <<= 1;

       if (e->bottom & (1 << 31))  /* detect carry */
         add_one_to_output(e->output);

       e->bottom <<= 1;        /* before shifting bottom */

       if (!--e->bit_count) {  /* write out high byte of bottom ... */

         *e->output++ = (uint8) (e->bottom >> 24);

         e->bottom &= (1 << 24) - 1;  /* ... keeping low 3 bytes */

         e->bit_count = 8;            /* 8 shifts until next output */
       }
     }
   }

   /* Call this function (exactly once) after encoding the last
      bool value for the partition being written */

   void flush_bool_encoder(bool_encoder *e)
   {
     int c = e->bit_count;
     uint32 v = e->bottom;

     if (v & (1 << (32 - c)))   /* propagate (unlikely) carry */
       add_one_to_output(e->output);
     v <<= c & 7;               /* before shifting remaining output */
     c >>= 3;                   /* to top of internal buffer */
     while (--c >= 0)
       v <<= 8;
     c = 4;
     while (--c >= 0) {    /* write remaining data, possibly padded */
       *e->output++ = (uint8) (v >> 24);
       v <<= 8;
     }
   }

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   /* Decoder state exactly parallels that of the encoder.
      "value", together with the remaining input, equals the
      complete encoded number x less the left endpoint of the
      current coding interval. */

   typedef struct {
     uint8   *input;     /* pointer to next compressed data byte */
     uint32  range;      /* always identical to encoder's range */
     uint32  value;      /* contains at least 8 significant bits */
     int     bit_count;  /* # of bits shifted out of
                            value, at most 7 */
   } bool_decoder;

   /* Call this function before reading any bools from the
      partition. */

   void init_bool_decoder(bool_decoder *d, uint8 *start_partition)
   {
     {
       int i = 0;
       d->value = 0;           /* value = first 2 input bytes */
       while (++i <= 2)

         d->value = (d->value << 8)  |  *start_partition++;
     }

     d->input = start_partition;  /* ptr to next byte to be read */
     d->range = 255;           /* initial range is full */
     d->bit_count = 0;         /* have not yet shifted out any bits */
   }

   /* Main function reads a bool encoded at probability prob/256,
      which of course must agree with the probability used when the
      bool was written. */

   int read_bool(bool_decoder *d, Prob prob)
   {
     /* range and split are identical to the corresponding values
        used by the encoder when this bool was written */

     uint32  split = 1 + (((d->range - 1) * prob) >> 8);
     uint32  SPLIT = split << 8;
     int     retval;           /* will be 0 or 1 */

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     if (d->value >= SPLIT) {  /* encoded a one */
       retval = 1;
       d->range -= split;  /* reduce range */
       d->value -= SPLIT;  /* subtract off left endpoint of interval */
     } else {              /* encoded a zero */
       retval = 0;
       d->range = split;  /* reduce range, no change in left endpoint */
     }

     while (d->range < 128) {  /* shift out irrelevant value bits */
       d->value <<= 1;
       d->range <<= 1;
       if (++d->bit_count == 8) {  /* shift in new bits 8 at a time */
         d->bit_count = 0;
         d->value |= *d->input++;
       }
     }
     return retval;
   }

   /* Convenience function reads a "literal", that is, a "num_bits"-
      wide unsigned value whose bits come high- to low-order, with
      each bit encoded at probability 128 (i.e., 1/2). */

   uint32 read_literal(bool_decoder *d, int num_bits)
   {
     uint32 v = 0;

     while (num_bits--)
       v = (v << 1) + read_bool(d, 128);
     return v;
   }

   /* Variant reads a signed number */

   int32 read_signed_literal(bool_decoder *d, int num_bits)
   {
     int32 v = 0;
     if (!num_bits)
       return 0;
     if (read_bool(d, 128))
       v = -1;
     while (--num_bits)
       v = (v << 1) + read_bool(d, 128);
     return v;
   }

   ---- End code block ----------------------------------------

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8.  Compressed Data Components

   At the lowest level, VP8's compressed data is simply a sequence of
   probabilistically encoded bools.  Most of this data is composed of
   (slightly) larger semantic units fashioned from bools, which we
   describe here.

   We sometimes use these descriptions in C expressions within data
   format specifications.  In this context, they refer to the return
   value of a call to an appropriate bool_decoder d, reading (as always)
   from its current reference point.

   +--------------+-------+--------------------------------------------+
   | Call         | Alt.  | Return                                     |
   +--------------+-------+--------------------------------------------+
   | Bool(p)      | B(p)  | Bool with probability p/256 of being 0.    |
   |              |       | Return value of read_bool(d, p).           |
   |              |       |                                            |
   | Flag         | F     | A one-bit flag (same thing as a B(128) or  |
   |              |       | an L(1)).  Abbreviated F.  Return value of |
   |              |       | read_bool(d, 128).                         |
   |              |       |                                            |
   | Lit(n)       | L(n)  | Unsigned n-bit number encoded as n flags   |
   |              |       | (a "literal").  Abbreviated L(n).  The     |
   |              |       | bits are read from high to low order.      |
   |              |       | Return value of read_literal(d, n).        |
   |              |       |                                            |
   | SignedLit(n) |       | Signed n-bit number encoded similarly to   |
   |              |       | an L(n).  Return value of                  |
   |              |       | read_signed_literal(d, n).  These are      |
   |              |       | rare.                                      |
   |              |       |                                            |
   | P(8)         |       | An 8-bit probability.  No different from   |
   |              |       | an L(8), but we sometimes use this         |
   |              |       | notation to emphasize that a probability   |
   |              |       | is being coded.                            |
   |              |       |                                            |
   | P(7)         |       | A 7-bit specification of an 8-bit          |
   |              |       | probability.  Coded as an L(7) number x;   |
   |              |       | the resulting 8-bit probability is x ? x   |
   |              |       | << 1 : 1.                                  |
   |              |       |                                            |
   | F?  X        |       | A flag that, if true, is followed by a     |
   |              |       | piece of data X.                           |
   |              |       |                                            |

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   | F?  X:Y      |       | A flag that, if true, is followed by X     |
   |              |       | and, if false, is followed by Y.  Also     |
   |              |       | used to express a value where Y is an      |
   |              |       | implicit default (not encoded in the data  |
   |              |       | stream), as in F?  P(8):255, which         |
   |              |       | expresses an optional probability: If the  |
   |              |       | flag is true, the probability is specified |
   |              |       | as an 8-bit literal, while if the flag is  |
   |              |       | false, the probability defaults to 255.    |
   |              |       |                                            |
   | B(p)?  X     | B(p)? | Variants of the above using a boolean      |
   |              | X:Y   | indicator whose probability is not         |
   |              |       | necessarily 128.                           |
   |              |       |                                            |
   | T            |       | Tree-encoded value from small alphabet.    |
   +--------------+-------+--------------------------------------------+

   The last type requires elaboration.  We often wish to encode
   something whose value is restricted to a small number of
   possibilities (the alphabet).

   This is done by representing the alphabet as the leaves of a small
   binary tree.  The (non-leaf) nodes of the tree have associated
   probabilities p and correspond to calls to read_bool(d, p).  We think
   of a zero as choosing the left branch below the node and a one as
   choosing the right branch.

   Thus, every value (leaf) whose tree depth is x is decoded after
   exactly x calls to read_bool.

   A tree representing an encoding of an alphabet of n possible values
   always contains n-1 non-leaf nodes, regardless of its shape (this is
   easily seen by induction on n).

   There are many ways that a given alphabet can be so represented.  The
   choice of tree has little impact on datarate but does affect decoder
   performance.  The trees used by VP8 are chosen to (on average)
   minimize the number of calls to read_bool.  This amounts to shaping
   the tree so that values that are more probable have smaller tree
   depth than do values that are less probable.

   Readers familiar with Huffman coding will notice that, given an
   alphabet together with probabilities for each value, the associated
   Huffman tree minimizes the expected number of calls to read_bool.

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   Such readers will also realize that the coding method described here
   never results in higher datarates than does the Huffman method and,
   indeed, often results in much lower datarates.  Huffman coding is, in
   fact, nothing more than a special case of this method in which each
   node probability is fixed at 128 (i.e., 1/2).

8.1.  Tree Coding Implementation

   We give a suggested implementation of a tree data structure followed
   by a couple of actual examples of its usage by VP8.

   It is most convenient to represent the values using small positive
   integers, typically an enum counting up from zero.  The largest
   alphabet (used to code DCT coefficients, described in Section 13)
   that is tree-coded by VP8 has only 12 values.  The tree for this
   alphabet adds 11 interior nodes and so has a total of 23 positions.
   Thus, an 8-bit number easily accommodates both a tree position and a
   return value.

   A tree may then be compactly represented as an array of (pairs of)
   8-bit integers.  Each (even) array index corresponds to an interior
   node of the tree; the 0th index of course corresponds to the root of
   the tree.  The array entries come in pairs corresponding to the left
   (0) and right (1) branches of the subtree below the interior node.
   We use the convention that a positive (even) branch entry is the
   index of a deeper interior node, while a nonpositive entry v
   corresponds to a leaf whose value is -v.

   The node probabilities associated to a tree-coded value are stored in
   an array whose indices are half the indices of the corresponding tree
   positions.  The length of the probability array is one less than the
   size of the alphabet.

   Here is C code implementing the foregoing.  The advantages of our
   data structure should be noted.  Aside from the smallness of the
   structure itself, the tree-directed reading algorithm is essentially
   a single line of code.

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   ---- Begin code block --------------------------------------

   /* A tree specification is simply an array of 8-bit integers. */

   typedef int8 tree_index;
   typedef const tree_index Tree[];

   /* Read and return a tree-coded value at the current decoder
      position. */

   int treed_read(
     bool_decoder * const d, /* bool_decoder always returns a 0 or 1 */
     Tree t,                 /* tree specification */
     const Prob p[]     /* corresponding interior node probabilities */
   ) {
     register tree_index i = 0;   /* begin at root */

     /* Descend tree until leaf is reached */

     while ((i = t[ i + read_bool(d, p[i>>1])]) > 0) {}

     return -i;     /* return value is negation of nonpositive index */
   }

   ---- End code block ----------------------------------------

   Tree-based decoding is implemented in the reference decoder file
   bool_decoder.h (Section 20.2).

8.2.  Tree Coding Example

   As a multi-part example, without getting too far into the semantics
   of macroblock decoding (which is of course taken up below), we look
   at the "mode" coding for intra-predicted macroblocks.

   It so happens that, because of a difference in statistics, the Y (or
   luma) mode encoding uses two different trees: one for key frames and
   another for interframes.  This is the only instance in VP8 of the
   same dataset being coded by different trees under different
   circumstances.  The UV (or chroma) modes are a proper subset of the Y
   modes and, as such, have their own decoding tree.

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   ---- Begin code block --------------------------------------

   typedef enum
   {
       DC_PRED, /* predict DC using row above and column to the left */
       V_PRED,  /* predict rows using row above */
       H_PRED,  /* predict columns using column to the left */
       TM_PRED, /* propagate second differences a la "True Motion" */

       B_PRED,  /* each Y subblock is independently predicted */

       num_uv_modes = B_PRED,  /* first four modes apply to chroma */
       num_ymodes   /* all modes apply to luma */
   }
   intra_mbmode;

   /* The aforementioned trees together with the implied codings as
      comments.
      Actual (i.e., positive) indices are always even.
      Value (i.e., nonpositive) indices are arbitrary. */

   const tree_index ymode_tree [2 * (num_ymodes - 1)] =
   {
    -DC_PRED, 2,        /* root: DC_PRED = "0", "1" subtree */
     4, 6,              /* "1" subtree has 2 descendant subtrees */
      -V_PRED, -H_PRED, /* "10" subtree: V_PRED = "100",
                           H_PRED = "101" */
      -TM_PRED, -B_PRED /* "11" subtree: TM_PRED = "110",
                           B_PRED = "111" */
   };

   const tree_index kf_ymode_tree [2 * (num_ymodes - 1)] =
   {
    -B_PRED, 2,            /* root: B_PRED = "0", "1" subtree */
     4, 6,                 /* "1" subtree has 2 descendant subtrees */
      -DC_PRED, -V_PRED,   /* "10" subtree: DC_PRED = "100",
                              V_PRED = "101" */
      -H_PRED, -TM_PRED    /* "11" subtree: H_PRED = "110",
                              TM_PRED = "111" */
   };

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   const tree_index uv_mode_tree [2 * (num_uv_modes - 1)] =
   {
    -DC_PRED, 2,          /* root: DC_PRED = "0", "1" subtree */
     -V_PRED, 4,          /* "1" subtree:  V_PRED = "10",
                             "11" subtree */
      -H_PRED, -TM_PRED   /* "11" subtree: H_PRED = "110",
                             TM_PRED = "111" */
   };

   /* Given a bool_decoder d, a Y mode might be decoded as follows. */

   const Prob pretend_its_huffman [num_ymodes - 1] =
     { 128, 128, 128, 128};

   Ymode = (intra_mbmode) treed_read(d, ymode_tree,
     pretend_its_huffman);

   ---- End code block ----------------------------------------

   Since it greatly facilitates re-use of reference code, and since
   there is no real reason to do otherwise, it is strongly suggested
   that any decoder implementation use exactly the same enumeration
   values and probability table layouts as those described in this
   document (and in the reference code) for all tree-coded data in VP8.

9.  Frame Header

   The uncompressed data chunk at the start of each frame and at the
   first part of the first data partition contains information
   pertaining to the frame as a whole.  We list the fields in the order
   of occurrence.  Most of the header decoding occurs in the reference
   decoder file dixie.c (Section 20.4).

9.1.  Uncompressed Data Chunk

   The uncompressed data chunk comprises a common (for key frames and
   interframes) 3-byte frame tag that contains four fields, as follows:

   1.  A 1-bit frame type (0 for key frames, 1 for interframes).

   2.  A 3-bit version number (0 - 3 are defined as four different
       profiles with different decoding complexity; other values may be
       defined for future variants of the VP8 data format).

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   3.  A 1-bit show_frame flag (0 when current frame is not for display,
       1 when current frame is for display).

   4.  A 19-bit field containing the size of the first data partition in
       bytes.

   The version number setting enables or disables certain features in
   the bitstream, as follows:

            +---------+-------------------------+-------------+
            | Version | Reconstruction Filter   | Loop Filter |
            +---------+-------------------------+-------------+
            | 0       | Bicubic                 | Normal      |
            |         |                         |             |
            | 1       | Bilinear                | Simple      |
            |         |                         |             |
            | 2       | Bilinear                | None        |
            |         |                         |             |
            | 3       | None                    | None        |
            |         |                         |             |
            | Other   | Reserved for future use |             |
            +---------+-------------------------+-------------+

   The reference software also adjusts the loop filter based on version
   number, as per the table above.  Version number 1 implies a "simple"
   loop filter, and version numbers 2 and 3 imply no loop filter.
   However, the "simple" filter setting in this context has no effect
   whatsoever on the decoding process, and the "no loop filter" setting
   only forces the reference encoder to set filter level equal to 0.
   Neither affect the decoding process.  In decoding, the only loop
   filter settings that matter are those in the frame header.

   For key frames, the frame tag is followed by a further 7 bytes of
   uncompressed data, as follows:

   ---- Begin code block --------------------------------------

   Start code byte 0     0x9d
   Start code byte 1     0x01
   Start code byte 2     0x2a

   16 bits      :     (2 bits Horizontal Scale << 14) | Width (14 bits)
   16 bits      :     (2 bits Vertical Scale << 14) | Height (14 bits)

   ---- End code block ----------------------------------------

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   The following source code segment illustrates validation of the start
   code and reading the width, height, and scale factors for a key
   frame.

   ---- Begin code block --------------------------------------

   unsigned char *c = pbi->source+3;

   // vet via sync code
   if (c[0]!=0x9d||c[1]!=0x01||c[2]!=0x2a)
       return -1;

   ---- End code block ----------------------------------------

   Where pbi->source points to the beginning of the frame.

   The following code reads the image dimension from the bitstream:

   ---- Begin code block --------------------------------------

   pc->Width      = swap2(*(unsigned short*)(c+3))&0x3fff;
   pc->horiz_scale = swap2(*(unsigned short*)(c+3))>>14;
   pc->Height     = swap2(*(unsigned short*)(c+5))&0x3fff;
   pc->vert_scale  = swap2(*(unsigned short*)(c+5))>>14;

   ---- End code block ----------------------------------------

   Where the swap2 macro takes care of the endian on a different
   platform:

   ---- Begin code block --------------------------------------

   #if defined(__ppc__) || defined(__ppc64__)
   # define swap2(d)  \
     ((d&0x000000ff)<<8) |  \
     ((d&0x0000ff00)>>8)
   #else
     # define swap2(d) d
   #endif

   ---- End code block ----------------------------------------

   While each frame is encoded as a raster scan of 16x16 macroblocks,
   the frame dimensions are not necessarily evenly divisible by 16.  In
   this case, write ew = 16 - (width & 15) and eh = 16 - (height & 15)
   for the excess width and height, respectively.  Although they are

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   encoded, the last ew columns and eh rows are not actually part of the
   image and should be discarded before final output.  However, these
   "excess pixels" should be maintained in the internal reconstruction
   buffer used to predict ensuing frames.

   The scaling specifications for each dimension are encoded as follows.

             +-------+--------------------------------------+
             | Value | Scaling                              |
             +-------+--------------------------------------+
             | 0     | No upscaling (the most common case). |
             |       |                                      |
             | 1     | Upscale by 5/4.                      |
             |       |                                      |
             | 2     | Upscale by 5/3.                      |
             |       |                                      |
             | 3     | Upscale by 2.                        |
             +-------+--------------------------------------+

   Upscaling does not affect the reconstruction buffer, which should be
   maintained at the encoded resolution.  Any reasonable method of
   upsampling (including any that may be supported by video hardware in
   the playback environment) may be used.  Since scaling has no effect
   on decoding, we do not discuss it any further.

   As discussed in Section 5, allocation (or re-allocation) of data
   structures (such as the reconstruction buffer) whose size depends on
   dimension will be triggered here.

9.2.  Color Space and Pixel Type (Key Frames Only)

           +-------+------------------------------------------+
           | Field | Value                                    |
           +-------+------------------------------------------+
           | L(1)  | 1-bit color space type specification     |
           |       |                                          |
           | L(1)  | 1-bit pixel value clamping specification |
           +-------+------------------------------------------+

   The color space type bit is encoded as follows:

   o  0 - YUV color space similar to the YCrCb color space defined in
      [ITU-R_BT.601]

   o  1 - Reserved for future use

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   The pixel value clamping type bit is encoded as follows:

   o  0 - Decoders are required to clamp the reconstructed pixel values
      to between 0 and 255 (inclusive).

   o  1 - Reconstructed pixel values are guaranteed to be between 0 and
      255; no clamping is necessary.

   Information in this subsection does not appear in interframes.

9.3.  Segment-Based Adjustments

   This subsection contains probability and value information for
   implementing segment adaptive adjustments to default decoder
   behavior.  The data in this subsection is used in the decoding of the
   ensuing per-segment information and applies to the entire frame.
   When segment adaptive adjustments are enabled, each macroblock will
   be assigned a segment ID.  Macroblocks with the same segment ID
   belong to the same segment and have the same adaptive adjustments
   over default baseline values for the frame.  The adjustments can be
   quantizer level or loop filter strength.

   The context for decoding this feature at the macroblock level is
   provided by a subsection in the frame header, which contains:

   1.  A segmentation_enabled flag that enables the feature for this
       frame if set to 1, and disables it if set to 0.  The following
       fields occur if the feature is enabled.

   2.  L(1) indicates if the segment map is updated for the current
       frame (update_mb_segmentation_map).

   3.  L(1) indicates if the segment feature data items are updated for
       the current frame (update_segment_feature_data).

   4.  If Item 3 above (update_segment_feature_data) is 1, the following
       fields occur:

       a.  L(1), the mode of segment feature data
           (segment_feature_mode), can be absolute-value mode (0) or
           delta value mode (1).

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       b.  Segment feature data items are decoded segment by segment for
           each segment feature.  For every data item, a one-bit flag
           indicates whether the item is 0, or a non-zero value to be
           decoded.  If the value is non-zero, then the value is decoded
           as a magnitude L(n), followed by a one-bit sign (L(1) -- 0
           for positive and 1 for negative).  The length n can be looked
           up from a pre-defined length table for all feature data.

   5.  If the L(1) flag as noted in Item 2 above is set to 1, the
       probabilities of the decoding tree for the segment map are
       decoded from the bitstream.  Each probability is decoded with a
       one-bit flag indicating whether the probability is the default
       value of 255 (flag is set to 0), or an 8-bit value, L(8), from
       the bitstream.

   The layout and semantics supporting this feature at the macroblock
   level are described in Section 10.

9.4.  Loop Filter Type and Levels

   VP8 supports two types of loop filters having different computational
   complexity.  The following bits occur in the header to support the
   selection of the baseline type, strength, and sharpness behavior of
   the loop filter used for the current frame.

                       +-------+-------------------+
                       | Index | Description       |
                       +-------+-------------------+
                       | L(1)  | filter_type       |
                       |       |                   |
                       | L(6)  | loop_filter_level |
                       |       |                   |
                       | L(3)  | sharpness_level   |
                       +-------+-------------------+

   The meaning of these numbers will be further explained in Section 15.

   VP8 has a feature in the bitstream that enables adjustment of the
   loop filter level based on a macroblock's prediction mode and
   reference frame.  The per-macroblock adjustment is done through delta
   values against the default loop filter level for the current frame.
   This subsection contains flag and value information for implementing
   per-macroblock loop filter level adjustment to default decoder
   behavior.  The data in this section is used in the decoding of the
   ensuing per-macroblock information and applies to the entire frame.

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   L(1) is a one-bit flag indicating if the macroblock loop filter
   adjustment is on for the current frame.  0 means that such a feature
   is not supported in the current frame, and 1 means this feature is
   enabled for the current frame.

   Whether the adjustment is based on a reference frame or encoding
   mode, the adjustment of the loop filter level is done via a delta
   value against a baseline loop filter value.  The delta values are
   updated for the current frame if an L(1) bit,
   mode_ref_lf_delta_update, takes the value 1.  There are two groups of
   delta values: One group of delta values is for reference frame-based
   adjustments, and the other group is for mode-based adjustments.  The
   number of delta values in the two groups is MAX_REF_LF_DELTAS and
   MAX_MODE_LF_DELTAS, respectively.  For every value within the two
   groups, there is a one-bit L(1) to indicate if the particular value
   is updated.  When one is updated (1), it is transmitted as a six-bit-
   magnitude L(6) followed by a one-bit sign flag (L(1) -- 0 for
   positive and 1 for negative).

9.5.  Token Partition and Partition Data Offsets

   VP8 allows DCT coefficients to be packed into multiple partitions,
   besides the first partition with header and per-macroblock prediction
   information, so the decoder can perform parallel decoding in an
   efficient manner.  A two-bit L(2) is used to indicate the number of
   coefficient data partitions within a compressed frame.  The two bits
   are defined in the following table:

                 +-------+-------+----------------------+
                 | Bit 1 | Bit 0 | Number of Partitions |
                 +-------+-------+----------------------+
                 | 0     | 0     | 1                    |
                 |       |       |                      |
                 | 0     | 1     | 2                    |
                 |       |       |                      |
                 | 1     | 0     | 4                    |
                 |       |       |                      |
                 | 1     | 1     | 8                    |
                 +-------+-------+----------------------+

   Offsets are embedded in the bitstream to provide the decoder direct
   access to token partitions.  If the number of data partitions is
   greater than 1, the size of each partition (except the last) is
   written in 3 bytes (24 bits).  The size of the last partition is the
   remainder of the data not used by any of the previous partitions.

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   The partitioned data are consecutive in the bitstream, so the size
   can also be used to calculate the offset of each partition.  The
   following pseudocode illustrates how the size/offset is defined by
   the three bytes in the bitstream.

   ---- Begin code block --------------------------------------

   Offset/size  =  (uint32)(byte0) + ((uint32)(byte1)<<8)
     + ((uint32)(byte2)<<16);

   ---- End code block ----------------------------------------

9.6.  Dequantization Indices

   All residue signals are specified via a quantized 4x4 DCT applied to
   the Y, U, V, or Y2 subblocks of a macroblock.  As detailed in
   Section 14, before inverting the transform, each decoded coefficient
   is multiplied by one of six dequantization factors, the choice of
   which depends on the plane (Y, chroma = U or V, Y2) and coefficient
   position (DC = coefficient 0, AC = coefficients 1-15).  The six
   values are specified using 7-bit indices into six corresponding fixed
   tables (the tables are given in Section 14).

   The first 7-bit index gives the dequantization table index for
   Y-plane AC coefficients, called yac_qi.  It is always coded and acts
   as a baseline for the other 5 quantization indices, each of which is
   represented by a delta from this baseline index.  Pseudocode for
   reading the indices follows:

   ---- Begin code block --------------------------------------

   yac_qi     = L(7);           /* Y ac index always specified */
   ydc_delta  = F? delta(): 0;  /* Y dc delta specified if
                                   flag is true */

   y2dc_delta = F? delta(): 0;  /* Y2 dc delta specified if
                                   flag is true */
   y2ac_delta = F? delta(): 0;  /* Y2 ac delta specified if
                                   flag is true */

   uvdc_delta = F? delta(): 0;  /* chroma dc delta specified
                                   if flag is true */
   uvac_delta = F? delta(): 0;  /* chroma ac delta specified
                                   if flag is true */

   ---- End code block ----------------------------------------

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   Where delta() is the process to read 5 bits from the bitstream to
   determine a signed delta value:

       +-------+--------------------------------------------------+
       | Index | Description                                      |
       +-------+--------------------------------------------------+
       | L(4)  | Magnitude of delta                               |
       |       |                                                  |
       | L(1)  | Sign of delta, 0 for positive and 1 for negative |
       +-------+--------------------------------------------------+

9.7.  Refresh Golden Frame and Altref Frame

   For key frames, both the golden frame and the altref frame are
   refreshed/ replaced by the current reconstructed frame, by default.
   For non-key frames, VP8 uses two bits to indicate whether the two
   frame buffers are refreshed, using the reconstructed current frame:

   +-------+----------------------------------------------------------+
   | Index | Description                                              |
   +-------+----------------------------------------------------------+
   | L(1)  | Whether golden frame is refreshed (0 for no, 1 for yes). |
   |       |                                                          |
   | L(1)  | Whether altref frame is refreshed (0 for no, 1 for yes). |
   +-------+----------------------------------------------------------+

   When the flag for the golden frame is 0, VP8 uses 2 more bits in the
   bitstream to indicate whether the buffer (and which buffer) is copied
   to the golden frame, or if no buffer is copied:

           +-------+------------------------------------------+
           | Index | Description                              |
           +-------+------------------------------------------+
           | L(2)  | Buffer copy flag for golden frame buffer |
           +-------+------------------------------------------+

   Where:

   o  0 means no buffer is copied to the golden frame

   o  1 means last_frame is copied to the golden frame

   o  2 means alt_ref_frame is copied to the golden frame

   Similarly, when the flag for altref is 0, VP8 uses 2 bits in the
   bitstream to indicate which buffer is copied to alt_ref_frame.

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           +-------+------------------------------------------+
           | Index | Description                              |
           +-------+------------------------------------------+
           | L(2)  | Buffer copy flag for altref frame buffer |
           +-------+------------------------------------------+

   Where:

   o  0 means no buffer is copied to the altref frame

   o  1 means last_frame is copied to the altref frame

   o  2 means golden_frame is copied to the altref frame

   Two bits are transmitted for ref_frame_sign_bias for golden_frame and
   alt_ref_frame, respectively.

                +-------+---------------------------------+
                | Index | Description                     |
                +-------+---------------------------------+
                | L(1)  | Sign bias flag for golden frame |
                |       |                                 |
                | L(1)  | Sign bias flag for altref frame |
                +-------+---------------------------------+

   These values are used to control the sign of the motion vectors when
   a golden frame or an altref frame is used as the reference frame for
   a macroblock.

9.8.  Refresh Last Frame Buffer

   VP8 uses one bit, L(1), to indicate if the last frame reference
   buffer is refreshed using the constructed current frame.  On a key
   frame, this bit is overridden, and the last frame buffer is always
   refreshed.

9.9.  DCT Coefficient Probability Update

   This field contains updates to the probability tables used to decode
   DCT coefficients.  For each of the probabilities in the tables, there
   is an L(1) flag indicating if the probability is updated for the
   current frame, and if the L(1) flag is set to 1, there follows an
   additional 8-bit value representing the new probability value.  These
   tables are maintained across interframes but are of course replaced
   with their defaults at the beginning of every key frame.

   The layout and semantics of this field will be taken up in
   Section 13.

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9.10.  Remaining Frame Header Data (Non-Key Frame)

   +-------+-----------------------------------------------------------+
   | Index | Description                                               |
   +-------+-----------------------------------------------------------+
   | L(1)  | mb_no_skip_coeff.  This flag indicates at the frame level |
   |       | if skipping of macroblocks with no non-zero coefficients  |
   |       | is enabled.  If it is set to 0, then prob_skip_false is   |
   |       | not read and mb_skip_coeff is forced to 0 for all         |
   |       | macroblocks (see Sections 11.1 and 12.1).                 |
   |       |                                                           |
   | L(8)  | prob_skip_false = probability used for decoding a         |
   |       | macroblock-level flag, which indicates if a macroblock    |
   |       | has any non-zero coefficients.  Only read if              |
   |       | mb_no_skip_coeff is 1.                                    |
   |       |                                                           |
   | L(8)  | prob_intra = probability that a macroblock is "intra"     |
   |       | predicted (that is, predicted from the already-encoded    |
   |       | portions of the current frame), as opposed to "inter"     |
   |       | predicted (that is, predicted from the contents of a      |
   |       | prior frame).                                             |
   |       |                                                           |
   | L(8)  | prob_last = probability that an inter-predicted           |
   |       | macroblock is predicted from the immediately previous     |
   |       | frame, as opposed to the most recent golden frame or      |
   |       | altref frame.                                             |
   |       |                                                           |
   | L(8)  | prob_gf = probability that an inter-predicted macroblock  |
   |       | is predicted from the most recent golden frame, as        |
   |       | opposed to the altref frame.                              |
   |       |                                                           |
   | F     | If true, followed by four L(8)s updating the              |
   |       | probabilities for the different types of intra-prediction |
   |       | for the Y plane.  These probabilities correspond to the   |
   |       | four interior nodes of the decoding tree for intra-Y      |
   |       | modes in an interframe, that is, the even positions in    |
   |       | the ymode_tree array given above.                         |
   |       |                                                           |
   | F     | If true, followed by three L(8)s updating the             |
   |       | probabilities for the different types of intra-prediction |
   |       | for the chroma planes.  These probabilities correspond to |
   |       | the even positions in the uv_mode_tree array given above. |
   |       |                                                           |
   | X     | Motion vector probability update.  Details are given in   |
   |       | Section 17.2, "Probability Updates".                      |
   +-------+-----------------------------------------------------------+

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   Decoding of this portion of the frame header is handled in the
   reference decoder file dixie.c (Section 20.4).

9.11.  Remaining Frame Header Data (Key Frame)

   +-------+-----------------------------------------------------------+
   | Index | Description                                               |
   +-------+-----------------------------------------------------------+
   | L(1)  | mb_no_skip_coeff.  This flag indicates at the frame level |
   |       | if skipping of macroblocks with no non-zero coefficients  |
   |       | is enabled.  If it is set to 0, then prob_skip_false is   |
   |       | not read and mb_skip_coeff is forced to 0 for all         |
   |       | macroblocks (see Sections 11.1 and 12.1).                 |
   |       |                                                           |
   | L(8)  | prob_skip_false = Probability used for decoding a         |
   |       | macroblock-level flag, which indicates if a macroblock    |
   |       | has any non-zero coefficients.  Only read if              |
   |       | mb_no_skip_coeff is 1.                                    |
   +-------+-----------------------------------------------------------+

   Decoding of this portion of the frame header is handled in the
   reference decoder file modemv.c (Section 20.11).

   This completes the layout of the frame header.  The remainder of the
   first data partition consists of macroblock-level prediction data.

   After the frame header is processed, all probabilities needed to
   decode the prediction and residue data are known and will not change
   until the next frame.

10.  Segment-Based Feature Adjustments

   Every macroblock may optionally override some of the default
   behaviors of the decoder.  Specifically, VP8 uses segment-based
   adjustments to support changing quantizer level and loop filter level
   for a macroblock.  When the segment-based adjustment feature is
   enabled for a frame, each macroblock within the frame is coded with a
   segment_id.  This effectively segments all the macroblocks in the
   current frame into a number of different segments.  Macroblocks
   within the same segment behave exactly the same for quantizer and
   loop filter level adjustments.

   If both the segmentation_enabled and update_mb_segmentation_map flags
   in subsection B of the frame header take a value of 1, the prediction
   data for each (intra- or inter-coded) macroblock begins with a
   specification of segment_id for the current macroblock.  It is
   decoded using this simple tree ...

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   ---- Begin code block --------------------------------------

   const tree_index mb_segment_tree [2 * (4-1)] =
     {
       2,  4,     /* root: "0", "1" subtrees */
       -0, -1,    /* "00" = 0th value, "01" = 1st value */
        -2, -3    /* "10" = 2nd value, "11" = 3rd value */
     }

   ---- End code block ----------------------------------------

   ... combined with a 3-entry probability table,
   mb_segment_tree_probs[3].  The macroblock's segment_id is used later
   in the decoding process to look into the segment_feature_data table
   and determine how the quantizer and loop filter levels are adjusted.

   The decoding of segment_id, together with the parsing of
   intra-prediction modes (which is taken up next), is implemented in
   the reference decoder file modemv.c.

11.  Key Frame Macroblock Prediction Records

   After specifying the features described above, the macroblock
   prediction record next specifies the prediction mode used for the
   macroblock.

11.1.  mb_skip_coeff

   The single bool flag is decoded using prob_skip_false if and only if
   mb_no_skip_coeff is set to 1 (see Sections 9.10 and 9.11).  If
   mb_no_skip_coeff is set to 0, then this value defaults to 0.

11.2.  Luma Modes

   First comes the luma specification of type intra_mbmode, coded using
   the kf_ymode_tree, as described in Section 8 and repeated here for
   convenience:

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   ---- Begin code block --------------------------------------

   typedef enum
   {
       DC_PRED, /* predict DC using row above and column to the left */
       V_PRED,  /* predict rows using row above */
       H_PRED,  /* predict columns using column to the left */
       TM_PRED, /* propagate second differences a la "True Motion" */

       B_PRED,  /* each Y subblock is independently predicted */

       num_uv_modes = B_PRED,  /* first four modes apply to chroma */
       num_ymodes   /* all modes apply to luma */
   }
   intra_mbmode;

   const tree_index kf_ymode_tree [2 * (num_ymodes - 1)] =
   {
    -B_PRED, 2,            /* root: B_PRED = "0", "1" subtree */
     4, 6,                 /* "1" subtree has 2 descendant subtrees */
      -DC_PRED, -V_PRED,   /* "10" subtree: DC_PRED = "100",
                              V_PRED = "101" */
      -H_PRED, -TM_PRED    /* "11" subtree: H_PRED = "110",
                              TM_PRED = "111" */
   };

   ---- End code block ----------------------------------------

   For key frames, the Y mode is decoded using a fixed probability array
   as follows:

   ---- Begin code block --------------------------------------

   const Prob kf_ymode_prob [num_ymodes - 1] = { 145, 156, 163, 128};
   Ymode = (intra_mbmode) treed_read(d, kf_ymode_tree, kf_ymode_prob);

   ---- End code block ----------------------------------------

   d is of course the bool_decoder being used to read the first data
   partition.

   If the Ymode is B_PRED, it is followed by a (tree-coded) mode for
   each of the 16 Y subblocks.  The 10 subblock modes and their coding
   tree are as follows:

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   ---- Begin code block --------------------------------------

   typedef enum
   {
       B_DC_PRED,  /* predict DC using row above and column
                      to the left */
       B_TM_PRED,  /* propagate second differences a la
                      "True Motion" */

       B_VE_PRED,  /* predict rows using row above */
       B_HE_PRED,  /* predict columns using column to the left */

       B_LD_PRED,  /* southwest (left and down) 45 degree diagonal
                      prediction */
       B_RD_PRED,  /* southeast (right and down) "" */

       B_VR_PRED,  /* SSE (vertical right) diagonal prediction */
       B_VL_PRED,  /* SSW (vertical left) "" */
       B_HD_PRED,  /* ESE (horizontal down) "" */
       B_HU_PRED,  /* ENE (horizontal up) "" */

       num_intra_bmodes
   }
   intra_bmode;

   /* Coding tree for the above, with implied codings as comments */

   const tree_index bmode_tree [2 * (num_intra_bmodes - 1)] =
   {
    -B_DC_PRED, 2,                   /* B_DC_PRED = "0" */
     -B_TM_PRED, 4,                  /* B_TM_PRED = "10" */
      -B_VE_PRED, 6,                 /* B_VE_PRED = "110" */
       8, 12,
        -B_HE_PRED, 10,              /* B_HE_PRED = "11100" */
         -B_RD_PRED, -B_VR_PRED,     /* B_RD_PRED = "111010",
                                        B_VR_PRED = "111011" */
        -B_LD_PRED, 14,              /* B_LD_PRED = "111110" */
          -B_VL_PRED, 16,            /* B_VL_PRED = "1111110" */
            -B_HD_PRED, -B_HU_PRED   /* HD = "11111110",
                                        HU = "11111111" */
   };

   ---- End code block ----------------------------------------

   The first four modes are smaller versions of the similarly named
   16x16 modes above, albeit with slightly different numbering.  The
   last six "diagonal" modes are unique to luma subblocks.

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11.3.  Subblock Mode Contexts

   The coding of subblock modes in key frames uses the modes already
   coded for the subblocks to the left of and above the subblock to
   select a probability array for decoding the current subblock mode.
   This is our first instance of contextual prediction, and there are
   several caveats associated with it:

   1.  The adjacency relationships between subblocks are based on the
       normal default raster placement of the subblocks.

   2.  The adjacent subblocks need not lie in the current macroblock.
       The subblocks to the left of the left-edge subblocks 0, 4, 8, and
       12 are the right-edge subblocks 3, 7, 11, and 15, respectively,
       of the (already coded) macroblock immediately to the left.
       Similarly, the subblocks above the top-edge subblocks 0, 1, 2,
       and 3 are the bottom-edge subblocks 12, 13, 14, and 15 of the
       already-coded macroblock immediately above us.

   3.  For macroblocks on the top row or left edge of the image, some of
       the predictors will be non-existent.  Such predictors are taken
       to have had the value B_DC_PRED, which, perhaps conveniently,
       takes the value 0 in the enumeration above.  A simple management
       scheme for these contexts might maintain a row of above
       predictors and four left predictors.  Before decoding the frame,
       the entire row is initialized to B_DC_PRED; before decoding each
       row of macroblocks, the four left predictors are also set to
       B_DC_PRED.  After decoding a macroblock, the bottom four subblock
       modes are copied into the row predictor (at the current position,
       which then advances to be above the next macroblock), and the
       right four subblock modes are copied into the left predictor.

   4.  Many macroblocks will of course be coded using a 16x16 luma
       prediction mode.  For the purpose of predicting ensuing subblock
       modes (only), such macroblocks derive a subblock mode, constant
       throughout the macroblock, from the 16x16 luma mode as follows:
       DC_PRED uses B_DC_PRED, V_PRED uses B_VE_PRED, H_PRED uses
       B_HE_PRED, and TM_PRED uses B_TM_PRED.

   5.  Although we discuss interframe modes in Section 16, we remark
       here that, while interframes do use all the intra-coding modes
       described here and below, the subblock modes in an interframe are
       coded using a single constant probability array that does not
       depend on any context.

   The dependence of subblock mode probability on the nearby subblock
   mode context is most easily handled using a three-dimensional
   constant array:

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   ---- Begin code block --------------------------------------

   const Prob kf_bmode_prob [num_intra_bmodes] [num_intra_bmodes]
     [num_intra_bmodes-1];

   ---- End code block ----------------------------------------

   The outer two dimensions of this array are indexed by the already-
   coded subblock modes above and to the left of the current block,
   respectively.  The inner dimension is a typical tree probability list
   whose indices correspond to the even indices of the bmode_tree above.
   The mode for the j^(th) luma subblock is then

   ---- Begin code block --------------------------------------

   Bmode = (intra_bmode) treed_read(d, bmode_tree, kf_bmode_prob
     [A] [L]);

   ---- End code block ----------------------------------------

   Where the 4x4 Y subblock index j varies from 0 to 15 in raster order,
   and A and L are the modes used above and to the left of the j^(th)
   subblock.

   The contents of the kf_bmode_prob array are given at the end of this
   section.

11.4.  Chroma Modes

   After the Y mode (and optional subblock mode) specification comes the
   chroma mode.  The chroma modes are a subset of the Y modes and are
   coded using the uv_mode_tree, as described in Section 8 and repeated
   here for convenience:

   ---- Begin code block --------------------------------------

   const tree_index uv_mode_tree [2 * (num_uv_modes - 1)] =
   {
    -DC_PRED, 2,           /* root: DC_PRED = "0", "1" subtree */
     -V_PRED, 4,           /* "1" subtree:  V_PRED = "10",
                              "11" subtree */
      -H_PRED, -TM_PRED    /* "11" subtree: H_PRED = "110",
                              TM_PRED = "111" */
   };

   ---- End code block ----------------------------------------

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   As for the Y modes (in a key frame), the chroma modes are coded using
   a fixed, contextless probability table:

   ---- Begin code block --------------------------------------

   const Prob kf_uv_mode_prob [num_uv_modes - 1] = { 142, 114, 183};
   uv_mode = (intra_mbmode) treed_read(d, uv_mode_tree,
     kf_uv_mode_prob);

   ---- End code block ----------------------------------------

   This completes the description of macroblock prediction coding for
   key frames.  As will be discussed in Section 16, the coding of intra
   modes within interframes is similar, but not identical, to that
   described here (and in the reference code) for prediction modes and,
   indeed, for all tree-coded data in VP8.

11.5.  Subblock Mode Probability Table

   Finally, here is the fixed probability table used to decode subblock
   modes in key frames.

   ---- Begin code block --------------------------------------

   const Prob kf_bmode_prob [num_intra_bmodes] [num_intra_bmodes]
     [num_intra_bmodes-1] =
   {
     {
       { 231, 120,  48,  89, 115, 113, 120, 152, 112},
       { 152, 179,  64, 126, 170, 118,  46,  70,  95},
       { 175,  69, 143,  80,  85,  82,  72, 155, 103},
       {  56,  58,  10, 171, 218, 189,  17,  13, 152},
       { 144,  71,  10,  38, 171, 213, 144,  34,  26},
       { 114,  26,  17, 163,  44, 195,  21,  10, 173},
       { 121,  24,  80, 195,  26,  62,  44,  64,  85},
       { 170,  46,  55,  19, 136, 160,  33, 206,  71},
       {  63,  20,   8, 114, 114, 208,  12,   9, 226},
       {  81,  40,  11,  96, 182,  84,  29,  16,  36}
     },

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     {
       { 134, 183,  89, 137,  98, 101, 106, 165, 148},
       {  72, 187, 100, 130, 157, 111,  32,  75,  80},
       {  66, 102, 167,  99,  74,  62,  40, 234, 128},
       {  41,  53,   9, 178, 241, 141,  26,   8, 107},
       { 104,  79,  12,  27, 217, 255,  87,  17,   7},
       {  74,  43,  26, 146,  73, 166,  49,  23, 157},
       {  65,  38, 105, 160,  51,  52,  31, 115, 128},
       {  87,  68,  71,  44, 114,  51,  15, 186,  23},
       {  47,  41,  14, 110, 182, 183,  21,  17, 194},
       {  66,  45,  25, 102, 197, 189,  23,  18,  22}
     },
     {
       {  88,  88, 147, 150,  42,  46,  45, 196, 205},
       {  43,  97, 183, 117,  85,  38,  35, 179,  61},
       {  39,  53, 200,  87,  26,  21,  43, 232, 171},
       {  56,  34,  51, 104, 114, 102,  29,  93,  77},
       { 107,  54,  32,  26,  51,   1,  81,  43,  31},
       {  39,  28,  85, 171,  58, 165,  90,  98,  64},
       {  34,  22, 116, 206,  23,  34,  43, 166,  73},
       {  68,  25, 106,  22,  64, 171,  36, 225, 114},
       {  34,  19,  21, 102, 132, 188,  16,  76, 124},
       {  62,  18,  78,  95,  85,  57,  50,  48,  51}
     },
     {
       { 193, 101,  35, 159, 215, 111,  89,  46, 111},
       {  60, 148,  31, 172, 219, 228,  21,  18, 111},
       { 112, 113,  77,  85, 179, 255,  38, 120, 114},
       {  40,  42,   1, 196, 245, 209,  10,  25, 109},
       { 100,  80,   8,  43, 154,   1,  51,  26,  71},
       {  88,  43,  29, 140, 166, 213,  37,  43, 154},
       {  61,  63,  30, 155,  67,  45,  68,   1, 209},
       { 142,  78,  78,  16, 255, 128,  34, 197, 171},
       {  41,  40,   5, 102, 211, 183,   4,   1, 221},
       {  51,  50,  17, 168, 209, 192,  23,  25,  82}
     },
     {
       { 125,  98,  42,  88, 104,  85, 117, 175,  82},
       {  95,  84,  53,  89, 128, 100, 113, 101,  45},
       {  75,  79, 123,  47,  51, 128,  81, 171,   1},
       {  57,  17,   5,  71, 102,  57,  53,  41,  49},
       { 115,  21,   2,  10, 102, 255, 166,  23,   6},
       {  38,  33,  13, 121,  57,  73,  26,   1,  85},
       {  41,  10,  67, 138,  77, 110,  90,  47, 114},
       { 101,  29,  16,  10,  85, 128, 101, 196,  26},
       {  57,  18,  10, 102, 102, 213,  34,  20,  43},
       { 117,  20,  15,  36, 163, 128,  68,   1,  26}
     },

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     {
       { 138,  31,  36, 171,  27, 166,  38,  44, 229},
       {  67,  87,  58, 169,  82, 115,  26,  59, 179},
       {  63,  59,  90, 180,  59, 166,  93,  73, 154},
       {  40,  40,  21, 116, 143, 209,  34,  39, 175},
       {  57,  46,  22,  24, 128,   1,  54,  17,  37},
       {  47,  15,  16, 183,  34, 223,  49,  45, 183},
       {  46,  17,  33, 183,   6,  98,  15,  32, 183},
       {  65,  32,  73, 115,  28, 128,  23, 128, 205},
       {  40,   3,   9, 115,  51, 192,  18,   6, 223},
       {  87,  37,   9, 115,  59,  77,  64,  21,  47}
     },
     {
       { 104,  55,  44, 218,   9,  54,  53, 130, 226},
       {  64,  90,  70, 205,  40,  41,  23,  26,  57},
       {  54,  57, 112, 184,   5,  41,  38, 166, 213},
       {  30,  34,  26, 133, 152, 116,  10,  32, 134},
       {  75,  32,  12,  51, 192, 255, 160,  43,  51},
       {  39,  19,  53, 221,  26, 114,  32,  73, 255},
       {  31,   9,  65, 234,   2,  15,   1, 118,  73},
       {  88,  31,  35,  67, 102,  85,  55, 186,  85},
       {  56,  21,  23, 111,  59, 205,  45,  37, 192},
       {  55,  38,  70, 124,  73, 102,   1,  34,  98}
     },
     {
       { 102,  61,  71,  37,  34,  53,  31, 243, 192},
       {  69,  60,  71,  38,  73, 119,  28, 222,  37},
       {  68,  45, 128,  34,   1,  47,  11, 245, 171},
       {  62,  17,  19,  70, 146,  85,  55,  62,  70},
       {  75,  15,   9,   9,  64, 255, 184, 119,  16},
       {  37,  43,  37, 154, 100, 163,  85, 160,   1},
       {  63,   9,  92, 136,  28,  64,  32, 201,  85},
       {  86,   6,  28,   5,  64, 255,  25, 248,   1},
       {  56,   8,  17, 132, 137, 255,  55, 116, 128},
       {  58,  15,  20,  82, 135,  57,  26, 121,  40}
     },
     {
       { 164,  50,  31, 137, 154, 133,  25,  35, 218},
       {  51, 103,  44, 131, 131, 123,  31,   6, 158},
       {  86,  40,  64, 135, 148, 224,  45, 183, 128},
       {  22,  26,  17, 131, 240, 154,  14,   1, 209},
       {  83,  12,  13,  54, 192, 255,  68,  47,  28},
       {  45,  16,  21,  91,  64, 222,   7,   1, 197},
       {  56,  21,  39, 155,  60, 138,  23, 102, 213},
       {  85,  26,  85,  85, 128, 128,  32, 146, 171},
       {  18,  11,   7,  63, 144, 171,   4,   4, 246},
       {  35,  27,  10, 146, 174, 171,  12,  26, 128}
     },

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     {
       { 190,  80,  35,  99, 180,  80, 126,  54,  45},
       {  85, 126,  47,  87, 176,  51,  41,  20,  32},
       { 101,  75, 128, 139, 118, 146, 116, 128,  85},
       {  56,  41,  15, 176, 236,  85,  37,   9,  62},
       { 146,  36,  19,  30, 171, 255,  97,  27,  20},
       {  71,  30,  17, 119, 118, 255,  17,  18, 138},
       { 101,  38,  60, 138,  55,  70,  43,  26, 142},
       { 138,  45,  61,  62, 219,   1,  81, 188,  64},
       {  32,  41,  20, 117, 151, 142,  20,  21, 163},
       { 112,  19,  12,  61, 195, 128,  48,   4,  24}
     }
   };

   ---- End code block ----------------------------------------

12.  Intraframe Prediction

   Intraframe prediction uses already-coded macroblocks within the
   current frame to approximate the contents of the current macroblock.
   It applies to intra-coded macroblocks in an interframe and to all
   macroblocks in a key frame.

   Relative to the current macroblock "M", the already-coded macroblocks
   include all macroblocks above M together with the macroblocks on the
   same row as, and to the left of, M, though at most four of these
   macroblocks are actually used: the block "A" directly above M, the
   blocks immediately to the left and right of A, and the block
   immediately to the left of M.

   Each of the prediction modes (i.e., means of extrapolation from
   already-calculated values) uses fairly simple arithmetic on pixel
   values whose positions, relative to the current position, are defined
   by the mode.

   The chroma (U and V) and luma (Y) predictions are independent of each
   other.

   The relative addressing of pixels applied to macroblocks on the upper
   row or left column of the frame will sometimes cause pixels outside
   the visible frame to be referenced.  Usually such out-of-bounds
   pixels have an assumed value of 129 for pixels to the left of the
   leftmost column of the visible frame and 127 for pixels above the top
   row of the visible frame (including the special case of the pixel
   above and to the left of the top-left pixel in the visible frame).
   Exceptions to this (associated to certain modes) will be noted below.

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   The already-coded macroblocks referenced by intra-prediction have
   been "reconstructed", that is, have been predicted and residue-
   adjusted (as described in Section 14), but have not been loop-
   filtered.  While it does process the edges between individual
   macroblocks and individual subblocks, loop filtering (described in
   Section 15) is applied to the frame as a whole, after all of the
   macroblocks have been reconstructed.

12.1.  mb_skip_coeff

   The single bool flag is decoded using prob_skip_false if and only if
   mb_no_skip_coeff is set to 1 (see Sections 9.10 and 9.11).  If
   mb_no_skip_coeff is set to 0, then this value defaults to 0.

12.2.  Chroma Prediction

   The chroma prediction is a little simpler than the luma prediction,
   so we treat it first.  Each of the chroma modes treats U and V
   identically; that is, the U and V prediction values are calculated in
   parallel, using the same relative addressing and arithmetic in each
   of the two planes.

   The modes extrapolate prediction values using the 8-pixel row "A"
   lying immediately above the block (that is, the bottom chroma row of
   the macroblock immediately above the current macroblock) and the
   8-pixel column "L" immediately to the left of the block (that is, the
   rightmost chroma column of the macroblock immediately to the left of
   the current macroblock).

   Vertical prediction (chroma mode V_PRED) simply fills each 8-pixel
   row of the 8x8 chroma block with a copy of the "above" row (A).  If
   the current macroblock lies on the top row of the frame, all 8 of the
   pixel values in A are assigned the value 127.

   Similarly, horizontal prediction (H_PRED) fills each 8-pixel column
   of the 8x8 chroma block with a copy of the "left" column (L).  If the
   current macroblock is in the left column of the frame, all 8 pixel
   values in L are assigned the value 129.

   DC prediction (DC_PRED) fills the 8x8 chroma block with a single
   value.  In the generic case of a macroblock lying below the top row
   and right of the leftmost column of the frame, this value is the
   average of the 16 (genuinely visible) pixels in the (union of the)
   above row A and left column L.

   Otherwise, if the current macroblock lies on the top row of the
   frame, the average of the 8 pixels in L is used; if it lies in the
   left column of the frame, the average of the 8 pixels in A is used.

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   Note that the averages used in these exceptional cases are not the
   same as those that would be arrived at by using the out-of-bounds A
   and L values defined for V_PRED and H_PRED.  In the case of the
   leftmost macroblock on the top row of the frame, the 8x8 block is
   simply filled with the constant value 128.

   For DC_PRED, apart from the exceptional case of the top-left
   macroblock, we are averaging either 16 or 8 pixel values to get a
   single prediction value that fills the 8x8 block.  The rounding is
   done as follows:

   ---- Begin code block --------------------------------------

   int sum;  /* sum of 8 or 16 pixels at (at least) 16-bit precision */
   int shf;  /* base 2 logarithm of the number of pixels (3 or 4) */

   Pixel DCvalue = (sum + (1 << (shf-1))) >> shf;

   ---- End code block ----------------------------------------

   Because the summands are all valid pixels, no "clamp" is necessary in
   the calculation of DCvalue.

   The remaining "True Motion" (TM_PRED) chroma mode gets its name from
   an older technique of video compression used by On2 Technologies, to
   which it bears some relation.  In addition to the row "A" and column
   "L", TM_PRED uses the pixel "P" above and to the left of the chroma
   block.

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   The following figure gives an example of how TM_PRED works:

   ---- Begin code block --------------------------------------

   |-----|-----|-----|-----|-----|-----|-----|-----|-----|
   | P   | A0  | A1  | A2  | A3  | A4  | A5  | A6  | A7  |
   |-----|-----|-----|-----|-----|-----|-----|-----|-----|
   | L0  | X00 | X01 | X02 | X03 | X04 | X05 | X06 | X07 |
   |-----|-----|-----|-----|-----|-----|-----|-----|-----|
   | L1  | X10 | X11 | X12 | X13 | X14 | X15 | X16 | X17 |
   |-----|-----|-----|-----|-----|-----|-----|-----|-----|
   | L2  | X20 | X21 | X22 | X23 | X24 | X25 | X26 | X27 |
   |-----|-----|-----|-----|-----|-----|-----|-----|-----|
   | L3  | X30 | X31 | X32 | X33 | X34 | X35 | X36 | X37 |
   |-----|-----|-----|-----|-----|-----|-----|-----|-----|
   | L4  | X40 | X41 | X42 | X43 | X44 | X45 | X46 | X47 |
   |-----|-----|-----|-----|-----|-----|-----|-----|-----|
   | L5  | X50 | X51 | X52 | X53 | X54 | X55 | X56 | X57 |
   |-----|-----|-----|-----|-----|-----|-----|-----|-----|
   | L6  | X60 | X61 | X62 | X63 | X64 | X65 | X66 | X67 |
   |-----|-----|-----|-----|-----|-----|-----|-----|-----|
   | L7  | X70 | X71 | X72 | X73 | X74 | X75 | X76 | X77 |
   |-----|-----|-----|-----|-----|-----|-----|-----|-----|

   ---- End code block ----------------------------------------

   Where P, As, and Ls represent reconstructed pixel values from
   previously coded blocks, and X00 through X77 represent predicted
   values for the current block.  TM_PRED uses the following equation to
   calculate X_ij:

   X_ij = L_i + A_j - P (i, j=0, 1, 2, 3)

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   The exact algorithm is as follows:

   ---- Begin code block --------------------------------------

   void TMpred(
       Pixel b[8][8],      /* chroma (U or V) prediction block */
       const Pixel A[8],   /* row of already-constructed pixels
                              above block */
       const Pixel L[8],   /* column of "" just to the left of
                              block */
       const Pixel P       /* pixel just to the left of A and
                              above L*/
   ) {
       int r = 0;          /* row */
       do {
           int c = 0;      /* column */
           do {
               b[r][c] = clamp255(L[r]+ A[c] - P);
           } while (++c < 8);
       } while (++r < 8);
   }

   ---- End code block ----------------------------------------

   Note that the process could equivalently be described as propagating
   the vertical differences between pixels in L (starting from P), using
   the pixels from A to start each column.

   An implementation of chroma intra-prediction may be found in the
   reference decoder file predict.c (Section 20.14).

   Unlike DC_PRED, for macroblocks on the top row or left edge, TM_PRED
   does use the out-of-bounds values of 127 and 129 (respectively)
   defined for V_PRED and H_PRED.

12.3.  Luma Prediction

   The prediction processes for the first four 16x16 luma modes
   (DC_PRED, V_PRED, H_PRED, and TM_PRED) are essentially identical to
   the corresponding chroma prediction processes described above, the
   only difference being that we are predicting a single 16x16 luma
   block instead of two 8x8 chroma blocks.

   Thus, the row "A" and column "L" here contain 16 pixels, the DC
   prediction is calculated using 16 or 32 pixels (and shf is 4 or 5),
   and we of course fill the entire prediction buffer, that is, 16 rows
   (or columns) containing 16 pixels each.  The reference implementation
   of 16x16 luma prediction is also in predict.c.

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   In the remaining luma mode (B_PRED), each 4x4 Y subblock is
   independently predicted using one of ten modes (listed, along with
   their encodings, in Section 11).

   Also, unlike the full-macroblock modes already described, some of the
   subblock modes use prediction pixels above and to the right of the
   current subblock.  In detail, each 4x4 subblock "B" is predicted
   using (at most) the 4-pixel column "L" immediately to the left of B
   and the 8-pixel row "A" immediately above B, consisting of the 4
   pixels above B followed by the 4 adjacent pixels above and to the
   right of B, together with the single pixel "P" immediately to the
   left of A (and immediately above L).

   For the purpose of subblock intra-prediction, the pixels immediately
   to the left and right of a pixel in a subblock are the same as the
   pixels immediately to the left and right of the corresponding pixel
   in the frame buffer "F".  Vertical offsets behave similarly: The
   above row A lies immediately above B in F, and the adjacent pixels in
   the left column L are separated by a single row in F.

   Because entire macroblocks (as opposed to their constituent
   subblocks) are reconstructed in raster-scan order, for subblocks
   lying along the right edge (and not along the top row) of the current
   macroblock, the four "extra" prediction pixels in A above and to the
   right of B have not yet actually been constructed.

   Subblocks 7, 11, and 15 are affected.  All three of these subblocks
   use the same extra pixels as does subblock 3 (at the upper right
   corner of the macroblock), namely the 4 pixels immediately above and
   to the right of subblock 3.  Writing (R,C) for a frame buffer
   position offset from the upper left corner of the current macroblock
   by R rows and C columns, the extra pixels for all the right-edge
   subblocks (3, 7, 11, and 15) are at positions (-1,16), (-1,17),
   (-1,18), and (-1,19).  For the rightmost macroblock in each
   macroblock row except the top row, the extra pixels shall use the
   same value as the pixel at position (-1,15), which is the rightmost
   visible pixel on the line immediately above the macroblock row.  For
   the top macroblock row, all the extra pixels assume a value of 127.

   The details of the prediction modes are most easily described in
   code.

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   ---- Begin code block --------------------------------------

   /* Result pixels are often averages of two or three predictor
      pixels.  The following subroutines are used to calculate
      these averages.  Because the arguments are valid pixels, no
      clamping is necessary.  An actual implementation would
      probably use inline functions or macros. */

   /* Compute weighted average centered at y w/adjacent x, z */

   Pixel avg3(Pixel x, Pixel y, Pixel z) {
     return (x + y + y + z + 2) >> 2;}

   /* Weighted average of 3 adjacent pixels centered at p */

   Pixel avg3p(const Pixel *p) { return avg3(p[-1], p[0], p[1]);}

   /* Simple average of x and y */

   Pixel avg2(Pixel x, Pixel y) { return (x + y + 1) >> 1;}

   /* Average of p[0] and p[1] may be considered to be a synthetic
      pixel lying between the two, that is, one half-step past p. */

   Pixel avg2p(const Pixel *p) { return avg2(p[0], p[1]);}

   void subblock_intra_predict(
       Pixel B[4][4],     /* Y subblock prediction buffer */
       const Pixel *A,    /* A[0]...A[7] = above row, A[-1] = P */
       const Pixel *L,    /* L[0]...L[3] = left column, L[-1] = P */
       intra_bmode mode   /* enum is in Section 11.2 */
   ) {
       Pixel E[9];        /* 9 already-constructed edge pixels */
       E[0] = L[3];  E[1] = L[2];  E[2] = L[1];  E[3] = L[0];
       E[4] = A[-1];      /* == L[-1] == P */
       E[5] = A[0];  E[6] = A[1];  E[7] = A[2];  E[8] = A[3];

     switch(mode) {
       /* First four modes are similar to corresponding
          full-block modes. */

       case B_DC_PRED:
       {
           int v = 4;      /* DC sum/avg, 4 is rounding adjustment */
           int i = 0;  do { v += A[i] + L[i];}  while (++i < 4);
           v >>= 3;        /* averaging 8 pixels */
           i = 0;  do {    /* fill prediction buffer with constant DC
                              value */

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               int j = 0;  do { B[i][j] = v;}  while (++j < 4);
           } while (++i < 4);
           break;
       }

       case B_TM_PRED: /* just like 16x16 TM_PRED */
       {
           int r = 0;  do {
               int c = 0;  do {
                   B[r][c] = clamp255(L[r] + A[c] - A[-1]);
               } while (++c < 4);
           } while (++r < 4);
           break;
       }

       case B_VE_PRED: /* like 16x16 V_PRED except using averages */
       {
           int c = 0;  do { /* all 4 rows = smoothed top row */
               B[0][c] = B[1][c] = B[2][c] = B[3][c] = avg3p(A + c);
           } while (++c < 4);
           break;
       }

       case B_HE_PRED: /* like 16x16 H_PRED except using averages */
       {
           /* Bottom row is exceptional because L[4] does not exist */
           int v = avg3(L[2], L[3], L[3]);
           int r = 3;  while (1) {  /* all 4 columns = smoothed left
                                       column */
               B[r][0] = B[r][1] = B[r][2] = B[r][3] = v;
               if (--r < 0)
                   break;
               v = avg3p(L + r);  /* upper 3 rows use average of
                                      3 pixels */
           }
           break;
       }

       /* The remaining six "diagonal" modes subdivide the
          prediction buffer into diagonal lines.  All the pixels
          on each line are assigned the same value; this value is
          (a smoothed or synthetic version of) an
          already-constructed predictor value lying on the same
          line.  For clarity, in the comments, we express the
          positions of these predictor pixels relative to the
          upper left corner of the destination array B.

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          These modes are unique to subblock prediction and have
          no full-block analogs.  The first two use lines at
          +|- 45 degrees from horizontal (or, equivalently,
          vertical), that is, lines whose slopes are +|- 1. */

       case B_LD_PRED:    /* southwest (left and down) step =
                             (-1, 1) or (1,-1) */
           /* avg3p(A + j) is the "smoothed" pixel at (-1,j) */
           B[0][0] = avg3p(A + 1);
           B[0][1] = B[1][0] = avg3p(A + 2);
           B[0][2] = B[1][1] = B[2][0] = avg3p(A + 3);
           B[0][3] = B[1][2] = B[2][1] = B[3][0] = avg3p(A + 4);
           B[1][3] = B[2][2] = B[3][1] = avg3p(A + 5);
           B[2][3] = B[3][2] = avg3p(A + 6);
           B[3][3] = avg3(A[6], A[7], A[7]); /* A[8] does not exist */
           break;

       case B_RD_PRED: /* southeast (right and down) step =
                          (1,1) or (-1,-1) */
           B[3][0] = avg3p(E + 1);  /* predictor is from (2, -1) */
           B[3][1] = B[2][0] = avg3p(E + 2);  /* (1, -1) */
           B[3][2] = B[2][1] = B[1][0] = avg3p(E + 3);  /* (0, -1) */
           B[3][3] = B[2][2] = B[1][1] = B[0][0] =
             avg3p(E + 4);  /* (-1, -1) */
           B[2][3] = B[1][2] = B[0][1] = avg3p(E + 5);  /* (-1, 0) */
           B[1][3] = B[0][2] = avg3p(E + 6);  /* (-1, 1) */
           B[0][3] = avg3p(E + 7);  /* (-1, 2) */
           break;

       /* The remaining 4 diagonal modes use lines whose slopes are
          +|- 2 and +|- 1/2.  The angles of these lines are roughly
          +|- 27 degrees from horizontal or vertical.

          Unlike the 45 degree diagonals, here we often need to
          "synthesize" predictor pixels midway between two actual
          predictors using avg2p(p), which we think of as returning
          the pixel "at" p[1/2]. */

       case B_VR_PRED:    /* SSE (vertical right) step =
                             (2,1) or (-2,-1) */
           B[3][0] = avg3p(E + 2);  /* predictor is from (1, -1) */
           B[2][0] = avg3p(E + 3);  /* (0, -1) */
           B[3][1] = B[1][0] = avg3p(E + 4);  /* (-1,   -1) */
           B[2][1] = B[0][0] = avg2p(E + 4);  /* (-1, -1/2) */
           B[3][2] = B[1][1] = avg3p(E + 5);  /* (-1,    0) */
           B[2][2] = B[0][1] = avg2p(E + 5);  /* (-1,  1/2) */
           B[3][3] = B[1][2] = avg3p(E + 6);  /* (-1,    1) */
           B[2][3] = B[0][2] = avg2p(E + 6);  /* (-1,  3/2) */

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           B[1][3] = avg3p(E + 7);  /* (-1, 2) */
           B[0][3] = avg2p(E + 7);  /* (-1, 5/2) */
           break;

       case B_VL_PRED:    /* SSW (vertical left) step =
                             (2,-1) or (-2,1) */
           B[0][0] = avg2p(A);  /* predictor is from (-1, 1/2) */
           B[1][0] = avg3p(A + 1);  /* (-1, 1) */
           B[2][0] = B[0][1] = avg2p(A + 1);  /* (-1, 3/2) */
           B[1][1] = B[3][0] = avg3p(A + 2);  /* (-1,   2) */
           B[2][1] = B[0][2] = avg2p(A + 2);  /* (-1, 5/2) */
           B[3][1] = B[1][2] = avg3p(A + 3);  /* (-1,   3) */
           B[2][2] = B[0][3] = avg2p(A + 3);  /* (-1, 7/2) */
           B[3][2] = B[1][3] = avg3p(A + 4);  /* (-1,   4) */
           /* Last two values do not strictly follow the pattern. */
           B[2][3] = avg3p(A + 5);  /* (-1, 5) [avg2p(A + 4) =
                                        (-1,9/2)] */
           B[3][3] = avg3p(A + 6);  /* (-1, 6) [avg3p(A + 5) =
                                        (-1,5)] */
           break;

       case B_HD_PRED:    /* ESE (horizontal down) step =
                             (1,2) or (-1,-2) */
           B[3][0] = avg2p(E);  /* predictor is from (5/2, -1) */
           B[3][1] = avg3p(E + 1);  /* (2, -1) */
           B[2][0] = B[3][2] = svg2p(E + 1);  /* ( 3/2, -1) */
           B[2][1] = B[3][3] = avg3p(E + 2);  /* (   1, -1) */
           B[2][2] = B[1][0] = avg2p(E + 2);  /* ( 1/2, -1) */
           B[2][3] = B[1][1] = avg3p(E + 3);  /* (   0, -1) */
           B[1][2] = B[0][0] = avg2p(E + 3);  /* (-1/2, -1) */
           B[1][3] = B[0][1] = avg3p(E + 4);  /* (  -1, -1) */
           B[0][2] = avg3p(E + 5);  /* (-1, 0) */
           B[0][3] = avg3p(E + 6);  /* (-1, 1) */
           break;

       case B_HU_PRED:    /* ENE (horizontal up) step = (1,-2)
                             or (-1,2) */
           B[0][0] = avg2p(L);  /* predictor is from (1/2, -1) */
           B[0][1] = avg3p(L + 1);  /* (1, -1) */
           B[0][2] = B[1][0] = avg2p(L + 1);  /* (3/2, -1) */
           B[0][3] = B[1][1] = avg3p(L + 2);  /* (  2, -1) */
           B[1][2] = B[2][0] = avg2p(L + 2);  /* (5/2, -1) */
           B[1][3] = B[2][1] = avg3(L[2], L[3], L[3]);  /* (3, -1) */

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           /* Not possible to follow pattern for much of the bottom
              row because no (nearby) already-constructed pixels lie
              on the diagonals in question. */
           B[2][2] = B[2][3] = B[3][0] = B[3][1] = B[3][2] = B[3][3]
             = L[3];
     }
   }

   ---- End code block ----------------------------------------

   The reference decoder implementation of subblock intra-prediction may
   be found in predict.c (Section 20.14).

13.  DCT Coefficient Decoding

   The second data partition consists of an encoding of the quantized
   DCT (and WHT) coefficients of the residue signal.  As discussed in
   the format overview (Section 2), for each macroblock, the residue is
   added to the (intra- or inter-generated) prediction buffer to produce
   the final (except for loop filtering) reconstructed macroblock.

   VP8 works exclusively with 4x4 DCTs and WHTs, applied to the 24 (or
   25 with the Y2 subblock) 4x4 subblocks of a macroblock.  The ordering
   of macroblocks within any of the "residue" partitions in general
   follows the same raster scan as used in the first "prediction"
   partition.

   For all intra- and inter-prediction modes apart from B_PRED (intra:
   whose Y subblocks are independently predicted) and SPLITMV (inter),
   each macroblock's residue record begins with the Y2 component of the
   residue, coded using a WHT.  B_PRED and SPLITMV coded macroblocks
   omit this WHT and specify the 0th DCT coefficient in each of the 16 Y
   subblocks.

   After the optional Y2 block, the residue record continues with 16
   DCTs for the Y subblocks, followed by 4 DCTs for the U subblocks,
   ending with 4 DCTs for the V subblocks.  The subblocks occur in the
   usual order.

   The DCTs and WHT are tree-coded using a 12-element alphabet whose
   members we call "tokens".  Except for the end-of-block token (which
   sets the remaining subblock coefficients to zero and is followed by
   the next block), each token (sometimes augmented with data
   immediately following the token) specifies the value of the single
   coefficient at the current (implicit) position and is followed by a
   token applying to the next (implicit) position.

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   For all the Y and chroma subblocks, the ordering of the coefficients
   follows a so-called zig-zag order.  DCTs begin at coefficient 1 if Y2
   is present, and begin at coefficient 0 if Y2 is absent.  The WHT for
   a Y2 subblock always begins at coefficient 0.

13.1.  Macroblock without Non-Zero Coefficient Values

   If the flag within macroblock (MB) MODE_INFO indicates that a
   macroblock does not have any non-zero coefficients, the decoding
   process of DCT coefficients is skipped for the macroblock.

13.2.  Coding of Individual Coefficient Values

   The coding of coefficient tokens is the same for the DCT and WHT, and
   for the remainder of this section "DCT" should be taken to mean
   either DCT or WHT.

   All tokens (except end-of-block) specify either a single unsigned
   value or a range of unsigned values (immediately) followed by a
   simple probabilistic encoding of the offset of the value from the
   base of that range.

   Non-zero values (of either type) are then followed by a flag
   indicating the sign of the coded value (negative if 1, positive
   if 0).

   Below are the tokens and decoding tree.

   ---- Begin code block --------------------------------------

   typedef enum
   {
       DCT_0,      /* value 0 */
       DCT_1,      /* 1 */
       DCT_2,      /* 2 */
       DCT_3,      /* 3 */
       DCT_4,      /* 4 */
       dct_cat1,   /* range 5 - 6  (size 2) */
       dct_cat2,   /* 7 - 10   (4) */
       dct_cat3,   /* 11 - 18  (8) */
       dct_cat4,   /* 19 - 34  (16) */
       dct_cat5,   /* 35 - 66  (32) */
       dct_cat6,   /* 67 - 2048  (1982) */
       dct_eob,    /* end of block */

       num_dct_tokens   /* 12 */
   }
   dct_token;

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   const tree_index coeff_tree [2 * (num_dct_tokens - 1)] =
   {
    -dct_eob, 2,               /* eob = "0"   */
     -DCT_0, 4,                /* 0   = "10"  */
      -DCT_1, 6,               /* 1   = "110" */
       8, 12,
        -DCT_2, 10,            /* 2   = "11100" */
         -DCT_3, -DCT_4,       /* 3   = "111010", 4 = "111011" */
        14, 16,
         -dct_cat1, -dct_cat2, /* cat1 =  "111100",
                                  cat2 = "111101" */
        18, 20,
         -dct_cat3, -dct_cat4, /* cat3 = "1111100",
                                  cat4 = "1111101" */
         -dct_cat5, -dct_cat6  /* cat4 = "1111110",
                                  cat4 = "1111111" */
   };

   ---- End code block ----------------------------------------

   In general, all DCT coefficients are decoded using the same tree.
   However, if the preceding coefficient is a DCT_0, decoding will skip
   the first branch, since it is not possible for dct_eob to follow a
   DCT_0.

   The tokens dct_cat1 ... dct_cat6 specify ranges of unsigned values,
   the value within the range being formed by adding an unsigned offset
   (whose width is 1, 2, 3, 4, 5, or 11 bits, respectively) to the base
   of the range, using the following algorithm and fixed probability
   tables.

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   ---- Begin code block --------------------------------------

   uint DCTextra(bool_decoder *d, const Prob *p)
   {
       uint v = 0;
       do { v += v + read_bool(d, *p);}  while (*++p);
       return v;
   }

   const Prob Pcat1[] = { 159, 0};
   const Prob Pcat2[] = { 165, 145, 0};
   const Prob Pcat3[] = { 173, 148, 140, 0};
   const Prob Pcat4[] = { 176, 155, 140, 135, 0};
   const Prob Pcat5[] = { 180, 157, 141, 134, 130, 0};
   const Prob Pcat6[] =
       { 254, 254, 243, 230, 196, 177, 153, 140, 133, 130, 129, 0};

   ---- End code block ----------------------------------------

   If v -- the unsigned value decoded using the coefficient tree,
   possibly augmented by the process above -- is non-zero, its sign is
   set by simply reading a flag:

   ---- Begin code block --------------------------------------

   if (read_bool(d, 128))
       v = -v;

   ---- End code block ----------------------------------------

13.3.  Token Probabilities

   The probability specification for the token tree (unlike that for the
   "extra bits" described above) is rather involved.  It uses three
   pieces of context to index a large probability table, the contents of
   which may be incrementally modified in the frame header.  The full
   (non-constant) probability table is laid out as follows.

   ---- Begin code block --------------------------------------

   Prob coeff_probs [4] [8] [3] [num_dct_tokens-1];

   ---- End code block ----------------------------------------

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   Working from the outside in, the outermost dimension is indexed by
   the type of plane being decoded:

   o  0 - Y beginning at coefficient 1 (i.e., Y after Y2)

   o  1 - Y2

   o  2 - U or V

   o  3 - Y beginning at coefficient 0 (i.e., Y in the absence of Y2).

   The next dimension is selected by the position of the coefficient
   being decoded.  That position, c, steps by ones up to 15, starting
   from zero for block types 1, 2, or 3 and starting from one for block
   type 0.  The second array index is then

   ---- Begin code block --------------------------------------

   coeff_bands [c]

   ---- End code block ----------------------------------------

   Where:

   ---- Begin code block --------------------------------------

   const int coeff_bands [16] = {
        0, 1, 2, 3, 6, 4, 5, 6, 6, 6, 6, 6, 6, 6, 6, 7
   };

   ---- End code block ----------------------------------------

   is a fixed mapping of position to "band".

   The third dimension is the trickiest.  Roughly speaking, it measures
   the "local complexity" or extent to which nearby coefficients are
   non-zero.

   For the first coefficient (DC, unless the block type is 0), we
   consider the (already encoded) blocks within the same plane (Y2, Y,
   U, or V) above and to the left of the current block.  The context
   index is then the number (0, 1, or 2) of these blocks that had at
   least one non-zero coefficient in their residue record.  Specifically
   for Y2, because macroblocks above and to the left may or may not have
   a Y2 block, the block above is determined by the most recent
   macroblock in the same column that has a Y2 block, and the block to
   the left is determined by the most recent macroblock in the same row
   that has a Y2 block.

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   Beyond the first coefficient, the context index is determined by the
   absolute value of the most recently decoded coefficient (necessarily
   within the current block) and is 0 if the last coefficient was a
   zero, 1 if it was plus or minus one, and 2 if its absolute value
   exceeded one.

   Note that the intuitive meaning of this measure changes as
   coefficients are decoded.  For example, prior to the first token, a
   zero means that the neighbors are empty, suggesting that the current
   block may also be empty.  After the first token, because an end-of-
   block token must have at least one non-zero value before it, a zero
   means that we just decoded a zero and hence guarantees that a
   non-zero coefficient will appear later in this block.  However, this
   shift in meaning is perfectly okay because the complete context
   depends also on the coefficient band (and since band 0 is occupied
   exclusively by position 0).

   As with other contexts used by VP8, the "neighboring block" context
   described here needs a special definition for subblocks lying along
   the top row or left edge of the frame.  These "non-existent"
   predictors above and to the left of the image are simply taken to be
   empty -- that is, taken to contain no non-zero coefficients.

   The residue decoding of each macroblock then requires, in each of two
   directions (above and to the left), an aggregate coefficient
   predictor consisting of a single Y2 predictor, two predictors for
   each of U and V, and four predictors for Y.  In accordance with the
   scan-ordering of macroblocks, a decoder needs to maintain a single
   "left" aggregate predictor and a row of "above" aggregate predictors.

   Before decoding any residue, these maintained predictors may simply
   be cleared, in compliance with the definition of "non-existent"
   prediction.  After each block is decoded, the two predictors
   referenced by the block are replaced with the (empty or non-empty)
   state of the block, in preparation for the later decoding of the
   blocks below and to the right of the block just decoded.

   The fourth, and final, dimension of the token probability array is of
   course indexed by (half) the position in the token tree structure, as
   are all tree probability arrays.

   The pseudocode below illustrates the decoding process.  Note that
   criteria, functions, etc. delimited with ** are either dependent on
   decoder architecture or are elaborated on elsewhere in this document.

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   ---- Begin code block --------------------------------------

   int block[16] = { 0 }; /* current 4x4 block coeffs */
   int firstCoeff = 0;
   int plane;
   int ctx2;
   int ctx3 = 0; /* the 3rd context referred to in above description */
   Prob *probTable;
   int token;
   int sign;
   int absValue;
   int extraBits;
   bool prevCoeffWasZero = false;
   bool currentBlockHasCoeffs = false;
   /* base coeff abs values per each category, elem #0 is
      DCT_VAL_CATEGORY1, * #1 is DCT_VAL_CATEGORY2, etc. */
   int categoryBase[6] = { 5, 7, 11, 19, 35, 67 };

   /* Determine plane to use */
   if ( **current_block_is_Y2_block** )       plane = 0;
   else if ( **current_block_is_chroma** )   plane = 2;
   else if ( **current_macroblock_has_Y2** ) plane = 1;
   else                                      plane = 3;

   /* For luma blocks of a "Y2 macroblock" we skip coeff index #0 */
   if ( plane == 1 )
       firstCoeff++;

   /* Determine whether neighbor 4x4 blocks have coefficients.
      This is dependent on the plane we are currently decoding;
      i.e., we check only coefficients from the same plane as the
      current block. */
   if ( **left_neighbor_block_has_coefficients(plane)** )
       ctx3++;
   if ( **above_neighbor_block_has_coefficients(plane)** )
       ctx3++;

   for( i = firstCoeff; i < 16; ++i )
   {
       ctx2 = coeff_bands[i];
       probTable = coeff_probs[plane][ctx2][ctx3];

       /* skip first code (dct_eob) if previous token was DCT_0 */
       if ( prevCoeffWasZero )
           token = treed_read ( d, **coeff_tree_without_eob**,
             probTable );
       else
           token = treed_read ( d, coeff_tree, probTable );

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       if ( token == dct_eob )
           break;

       if ( token != DCT_0 )
       {
           currentBlockHasCoeffs = true;
     if ( **token_has_extra_bits(token)** )
     {
         extraBits = DCTextra( token );
         absValue =
             categoryBase[**token_to_cat_index(token)**] +
       extraBits;
     }
     else
     {
         absValue = **token_to_abs_value(token)**;
     }

     sign = read_bool(d, 128);
           block[i] = sign ? -absValue : absValue;
       }
       else
       {
           absValue = 0;
       }

       /* Set contexts and stuff for next coeff */
       if ( absValue == 0 )         ctx3 = 0;
       else if ( absValue == 1 )   ctx3 = 1;
       else                        ctx3 = 2;
       prevCoeffWasZero = true;
   }

   /* Store current block status to decoder internals */
   **block_has_coefficients[currentMb][currentBlock]** =
     currentBlockHasCoeffs;

   ---- End code block ----------------------------------------

   While we have in fact completely described the coefficient decoding
   procedure, the reader will probably find it helpful to consult the
   reference implementation, which can be found in the file tokens.c
   (Section 20.16).

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13.4.  Token Probability Updates

   As mentioned above, the token-decoding probabilities may change from
   frame to frame.  After detection of a key frame, they are of course
   set to their defaults as shown in Section 13.5; this must occur
   before decoding the remainder of the header, as both key frames and
   interframes may adjust these probabilities.

   The layout and semantics of the coefficient probability update record
   (Section I of the frame header) are straightforward.  For each
   position in the coeff_probs array there occurs a fixed-probability
   bool indicating whether or not the corresponding probability should
   be updated.  If the bool is true, there follows a P(8) replacing that
   probability.  Note that updates are cumulative; that is, a
   probability updated on one frame is in effect for all ensuing frames
   until the next key frame, or until the probability is explicitly
   updated by another frame.

   The algorithm to effect the foregoing is simple:

   ---- Begin code block --------------------------------------

   int i = 0;  do {
    int j = 0;  do {
     int k = 0;  do {
      int t = 0;  do {

           if (read_bool(d, coeff_update_probs [i] [j] [k] [t]))
               coeff_probs [i] [j] [k] [t] = read_literal(d, 8);

      } while (++t < num_dct_tokens - 1);
     } while (++k < 3);
    } while (++j < 8);
   } while (++i < 4);

   ---- End code block ----------------------------------------

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   The (constant) update probabilities are as follows:

   ---- Begin code block --------------------------------------

   const Prob coeff_update_probs [4] [8] [3] [num_dct_tokens-1] =
   {
    {
     {
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 176, 246, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 223, 241, 252, 255, 255, 255, 255, 255, 255, 255, 255},
      { 249, 253, 253, 255, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 244, 252, 255, 255, 255, 255, 255, 255, 255, 255},
      { 234, 254, 254, 255, 255, 255, 255, 255, 255, 255, 255},
      { 253, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 246, 254, 255, 255, 255, 255, 255, 255, 255, 255},
      { 239, 253, 254, 255, 255, 255, 255, 255, 255, 255, 255},
      { 254, 255, 254, 255, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 248, 254, 255, 255, 255, 255, 255, 255, 255, 255},
      { 251, 255, 254, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 253, 254, 255, 255, 255, 255, 255, 255, 255, 255},
      { 251, 254, 254, 255, 255, 255, 255, 255, 255, 255, 255},
      { 254, 255, 254, 255, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 254, 253, 255, 254, 255, 255, 255, 255, 255, 255},
      { 250, 255, 254, 255, 254, 255, 255, 255, 255, 255, 255},
      { 254, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255}
     }
    },

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    {
     {
      { 217, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 225, 252, 241, 253, 255, 255, 254, 255, 255, 255, 255},
      { 234, 250, 241, 250, 253, 255, 253, 254, 255, 255, 255}
     },
     {
      { 255, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 223, 254, 254, 255, 255, 255, 255, 255, 255, 255, 255},
      { 238, 253, 254, 254, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 248, 254, 255, 255, 255, 255, 255, 255, 255, 255},
      { 249, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 253, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 247, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 253, 254, 255, 255, 255, 255, 255, 255, 255, 255},
      { 252, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 254, 254, 255, 255, 255, 255, 255, 255, 255, 255},
      { 253, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 254, 253, 255, 255, 255, 255, 255, 255, 255, 255},
      { 250, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 254, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255}
     }
    },
    {
     {
      { 186, 251, 250, 255, 255, 255, 255, 255, 255, 255, 255},
      { 234, 251, 244, 254, 255, 255, 255, 255, 255, 255, 255},
      { 251, 251, 243, 253, 254, 255, 254, 255, 255, 255, 255}
     },

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     {
      { 255, 253, 254, 255, 255, 255, 255, 255, 255, 255, 255},
      { 236, 253, 254, 255, 255, 255, 255, 255, 255, 255, 255},
      { 251, 253, 253, 254, 254, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 254, 254, 255, 255, 255, 255, 255, 255, 255, 255},
      { 254, 254, 254, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 254, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 254, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 254, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255}
     }
    },
    {
     {
      { 248, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 250, 254, 252, 254, 255, 255, 255, 255, 255, 255, 255},
      { 248, 254, 249, 253, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 253, 253, 255, 255, 255, 255, 255, 255, 255, 255},
      { 246, 253, 253, 255, 255, 255, 255, 255, 255, 255, 255},
      { 252, 254, 251, 254, 254, 255, 255, 255, 255, 255, 255}
     },

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     {
      { 255, 254, 252, 255, 255, 255, 255, 255, 255, 255, 255},
      { 248, 254, 253, 255, 255, 255, 255, 255, 255, 255, 255},
      { 253, 255, 254, 254, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 251, 254, 255, 255, 255, 255, 255, 255, 255, 255},
      { 245, 251, 254, 255, 255, 255, 255, 255, 255, 255, 255},
      { 253, 253, 254, 255, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 251, 253, 255, 255, 255, 255, 255, 255, 255, 255},
      { 252, 253, 254, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 252, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 249, 255, 254, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 254, 255, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 255, 253, 255, 255, 255, 255, 255, 255, 255, 255},
      { 250, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255}
     },
     {
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 254, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
      { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255}
     }
    }
   };

   ---- End code block ----------------------------------------

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13.5.  Default Token Probability Table

   The default token probabilities are as follows.

   ---- Begin code block --------------------------------------

   const Prob default_coeff_probs [4] [8] [3] [num_dct_tokens - 1] =
   {
    {
     {
      { 128, 128, 128, 128, 128, 128, 128, 128, 128, 128, 128},
      { 128, 128, 128, 128, 128, 128, 128, 128, 128, 128, 128},
      { 128, 128, 128, 128, 128, 128, 128, 128, 128, 128, 128}
     },
     {
      { 253, 136, 254, 255, 228, 219, 128, 128, 128, 128, 128},
      { 189, 129, 242, 255, 227, 213, 255, 219, 128, 128, 128},
      { 106, 126, 227, 252, 214, 209, 255, 255, 128, 128, 128}
     },
     {
      {   1,  98, 248, 255, 236, 226, 255, 255, 128, 128, 128},
      { 181, 133, 238, 254, 221, 234, 255, 154, 128, 128, 128},
      {  78, 134, 202, 247, 198, 180, 255, 219, 128, 128, 128}
     },
     {
      {   1, 185, 249, 255, 243, 255, 128, 128, 128, 128, 128},
      { 184, 150, 247, 255, 236, 224, 128, 128, 128, 128, 128},
      {  77, 110, 216, 255, 236, 230, 128, 128, 128, 128, 128}
     },
     {
      {   1, 101, 251, 255, 241, 255, 128, 128, 128, 128, 128},
      { 170, 139, 241, 252, 236, 209, 255, 255, 128, 128, 128},
      {  37, 116, 196, 243, 228, 255, 255, 255, 128, 128, 128}
     },
     {
      {   1, 204, 254, 255, 245, 255, 128, 128, 128, 128, 128},
      { 207, 160, 250, 255, 238, 128, 128, 128, 128, 128, 128},
      { 102, 103, 231, 255, 211, 171, 128, 128, 128, 128, 128}
     },
     {
      {   1, 152, 252, 255, 240, 255, 128, 128, 128, 128, 128},
      { 177, 135, 243, 255, 234, 225, 128, 128, 128, 128, 128},
      {  80, 129, 211, 255, 194, 224, 128, 128, 128, 128, 128}
     },

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     {
      {   1,   1, 255, 128, 128, 128, 128, 128, 128, 128, 128},
      { 246,   1, 255, 128, 128, 128, 128, 128, 128, 128, 128},
      { 255, 128, 128, 128, 128, 128, 128, 128, 128, 128, 128}
     }
    },
    {
     {
      { 198,  35, 237, 223, 193, 187, 162, 160, 145, 155,  62},
      { 131,  45, 198, 221, 172, 176, 220, 157, 252, 221,   1},
      {  68,  47, 146, 208, 149, 167, 221, 162, 255, 223, 128}
     },
     {
      {   1, 149, 241, 255, 221, 224, 255, 255, 128, 128, 128},
      { 184, 141, 234, 253, 222, 220, 255, 199, 128, 128, 128},
      {  81,  99, 181, 242, 176, 190, 249, 202, 255, 255, 128}
     },
     {
      {   1, 129, 232, 253, 214, 197, 242, 196, 255, 255, 128},
      {  99, 121, 210, 250, 201, 198, 255, 202, 128, 128, 128},
      {  23,  91, 163, 242, 170, 187, 247, 210, 255, 255, 128}
     },
     {
      {   1, 200, 246, 255, 234, 255, 128, 128, 128, 128, 128},
      { 109, 178, 241, 255, 231, 245, 255, 255, 128, 128, 128},
      {  44, 130, 201, 253, 205, 192, 255, 255, 128, 128, 128}
     },
     {
      {   1, 132, 239, 251, 219, 209, 255, 165, 128, 128, 128},
      {  94, 136, 225, 251, 218, 190, 255, 255, 128, 128, 128},
      {  22, 100, 174, 245, 186, 161, 255, 199, 128, 128, 128}
     },
     {
      {   1, 182, 249, 255, 232, 235, 128, 128, 128, 128, 128},
      { 124, 143, 241, 255, 227, 234, 128, 128, 128, 128, 128},
      {  35,  77, 181, 251, 193, 211, 255, 205, 128, 128, 128}
     },
     {
      {   1, 157, 247, 255, 236, 231, 255, 255, 128, 128, 128},
      { 121, 141, 235, 255, 225, 227, 255, 255, 128, 128, 128},
      {  45,  99, 188, 251, 195, 217, 255, 224, 128, 128, 128}
     },
     {
      {   1,   1, 251, 255, 213, 255, 128, 128, 128, 128, 128},
      { 203,   1, 248, 255, 255, 128, 128, 128, 128, 128, 128},
      { 137,   1, 177, 255, 224, 255, 128, 128, 128, 128, 128}
     }
    },

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    {
     {
      { 253,   9, 248, 251, 207, 208, 255, 192, 128, 128, 128},
      { 175,  13, 224, 243, 193, 185, 249, 198, 255, 255, 128},
      {  73,  17, 171, 221, 161, 179, 236, 167, 255, 234, 128}
     },
     {
      {   1,  95, 247, 253, 212, 183, 255, 255, 128, 128, 128},
      { 239,  90, 244, 250, 211, 209, 255, 255, 128, 128, 128},
      { 155,  77, 195, 248, 188, 195, 255, 255, 128, 128, 128}
     },
     {
      {   1,  24, 239, 251, 218, 219, 255, 205, 128, 128, 128},
      { 201,  51, 219, 255, 196, 186, 128, 128, 128, 128, 128},
      {  69,  46, 190, 239, 201, 218, 255, 228, 128, 128, 128}
     },
     {
      {   1, 191, 251, 255, 255, 128, 128, 128, 128, 128, 128},
      { 223, 165, 249, 255, 213, 255, 128, 128, 128, 128, 128},
      { 141, 124, 248, 255, 255, 128, 128, 128, 128, 128, 128}
     },
     {
      {   1,  16, 248, 255, 255, 128, 128, 128, 128, 128, 128},
      { 190,  36, 230, 255, 236, 255, 128, 128, 128, 128, 128},
      { 149,   1, 255, 128, 128, 128, 128, 128, 128, 128, 128}
     },
     {
      {   1, 226, 255, 128, 128, 128, 128, 128, 128, 128, 128},
      { 247, 192, 255, 128, 128, 128, 128, 128, 128, 128, 128},
      { 240, 128, 255, 128, 128, 128, 128, 128, 128, 128, 128}
     },
     {
      {   1, 134, 252, 255, 255, 128, 128, 128, 128, 128, 128},
      { 213,  62, 250, 255, 255, 128, 128, 128, 128, 128, 128},
      {  55,  93, 255, 128, 128, 128, 128, 128, 128, 128, 128}
     },
     {
      { 128, 128, 128, 128, 128, 128, 128, 128, 128, 128, 128},
      { 128, 128, 128, 128, 128, 128, 128, 128, 128, 128, 128},
      { 128, 128, 128, 128, 128, 128, 128, 128, 128, 128, 128}
     }
    },
    {
     {
      { 202,  24, 213, 235, 186, 191, 220, 160, 240, 175, 255},
      { 126,  38, 182, 232, 169, 184, 228, 174, 255, 187, 128},
      {  61,  46, 138, 219, 151, 178, 240, 170, 255, 216, 128}
     },

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     {
      {   1, 112, 230, 250, 199, 191, 247, 159, 255, 255, 128},
      { 166, 109, 228, 252, 211, 215, 255, 174, 128, 128, 128},
      {  39,  77, 162, 232, 172, 180, 245, 178, 255, 255, 128}
     },
     {
      {   1,  52, 220, 246, 198, 199, 249, 220, 255, 255, 128},
      { 124,  74, 191, 243, 183, 193, 250, 221, 255, 255, 128},
      {  24,  71, 130, 219, 154, 170, 243, 182, 255, 255, 128}
     },
     {
      {   1, 182, 225, 249, 219, 240, 255, 224, 128, 128, 128},
      { 149, 150, 226, 252, 216, 205, 255, 171, 128, 128, 128},
      {  28, 108, 170, 242, 183, 194, 254, 223, 255, 255, 128}
     },
     {
      {   1,  81, 230, 252, 204, 203, 255, 192, 128, 128, 128},
      { 123, 102, 209, 247, 188, 196, 255, 233, 128, 128, 128},
      {  20,  95, 153, 243, 164, 173, 255, 203, 128, 128, 128}
     },
     {
      {   1, 222, 248, 255, 216, 213, 128, 128, 128, 128, 128},
      { 168, 175, 246, 252, 235, 205, 255, 255, 128, 128, 128},
      {  47, 116, 215, 255, 211, 212, 255, 255, 128, 128, 128}
     },
     {
      {   1, 121, 236, 253, 212, 214, 255, 255, 128, 128, 128},
      { 141,  84, 213, 252, 201, 202, 255, 219, 128, 128, 128},
      {  42,  80, 160, 240, 162, 185, 255, 205, 128, 128, 128}
     },
     {
      {   1,   1, 255, 128, 128, 128, 128, 128, 128, 128, 128},
      { 244,   1, 255, 128, 128, 128, 128, 128, 128, 128, 128},
      { 238,   1, 255, 128, 128, 128, 128, 128, 128, 128, 128}
     }
    }
   };

   ---- End code block ----------------------------------------

14.  DCT and WHT Inversion and Macroblock Reconstruction

14.1.  Dequantization

   After decoding the DCTs/WHTs as described above, each (quantized)
   coefficient in each subblock is multiplied by one of six
   dequantization factors, the choice of factor depending on the plane
   (Y2, Y, or chroma) and position (DC = coefficient zero, AC = any

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   other coefficient).  If the current macroblock has overridden the
   quantizer level (as described in Section 10), then the six factors
   are looked up from two dequantization tables with appropriate scaling
   and clamping using the single index supplied by the override.
   Otherwise, the frame-level dequantization factors (as described in
   Section 9.6) are used.  In either case, the multiplies are computed
   and stored using 16-bit signed integers.

   The two dequantization tables, which may also be found in the
   reference decoder file dequant_data.h (Section 20.3), are as follows.

   ---- Begin code block --------------------------------------

   static const int dc_qlookup[QINDEX_RANGE] =
   {
       4,   5,   6,   7,   8,   9,  10,  10,   11,  12,  13,  14,  15,
      16,  17,  17,  18,  19,  20,  20,  21,   21,  22,  22,  23,  23,
      24,  25,  25,  26,  27,  28,  29,  30,   31,  32,  33,  34,  35,
      36,  37,  37,  38,  39,  40,  41,  42,   43,  44,  45,  46,  46,
      47,  48,  49,  50,  51,  52,  53,  54,   55,  56,  57,  58,  59,
      60,  61,  62,  63,  64,  65,  66,  67,   68,  69,  70,  71,  72,
      73,  74,  75,  76,  76,  77,  78,  79,   80,  81,  82,  83,  84,
      85,  86,  87,  88,  89,  91,  93,  95,   96,  98, 100, 101, 102,
      104, 106, 108, 110, 112, 114, 116, 118, 122, 124, 126, 128, 130,
      132, 134, 136, 138, 140, 143, 145, 148, 151, 154, 157,
   };

   static const int ac_qlookup[QINDEX_RANGE] =
   {
       4,   5,   6,   7,   8,   9,  10,  11,  12,  13,  14,  15,  16,
      17,  18,  19,  20,  21,  22,  23,  24,  25,  26,  27,  28,  29,
      30,  31,  32,  33,  34,  35,  36,  37,  38,  39,  40,  41,  42,
      43,  44,  45,  46,  47,  48,  49,  50,  51,  52,  53,  54,  55,
      56,  57,  58,  60,  62,  64,  66,  68,  70,  72,  74,  76,  78,
      80,  82,  84,  86,  88,  90,  92,  94,  96,  98, 100, 102, 104,
     106, 108, 110, 112, 114, 116, 119, 122, 125, 128, 131, 134, 137,
     140, 143, 146, 149, 152, 155, 158, 161, 164, 167, 170, 173, 177,
     181, 185, 189, 193, 197, 201, 205, 209, 213, 217, 221, 225, 229,
     234, 239, 245, 249, 254, 259, 264, 269, 274, 279, 284,
   };

   ---- End code block ----------------------------------------

   Lookup values from the above two tables are directly used in the DC
   and AC coefficients in Y1, respectively.  For Y2 and chroma, values
   from the above tables undergo either scaling or clamping before the
   multiplies.  Details regarding these scaling and clamping processes
   can be found in related lookup functions in dixie.c (Section 20.4).

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14.2.  Inverse Transforms

   If the Y2 residue block exists (i.e., the macroblock luma mode is not
   SPLITMV or B_PRED), it is inverted first (using the inverse WHT) and
   the element of the result at row i, column j is used as the 0th
   coefficient of the Y subblock at position (i, j), that is, the Y
   subblock whose index is (i * 4) + j.  As discussed in Section 13, if
   the luma mode is B_PRED or SPLITMV, the 0th Y coefficients are part
   of the residue signal for the subblocks themselves.

   In either case, the inverse transforms for the sixteen Y subblocks
   and eight chroma subblocks are computed next.  All 24 of these
   inversions are independent of each other; their results may (at least
   conceptually) be stored in 24 separate 4x4 arrays.

   As is done by the reference decoder, an implementation may wish to
   represent the prediction and residue buffers as macroblock-sized
   arrays (that is, a 16x16 Y buffer and two 8x8 chroma buffers).
   Regarding the inverse DCT implementation given below, this requires a
   simple adjustment to the address calculation for the resulting
   residue pixels.

14.3.  Implementation of the WHT Inversion

   As previously discussed (see Sections 2 and 13), for macroblocks
   encoded using prediction modes other than B_PRED and SPLITMV, the DC
   values derived from the DCT transform on the 16 Y blocks are
   collected to construct a 25th block of a macroblock (16 Y, 4 U, 4 V
   constitute the 24 blocks).  This 25th block is transformed using a
   Walsh-Hadamard transform (WHT).

   The inputs to the inverse WHT (that is, the dequantized
   coefficients), the intermediate "horizontally detransformed" signal,
   and the completely detransformed residue signal are all stored as
   arrays of 16-bit signed integers.

   Following the tradition of specifying bitstream format using the
   decoding process, we specify the inverse WHT in the decoding process
   using the following C-style source code:

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   ---- Begin code block --------------------------------------

   void vp8_short_inv_walsh4x4_c(short *input, short *output)
   {
     int i;
     int a1, b1, c1, d1;
     int a2, b2, c2, d2;
     short *ip = input;
     short *op = output;
     int temp1, temp2;

     for(i=0;i<4;i++)
     {
       a1 = ip[0] + ip[12];
       b1 = ip[4] + ip[8];
       c1 = ip[4] - ip[8];
       d1 = ip[0] - ip[12];

       op[0] = a1 + b1;
       op[4] = c1 + d1;
       op[8] = a1 - b1;
       op[12]= d1 - c1;
       ip++;
       op++;
     }
     ip = output;
     op = output;
     for(i=0;i<4;i++)
     {
       a1 = ip[0] + ip[3];
       b1 = ip[1] + ip[2];
       c1 = ip[1] - ip[2];
       d1 = ip[0] - ip[3];

       a2 = a1 + b1;
       b2 = c1 + d1;
       c2 = a1 - b1;
       d2 = d1 - c1;

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       op[0] = (a2+3)>>3;
       op[1] = (b2+3)>>3;
       op[2] = (c2+3)>>3;
       op[3] = (d2+3)>>3;

       ip+=4;
       op+=4;
     }
   }

   ---- End code block ----------------------------------------

   In the case that there is only one non-zero DC value in input, the
   inverse transform can be simplified to the following:

   ---- Begin code block --------------------------------------

   void vp8_short_inv_walsh4x4_1_c(short *input, short *output)
   {
     int i;
     int a1;
     short *op=output;

     a1 = ((input[0] + 3)>>3);

     for(i=0;i<4;i++)
     {
       op[0] = a1;
       op[1] = a1;
       op[2] = a1;
       op[3] = a1;
       op+=4;
     }
   }

   ---- End code block ----------------------------------------

   It should be noted that a conforming decoder should implement the
   inverse transform using exactly the same rounding to achieve bit-wise
   matching output to the output of the process specified by the above
   C source code.

   The reference decoder WHT inversion may be found in the file
   idct_add.c (Section 20.8).

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14.4.  Implementation of the DCT Inversion

   All of the DCT inversions are computed in exactly the same way.  In
   principle, VP8 uses a classical 2-D inverse discrete cosine
   transform, implemented as two passes of 1-D inverse DCT.  The 1-D
   inverse DCT was calculated using a similar algorithm to what was
   described in [Loeffler].  However, the paper only provided the
   8-point and 16-point version of the algorithms, which was adapted by
   On2 to perform the 4-point 1-D DCT.

   Accurate calculation of 1-D DCT of the above algorithm requires
   infinite precision.  VP8 of course can use only a finite-precision
   approximation.  Also, the inverse DCT used by VP8 takes care of
   normalization of the standard unitary transform; that is, every
   dequantized coefficient has roughly double the size of the
   corresponding unitary coefficient.  However, at all but the highest
   datarates, the discrepancy between transmitted and ideal coefficients
   is due almost entirely to (lossy) compression and not to errors
   induced by finite-precision arithmetic.

   The inputs to the inverse DCT (that is, the dequantized
   coefficients), the intermediate "horizontally detransformed" signal,
   and the completely detransformed residue signal are all stored as
   arrays of 16-bit signed integers.  The details of the computation are
   as follows.

   It should also be noted that this implementation makes use of the
   16-bit fixed-point version of two multiplication constants:

   sqrt(2) * cos (pi/8)

   sqrt(2) * sin (pi/8)

   Because the first constant is bigger than 1, to maintain the same
   16-bit fixed-point precision as the second one, we make use of the
   fact that

   x * a = x + x*(a-1)

   therefore

   x * sqrt(2) * cos (pi/8) = x + x * (sqrt(2) * cos(pi/8)-1)

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   ---- Begin code block --------------------------------------

   /* IDCT implementation */
   static const int cospi8sqrt2minus1=20091;
   static const int sinpi8sqrt2      =35468;
   void short_idct4x4llm_c(short *input, short *output, int pitch)
   {
     int i;
     int a1, b1, c1, d1;

     short *ip=input;
     short *op=output;
     int temp1, temp2;
     int shortpitch = pitch>>1;

     for(i=0;i<4;i++)
     {
       a1 = ip[0]+ip[8];
       b1 = ip[0]-ip[8];

       temp1 = (ip[4] * sinpi8sqrt2)>>16;
       temp2 = ip[12]+((ip[12] * cospi8sqrt2minus1)>>16);
       c1 = temp1 - temp2;

       temp1 = ip[4] + ((ip[4] * cospi8sqrt2minus1)>>16);
       temp2 = (ip[12] * sinpi8sqrt2)>>16;
       d1 = temp1 + temp2;

       op[shortpitch*0] = a1+d1;
       op[shortpitch*3] = a1-d1;
       op[shortpitch*1] = b1+c1;
       op[shortpitch*2] = b1-c1;

       ip++;
       op++;
     }
     ip = output;
     op = output;
     for(i=0;i<4;i++)
     {
       a1 = ip[0]+ip[2];
       b1 = ip[0]-ip[2];

       temp1 = (ip[1] * sinpi8sqrt2)>>16;
       temp2 = ip[3]+((ip[3] * cospi8sqrt2minus1)>>16);
       c1 = temp1 - temp2;

       temp1 = ip[1] + ((ip[1] * cospi8sqrt2minus1)>>16);

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       temp2 = (ip[3] * sinpi8sqrt2)>>16;
       d1 = temp1 + temp2;

       op[0] = (a1+d1+4)>>3;
       op[3] = (a1-d1+4)>>3;
       op[1] = (b1+c1+4)>>3;
       op[2] = (b1-c1+4)>>3;

       ip+=shortpitch;
       op+=shortpitch;
     }
   }

   ---- End code block ----------------------------------------

   The reference decoder DCT inversion may be found in the file
   idct_add.c (Section 20.8).

14.5.  Summation of Predictor and Residue

   Finally, the prediction and residue signals are summed to form the
   reconstructed macroblock, which, except for loop filtering (taken up
   next), completes the decoding process.

   The summing procedure is fairly straightforward, having only a couple
   of details.  The prediction and residue buffers are both arrays of
   16-bit signed integers.  Each individual (Y, U, and V pixel) result
   is calculated first as a 32-bit sum of the prediction and residue,
   and is then saturated to 8-bit unsigned range (using, say, the
   clamp255 function defined above) before being stored as an 8-bit
   unsigned pixel value.

   VP8 also supports a mode where the encoding of a bitstream guarantees
   all reconstructed pixel values between 0 and 255; compliant
   bitstreams of such requirements have the clamp_type bit in the frame
   header set to 1.  In such a case, the clamp255 function is no longer
   required.

   The summation process is the same, regardless of the (intra or inter)
   mode of prediction in effect for the macroblock.  The reference
   decoder implementation of reconstruction may be found in the file
   idct_add.c.

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15.  Loop Filter

   Loop filtering is the last stage of frame reconstruction and the
   next-to-last stage of the decoding process.  The loop filter is
   applied to the entire frame after the summation of predictor and
   residue signals, as described in Section 14.

   The purpose of the loop filter is to eliminate (or at least reduce)
   visually objectionable artifacts associated with the semi-
   independence of the coding of macroblocks and their constituent
   subblocks.

   As was discussed in Section 5, the loop filter is "integral" to
   decoding, in that the results of loop filtering are used in the
   prediction of subsequent frames.  Consequently, a functional decoder
   implementation must perform loop filtering exactly as described here.
   This is distinct from any postprocessing that may be applied only to
   the image immediately before display; such postprocessing is entirely
   at the option of the implementor (and/or user) and has no effect on
   decoding per se.

   The baseline frame-level parameters controlling the loop filter are
   defined in the frame header (Section 9.4) along with a mechanism for
   adjustment based on a macroblock's prediction mode and/or reference
   frame.  The first is a flag (filter_type) selecting the type of
   filter (normal or simple); the other two are numbers
   (loop_filter_level and sharpness_level) that adjust the strength or
   sensitivity of the filter.  As described in Sections 9.3 and 10,
   loop_filter_level may also be overridden on a per-macroblock basis
   using segmentation.

   Loop filtering is one of the more computationally intensive aspects
   of VP8 decoding.  This is the reason for the existence of the
   optional, less-demanding simple filter type.

   Note carefully that loop filtering must be skipped entirely if
   loop_filter_level at either the frame header level or macroblock
   override level is 0.  In no case should the loop filter be run with a
   value of 0; it should instead be skipped.

   We begin by discussing the aspects of loop filtering that are
   independent of the controlling parameters and type of filter chosen.

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15.1.  Filter Geometry and Overall Procedure

   The Y, U, and V planes are processed independently and identically.

   The loop filter acts on the edges between adjacent macroblocks and on
   the edges between adjacent subblocks of a macroblock.  All such edges
   are horizontal or vertical.  For each pixel position on an edge, a
   small number (two or three) of pixels adjacent to either side of the
   position are examined and possibly modified.  The displacements of
   these pixels are at a right angle to the edge orientation; that is,
   for a horizontal edge, we treat the pixels immediately above and
   below the edge position, and for a vertical edge, we treat the pixels
   immediately to the left and right of the edge.

   We call this collection of pixels associated to an edge position a
   segment; the length of a segment is 2, 4, 6, or 8.  Excepting that
   the normal filter uses slightly different algorithms for, and either
   filter may apply different control parameters to, the edges between
   macroblocks and those between subblocks, the treatment of edges is
   quite uniform: All segments straddling an edge are treated
   identically; there is no distinction between the treatment of
   horizontal and vertical edges, whether between macroblocks or between
   subblocks.

   As a consequence, adjacent subblock edges within a macroblock may be
   concatenated and processed in their entirety.  There is a single
   8-pixel-long vertical edge horizontally centered in each of the U and
   V blocks (the concatenation of upper and lower 4-pixel edges between
   chroma subblocks), and three 16-pixel-long vertical edges at
   horizontal positions 1/4, 1/2, and 3/4 the width of the luma
   macroblock, each representing the concatenation of four 4-pixel
   sub-edges between pairs of Y subblocks.

   The macroblocks comprising the frame are processed in the usual
   raster-scan order.  Each macroblock is "responsible for" the
   inter-macroblock edges immediately above and to the left of it (but
   not the edges below and to the right of it), as well as the edges
   between its subblocks.

   For each macroblock M, there are four filtering steps, which are,
   (almost) in order:

   1.  If M is not on the leftmost column of macroblocks, filter across
       the left (vertical) inter-macroblock edge of M.

   2.  Filter across the vertical subblock edges within M.

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   3.  If M is not on the topmost row of macroblocks, filter across the
       top (horizontal) inter-macroblock edge of M.

   4.  Filter across the horizontal subblock edges within M.

   We write MY, MU, and MV for the planar constituents of M, that is,
   the 16x16 luma block, 8x8 U block, and 8x8 V block comprising M.

   In step 1, for each of the three blocks MY, MU, and MV, we filter
   each of the (16 luma or 8 chroma) segments straddling the column
   separating the block from the block immediately to the left of it,
   using the inter-macroblock filter and controls associated to the
   loop_filter_level and sharpness_level.

   In step 4, we filter across the (three luma and one each for U and V)
   vertical subblock edges described above, this time using the
   inter-subblock filter and controls.

   Steps 2 and 4 are skipped for macroblocks that satisfy both of the
   following two conditions:

   1.  Macroblock coding mode is neither B_PRED nor SPLITMV; and

   2.  There is no DCT coefficient coded for the whole macroblock.

   For these macroblocks, loop filtering for edges between subblocks
   internal to a macroblock is effectively skipped.  This skip strategy
   significantly reduces VP8 loop-filtering complexity.

   Edges between macroblocks and those between subblocks are treated
   with different control parameters (and, in the case of the normal
   filter, with different algorithms).  Except for pixel addressing,
   there is no distinction between the treatment of vertical and
   horizontal edges.  Luma edges are always 16 pixels long, chroma edges
   are always 8 pixels long, and the segments straddling an edge are
   treated identically; this of course facilitates vector processing.

   Because many pixels belong to segments straddling two or more edges,
   and so will be filtered more than once, the order in which edges are
   processed given above must be respected by any implementation.
   Within a single edge, however, the segments straddling that edge are
   disjoint, and the order in which these segments are processed is
   immaterial.

   Before taking up the filtering algorithms themselves, we should
   emphasize a point already made: Even though the pixel segments
   associated to a macroblock are antecedent to the macroblock (that is,
   lie within the macroblock or in already-constructed macroblocks), a

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   macroblock must not be filtered immediately after its
   "reconstruction" (described in Section 14).  Rather, the loop filter
   applies after all the macroblocks have been "reconstructed" (i.e.,
   had their predictor summed with their residue); correct decoding is
   predicated on the fact that already-constructed portions of the
   current frame referenced via intra-prediction (described in
   Section 12) are not yet filtered.

15.2.  Simple Filter

   Having described the overall procedure of, and pixels affected by,
   the loop filter, we turn our attention to the treatment of individual
   segments straddling edges.  We begin by describing the simple filter,
   which, as the reader might guess, is somewhat simpler than the normal
   filter.

   Note that the simple filter only applies to luma edges.  Chroma edges
   are left unfiltered.

   Roughly speaking, the idea of loop filtering is, within limits, to
   reduce the difference between pixels straddling an edge.  Differences
   in excess of a threshold (associated to the loop_filter_level) are
   assumed to be "natural" and are unmodified; differences below the
   threshold are assumed to be artifacts of quantization and the
   (partially) separate coding of blocks, and are reduced via the
   procedures described below.  While the loop_filter_level is in
   principle arbitrary, the levels chosen by a VP8 compressor tend to be
   correlated to quantizer levels.

   Most of the filtering arithmetic is done using 8-bit signed operands
   (having a range of -128 to +127, inclusive), supplemented by 16-bit
   temporaries holding results of multiplies.

   Sums and other temporaries need to be "clamped" to a valid signed
   8-bit range:

   ---- Begin code block --------------------------------------

   int8 c(int v)
   {
       return (int8) (v < -128 ? -128 : (v < 128 ? v : 127));
   }

   ---- End code block ----------------------------------------

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   Since pixel values themselves are unsigned 8-bit numbers, we need to
   convert between signed and unsigned values:

   ---- Begin code block --------------------------------------

   /* Convert pixel value (0 <= v <= 255) to an 8-bit signed
      number. */
   int8 u2s(Pixel v) { return (int8) (v - 128);}

   /* Clamp, then convert signed number back to pixel value. */
   Pixel s2u(int v) { return (Pixel) (c(v) + 128);}

   ---- End code block ----------------------------------------

   Filtering is often predicated on absolute-value thresholds.  The
   following function is the equivalent of the standard library function
   abs, whose prototype is found in the standard header file stdlib.h.
   For us, the argument v is always the difference between two pixels
   and lies in the range -255 <= v <= +255.

   ---- Begin code block --------------------------------------

   int abs(int v) { return v < 0?  -v : v;}

   ---- End code block ----------------------------------------

   An actual implementation would of course use inline functions or
   macros to accomplish these trivial procedures (which are used by both
   the normal and simple loop filters).  An optimal implementation would
   probably express them in machine language, perhaps using single
   instruction, multiple data (SIMD) vector instructions.  On many SIMD
   processors, the saturation accomplished by the above clamping
   function is often folded into the arithmetic instructions themselves,
   obviating the explicit step taken here.

   To simplify the specification of relative pixel positions, we use the
   word "before" to mean "immediately above" (for a vertical segment
   straddling a horizontal edge) or "immediately to the left of" (for a
   horizontal segment straddling a vertical edge), and the word "after"
   to mean "immediately below" or "immediately to the right of".

   Given an edge, a segment, and a limit value, the simple loop filter
   computes a value based on the four pixels that straddle the edge (two
   either side).  If that value is below a supplied limit, then, very
   roughly speaking, the two pixel values are brought closer to each
   other, "shaving off" something like a quarter of the difference.  The

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   same procedure is used for all segments straddling any type of edge,
   regardless of the nature (inter-macroblock, inter-subblock, luma, or
   chroma) of the edge; only the limit value depends on the edge type.

   The exact procedure (for a single segment) is as follows; the
   subroutine common_adjust is used by both the simple filter presented
   here and the normal filters discussed in Section 15.3.

   ---- Begin code block --------------------------------------

   int8 common_adjust(
       int use_outer_taps,   /* filter is 2 or 4 taps wide */
       const Pixel *P1,    /* pixel before P0 */
       Pixel *P0,          /* pixel before edge */
       Pixel *Q0,          /* pixel after edge */
       const Pixel *Q1     /* pixel after Q0 */
   ) {
       cint8 p1 = u2s(*P1);   /* retrieve and convert all 4 pixels */
       cint8 p0 = u2s(*P0);
       cint8 q0 = u2s(*Q0);
       cint8 q1 = u2s(*Q1);

       /* Disregarding clamping, when "use_outer_taps" is false,
          "a" is 3*(q0-p0).  Since we are about to divide "a" by
          8, in this case we end up multiplying the edge
          difference by 5/8.

          When "use_outer_taps" is true (as for the simple filter),
          "a" is p1 - 3*p0 + 3*q0 - q1, which can be thought of as
          a refinement of 2*(q0 - p0), and the adjustment is
          something like (q0 - p0)/4. */

       int8 a = c((use_outer_taps? c(p1 - q1) : 0) + 3*(q0 - p0));

       /* b is used to balance the rounding of a/8 in the case where
          the "fractional" part "f" of a/8 is exactly 1/2. */

       cint8 b = (c(a + 3)) >> 3;

       /* Divide a by 8, rounding up when f >= 1/2.
          Although not strictly part of the C language,
          the right shift is assumed to propagate the sign bit. */

       a = c(a + 4) >> 3;

       /* Subtract "a" from q0, "bringing it closer" to p0. */

       *Q0 = s2u(q0 - a);

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       /* Add "a" (with adjustment "b") to p0, "bringing it closer"
          to q0.

          The clamp of "a+b", while present in the reference decoder,
          is superfluous; we have -16 <= a <= 15 at this point. */

       *P0 = s2u(p0 + b);

       return a;
   }

   ---- End code block ----------------------------------------

   ---- Begin code block --------------------------------------

   void simple_segment(
       uint8 edge_limit,   /* do nothing if edge difference
                              exceeds limit */
       const Pixel *P1,    /* pixel before P0 */
       Pixel *P0,          /* pixel before edge */
       Pixel *Q0,          /* pixel after edge */
       const Pixel *Q1     /* pixel after Q0 */
   ) {
       if ((abs(*P0 - *Q0)*2 + abs(*P1 - *Q1)/2) <= edge_limit))
           common_adjust(1, P1, P0, Q0, Q1);   /* use outer taps */
   }

   ---- End code block ----------------------------------------

   We make a couple of remarks about the rounding procedure above.  When
   b is zero (that is, when the "fractional part" of a is not 1/2), we
   are (except for clamping) adding the same number to p0 as we are
   subtracting from q0.  This preserves the average value of p0 and q0,
   but the resulting difference between p0 and q0 is always even; in
   particular, the smallest non-zero gradation +-1 is not possible here.

   When b is one, the value we add to p0 (again except for clamping) is
   one less than the value we are subtracting from q0.  In this case,
   the resulting difference is always odd (and the small gradation +-1
   is possible), but the average value is reduced by 1/2, yielding, for
   instance, a very slight darkening in the luma plane.  (In the very
   unlikely event of appreciable darkening after a large number of
   interframes, a compressor would of course eventually compensate for
   this in the selection of predictor and/or residue.)

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   The derivation of the edge_limit value used above, which depends on
   the loop_filter_level and sharpness_level, as well as the type of
   edge being processed, will be taken up after we describe the normal
   loop filtering algorithm below.

15.3.  Normal Filter

   The normal loop filter is a refinement of the simple loop filter; all
   of the general discussion above applies here as well.  In particular,
   the functions c, u2s, s2u, abs, and common_adjust are used by both
   the normal and simple filters.

   As mentioned above, the normal algorithms for inter-macroblock and
   inter-subblock edges differ.  Nonetheless, they have a great deal in
   common: They use similar threshold algorithms to disable the filter
   and to detect high internal edge variance (which influences the
   filtering algorithm).  Both algorithms also use, at least
   conditionally, the simple filter adjustment procedure described
   above.

   The common thresholding algorithms are as follows.

   ---- Begin code block --------------------------------------

   /* All functions take (among other things) a segment (of length
      at most 4 + 4 = 8) symmetrically straddling an edge.

      The pixel values (or pointers) are always given in order,
      from the "beforemost" to the "aftermost".  So, for a
      horizontal edge (written "|"), an 8-pixel segment would be
      ordered p3 p2 p1 p0 | q0 q1 q2 q3. */

   /* Filtering is disabled if the difference between any two
      adjacent "interior" pixels in the 8-pixel segment exceeds
      the relevant threshold (I).  A more complex thresholding
      calculation is done for the group of four pixels that
      straddle the edge, in line with the calculation in
      simple_segment() above. */

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   int filter_yes(
       uint8 I,        /* limit on interior differences */
       uint8 E,        /* limit at the edge */

       cint8 p3, cint8 p2, cint8 p1, cint8 p0, /* pixels before
                                                  edge */
       cint8 q0, cint8 q1, cint8 q2, cint8 q3  /* pixels after
                                                  edge */
   ) {
       return  (abs(p0 - q0)*2 + abs(p1 - q1)/2) <= E
           &&  abs(p3 - p2) <= I  &&  abs(p2 - p1) <= I  &&
             abs(p1 - p0) <= I
           &&  abs(q3 - q2) <= I  &&  abs(q2 - q1) <= I  &&
             abs(q1 - q0) <= I;
   }

   ---- End code block ----------------------------------------

   ---- Begin code block --------------------------------------

   /* Filtering is altered if (at least) one of the differences
      on either side of the edge exceeds a threshold (we have
      "high edge variance"). */

   int hev(
       uint8 threshold,
       cint8 p1, cint8 p0, /* pixels before edge */
       cint8 q0, cint8 q1  /* pixels after edge */
   ) {
       return abs(p1 - p0) > threshold  ||  abs(q1 - q0) > threshold;
   }

   ---- End code block ----------------------------------------

   The subblock filter is a variant of the simple filter.  In fact, if
   we have high edge variance, the adjustment is exactly as for the
   simple filter.  Otherwise, the simple adjustment (without outer taps)
   is applied, and the two pixels one step in from the edge pixels are
   adjusted by roughly half the amount by which the two edge pixels are
   adjusted; since the edge adjustment here is essentially 3/8 the edge
   difference, the inner adjustment is approximately 3/16 the edge
   difference.

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   ---- Begin code block --------------------------------------

   void subblock_filter(
       uint8 hev_threshold,     /* detect high edge variance */
       uint8 interior_limit,    /* possibly disable filter */
       uint8 edge_limit,
       cint8 *P3, cint8 *P2, int8 *P1, int8 *P0,   /* pixels before
                                                      edge */
       int8 *Q0, int8 *Q1, cint8 *Q2, cint8 *Q3    /* pixels after
                                                      edge */
   ) {
       cint8 p3 = u2s(*P3), p2 = u2s(*P2), p1 = u2s(*P1),
         p0 = u2s(*P0);
       cint8 q0 = u2s(*Q0), q1 = u2s(*Q1), q2 = u2s(*Q2),
         q3 = u2s(*Q3);

       if (filter_yes(interior_limit, edge_limit, q3, q2, q1, q0,
         p0, p1, p2, p3))
       {
           const int hv = hev(hev_threshold, p1, p0, q0, q1);

           cint8 a = (common_adjust(hv, P1, P0, Q0, Q1) + 1) >> 1;

           if (!hv) {
               *Q1 = s2u(q1 - a);
               *P1 = s2u(p1 + a);
           }
       }
   }

   ---- End code block ----------------------------------------

   The inter-macroblock filter has potentially wider scope.  If the edge
   variance is high, it performs the simple adjustment (using the outer
   taps, just like the simple filter and the corresponding case of the
   normal subblock filter).  If the edge variance is low, we begin with
   the same basic filter calculation and apply multiples of it to pixel
   pairs symmetric about the edge; the magnitude of adjustment decays as
   we move away from the edge and six of the pixels in the segment are
   affected.

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   ---- Begin code block --------------------------------------

   void MBfilter(
       uint8 hev_threshold,     /* detect high edge variance */
       uint8 interior_limit,    /* possibly disable filter */
       uint8 edge_limit,
       cint8 *P3, int8 *P2, int8 *P1, int8 *P0,  /* pixels before
                                                    edge */
       int8 *Q0, int8 *Q1, int8 *Q2, cint8 *Q3   /* pixels after
                                                    edge */
   ) {
       cint8 p3 = u2s(*P3), p2 = u2s(*P2), p1 = u2s(*P1),
         p0 = u2s(*P0);
       cint8 q0 = u2s(*Q0), q1 = u2s(*Q1), q2 = u2s(*Q2),
         q3 = u2s(*Q3);

       if (filter_yes(interior_limit, edge_limit, q3, q2, q1, q0,
         p0, p1, p2, p3))
       {
           if (!hev(hev_threshold, p1, p0, q0, q1))
           {
               /* Same as the initial calculation in "common_adjust",
                  w is something like twice the edge difference */

               const int8 w = c(c(p1 - q1) + 3*(q0 - p0));

               /* 9/64 is approximately 9/63 = 1/7, and 1<<7 = 128 =
                  2*64.  So this a, used to adjust the pixels adjacent
                  to the edge, is something like 3/7 the edge
                  difference. */

               int8 a = c((27*w + 63) >> 7);

               *Q0 = s2u(q0 - a);  *P0 = s2u(p0 + a);

               /* Next two are adjusted by 2/7 the edge difference */

               a = c((18*w + 63) >> 7);

               *Q1 = s2u(q1 - a);  *P1 = s2u(p1 + a);

               /* Last two are adjusted by 1/7 the edge difference */

               a = c((9*w + 63) >> 7);

               *Q2 = s2u(q2 - a);  *P2 = s2u(p2 + a);

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           } else                      /* if hev, do simple filter */
               common_adjust(1, P1, P0, Q0, Q1);   /* using outer
                                                       taps */
       }
   }

   ---- End code block ----------------------------------------

15.4.  Calculation of Control Parameters

   We conclude the discussion of loop filtering by showing how the
   thresholds supplied to the procedures above are derived from the two
   control parameters sharpness_level (an unsigned 3-bit number having
   maximum value 7) and loop_filter_level (an unsigned 6-bit number
   having maximum value 63).

   While the sharpness_level is constant over the frame, individual
   macroblocks may override the loop_filter_level with one of four
   possibilities supplied in the frame header (as described in
   Section 10).

   Both the simple and normal filters disable filtering if a value
   derived from the four pixels that straddle the edge (2 either side)
   exceeds a threshold / limit value.

   ---- Begin code block --------------------------------------

   /* Luma and Chroma use the same inter-macroblock edge limit */
   uint8 mbedge_limit = ((loop_filter_level + 2) * 2) +
     interior_limit;

   /* Luma and Chroma use the same inter-subblock edge limit */
   uint8 sub_bedge_limit = (loop_filter_level * 2) + interior_limit;

   ---- End code block ----------------------------------------

   The remaining thresholds are used only by the normal filters.  The
   filter-disabling interior difference limit is the same for all edges
   (luma, chroma, inter-subblock, inter-macroblock) and is given by the
   following.

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   ---- Begin code block --------------------------------------

   uint8 interior_limit = loop_filter_level;

   if (sharpness_level)
   {
       interior_limit  >>=  sharpness_level > 4 ?  2 : 1;
       if (interior_limit > 9 - sharpness_level)
           interior_limit = 9 - sharpness_level;
   }
   if (!interior_limit)
       interior_limit = 1;

   ---- End code block ----------------------------------------

   Finally, we give the derivation of the high edge-variance threshold,
   which is also the same for all edge types.

   ---- Begin code block --------------------------------------

   uint8 hev_threshold = 0;

   if (we_are_decoding_akey_frame)   /* current frame is a key frame */
   {
       if (loop_filter_level >= 40)
           hev_threshold = 2;
       else if (loop_filter_level >= 15)
           hev_threshold = 1;
   }
   else                            /* current frame is an interframe */
   {
       if (loop_filter_level >= 40)
           hev_threshold = 3;
       else if (loop_filter_level >= 20)
           hev_threshold = 2;
       else if (loop_filter_level >= 15)
           hev_threshold = 1;
   }

   ---- End code block ----------------------------------------

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16.  Interframe Macroblock Prediction Records

   We describe the layout and semantics of the prediction records for
   macroblocks in an interframe.

   After the feature specification (which is described in Section 10 and
   is identical for intraframes and interframes), there comes a
   Bool(prob_intra), which indicates inter-prediction (i.e., prediction
   from prior frames) when true and intra-prediction (i.e., prediction
   from already-coded portions of the current frame) when false.  The
   zero-probability prob_intra is set by field J of the frame header.

16.1.  Intra-Predicted Macroblocks

   For intra-prediction, the layout of the prediction data is
   essentially the same as the layout for key frames, although the
   contexts used by the decoding process are slightly different.

   As discussed in Section 8, the "outer" Y mode here uses a different
   tree from that used in key frames, repeated here for convenience.

   ---- Begin code block --------------------------------------

   const tree_index ymode_tree [2 * (num_ymodes - 1)] =
   {
    -DC_PRED, 2,           /* root: DC_PRED = "0", "1" subtree */
     4, 6,                 /* "1" subtree has 2 descendant subtrees */
      -V_PRED, -H_PRED,    /* "10" subtree:  V_PRED = "100",
                              H_PRED = "101" */
      -TM_PRED, -B_PRED    /* "11" subtree:  TM_PRED = "110",
                              B_PRED = "111" */
   };

   ---- End code block ----------------------------------------

   The probability table used to decode this tree is variable.  As
   described in Section 11, it (along with the similarly treated UV
   table) can be updated by field J of the frame header.  Similar to the
   coefficient-decoding probabilities, such updates are cumulative and
   affect all ensuing frames until the next key frame or explicit
   update.  The default probabilities for the Y and UV tables are:

   ---- Begin code block --------------------------------------

   Prob ymode_prob [num_ymodes - 1] = { 112, 86, 140, 37};
   Prob uv_mode_prob [num_uv_modes - 1] = { 162, 101, 204};

   ---- End code block ----------------------------------------

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   These defaults must be restored after detection of a key frame.

   Just as for key frames, if the Y mode is B_PRED, there next comes an
   encoding of the intra_bpred mode used by each of the sixteen Y
   subblocks.  These encodings use the same tree as does that for key
   frames but, in place of the contexts used in key frames, these
   encodings use the single fixed probability table.

   ---- Begin code block --------------------------------------

   const Prob bmode_prob [num_intra_bmodes - 1] = {
       120, 90, 79, 133, 87, 85, 80, 111, 151
   };

   ---- End code block ----------------------------------------

   Last comes the chroma mode, again coded using the same tree as that
   used for key frames, this time using the dynamic uv_mode_prob table
   described above.

   The calculation of the intra-prediction buffer is identical to that
   described for key frames in Section 12.

16.2.  Inter-Predicted Macroblocks

   Otherwise (when the above bool is true), we are using
   inter-prediction (which of course only happens for interframes), to
   which we now restrict our attention.

   The next datum is then another bool, B(prob_last), selecting the
   reference frame.  If 0, the reference frame is the previous frame
   (the last frame); if 1, another bool (prob_gf) selects the reference
   frame between the golden frame (0) and the altref frame (1).  The
   probabilities prob_last and prob_gf are set in field J of the frame
   header.

   Together with setting the reference frame, the purpose of inter-mode
   decoding is to set a motion vector for each of the sixteen Y
   subblocks of the current macroblock.  These settings then define the
   calculation of the inter-prediction buffer (detailed in Section 18).
   While the net effect of inter-mode decoding is straightforward, the
   implementation is somewhat complex; the (lossless) compression
   achieved by this method justifies the complexity.

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   After the reference frame selector comes the mode (or motion vector
   reference) applied to the macroblock as a whole, coded using the
   following enumeration and tree.  Setting mv_nearest = num_ymodes is a
   convenience that allows a single variable to unambiguously hold an
   inter- or intra-prediction mode.

   ---- Begin code block --------------------------------------

   typedef enum
   {
       mv_nearest = num_ymodes, /* use "nearest" motion vector
                                   for entire MB */
       mv_near,                 /* use "next nearest" "" */
       mv_zero,                 /* use zero "" */
       mv_new,                  /* use explicit offset from
                                   implicit "" */
       mv_split,                /* use multiple motion vectors */

       num_mv_refs = mv_split + 1 - mv_nearest
   }
   mv_ref;

   const tree_index mv_ref_tree [2 * (num_mv_refs - 1)] =
   {
    -mv_zero, 2,                /* zero = "0" */
     -mv_nearest, 4,            /* nearest = "10" */
      -mv_near, 6,              /* near = "110" */
        -mv_new, -mv_split      /* new = "1110", split = "1111" */
   };

   ---- End code block ----------------------------------------

16.3.  Mode and Motion Vector Contexts

   The probability table used to decode the mv_ref, along with three
   reference motion vectors used by the selected mode, is calculated via
   a survey of the already-decoded motion vectors in (up to) 3 nearby
   macroblocks.

   The algorithm generates a sorted list of distinct motion vectors
   adjacent to the search site.  The best_mv is the vector with the
   highest score.  The mv_nearest is the non-zero vector with the
   highest score.  The mv_near is the non-zero vector with the next
   highest score.  The number of motion vectors coded using the SPLITMV
   mode is scored using the same weighting and is returned with the
   scores of the best, nearest, and near vectors.

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   The three adjacent macroblocks above, left, and above-left are
   considered in order.  If the macroblock is intra-coded, no action is
   taken.  Otherwise, the motion vector is compared to other previously
   found motion vectors to determine if it has been seen before, and if
   so contributes its weight to that vector; otherwise, it enters a new
   vector in the list.  The above and left vectors have twice the weight
   of the above-left vector.

   As is the case with many contexts used by VP8, it is possible for
   macroblocks near the top or left edges of the image to reference
   blocks that are outside the visible image.  VP8 provides a border of
   1 macroblock filled with 0x0 motion vectors left of the left edge,
   and a border filled with 0,0 motion vectors of 1 macroblocks above
   the top edge.

   Much of the process is more easily described in C than in English.
   The reference code for this can be found in modemv.c (Section 20.11).
   The calculation of reference vectors, probability table, and,
   finally, the inter-prediction mode itself is implemented as follows.

   ---- Begin code block --------------------------------------

   typedef union
   {
       unsigned int as_int;
       MV           as_mv;
   } int_mv;        /* facilitates rapid equality tests */

   static void mv_bias(MODE_INFO *x,int refframe, int_mv *mvp,
     int * ref_frame_sign_bias)
   {
       MV xmv;
       xmv = x->mbmi.mv.as_mv;
       if ( ref_frame_sign_bias[x->mbmi.ref_frame] !=
         ref_frame_sign_bias[refframe] )
       {
           xmv.row*=-1;
           xmv.col*=-1;
       }
       mvp->as_mv = xmv;
   }

   ---- End code block ----------------------------------------

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   ---- Begin code block --------------------------------------

   void vp8_clamp_mv(MV *mv, const MACROBLOCKD *xd)
   {
       if ( mv->col < (xd->mb_to_left_edge - LEFT_TOP_MARGIN) )
           mv->col = xd->mb_to_left_edge - LEFT_TOP_MARGIN;
       else if ( mv->col > xd->mb_to_right_edge + RIGHT_BOTTOM_MARGIN )
           mv->col = xd->mb_to_right_edge + RIGHT_BOTTOM_MARGIN;

       if ( mv->row < (xd->mb_to_top_edge - LEFT_TOP_MARGIN) )
           mv->row = xd->mb_to_top_edge - LEFT_TOP_MARGIN;
       else if ( mv->row > xd->mb_to_bottom_edge + RIGHT_BOTTOM_MARGIN )
           mv->row = xd->mb_to_bottom_edge + RIGHT_BOTTOM_MARGIN;
   }

   ---- End code block ----------------------------------------

   In the function vp8_find_near_mvs(), the vectors "nearest" and "near"
   are used by the corresponding modes.

   The vector best_mv is used as a base for explicitly coded motion
   vectors.

   The first three entries in the return value cnt are (in order)
   weighted census values for "zero", "nearest", and "near" vectors.
   The final value indicates the extent to which SPLITMV was used by the
   neighboring macroblocks.  The largest possible "weight" value in each
   case is 5.

   ---- Begin code block --------------------------------------

   void vp8_find_near_mvs
   (
       MACROBLOCKD *xd,
       const MODE_INFO *here,
       MV *nearest,
       MV *near,
       MV *best_mv,
       int cnt[4],
       int refframe,
       int * ref_frame_sign_bias
   )

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   {
       const MODE_INFO *above = here - xd->mode_info_stride;
       const MODE_INFO *left = here - 1;
       const MODE_INFO *aboveleft = above - 1;
       int_mv            near_mvs[4];
       int_mv           *mv = near_mvs;
       int             *cntx = cnt;
       enum {CNT_ZERO, CNT_NEAREST, CNT_NEAR, CNT_SPLITMV};

       /* Zero accumulators */
       mv[0].as_int = mv[1].as_int = mv[2].as_int = 0;
       cnt[0] = cnt[1] = cnt[2] = cnt[3] = 0;

       /* Process above */
       if (above->mbmi.ref_frame != INTRA_FRAME) {
           if (above->mbmi.mv.as_int) {
               (++mv)->as_int = above->mbmi.mv.as_int;
               mv_bias(above, refframe, mv, ref_frame_sign_bias);
               ++cntx;
           }
           *cntx += 2;
       }

       /* Process left */
       if (left->mbmi.ref_frame != INTRA_FRAME) {
           if (left->mbmi.mv.as_int) {
               int_mv this_mv;

               this_mv.as_int = left->mbmi.mv.as_int;
               mv_bias(left, refframe, &this_mv, ref_frame_sign_bias);

               if (this_mv.as_int != mv->as_int) {
                   (++mv)->as_int = this_mv.as_int;
                   ++cntx;
               }
               *cntx += 2;
           } else
               cnt[CNT_ZERO] += 2;
       }

       /* Process above left */
       if (aboveleft->mbmi.ref_frame != INTRA_FRAME) {
           if (aboveleft->mbmi.mv.as_int) {
               int_mv this_mv;

               this_mv.as_int = aboveleft->mbmi.mv.as_int;
               mv_bias(aboveleft, refframe, &this_mv,
                 ref_frame_sign_bias);

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               if (this_mv.as_int != mv->as_int) {
                   (++mv)->as_int = this_mv.as_int;
                   ++cntx;
               }
               *cntx += 1;
           } else
               cnt[CNT_ZERO] += 1;
       }

       /* If we have three distinct MVs ... */
       if (cnt[CNT_SPLITMV]) {
           /* See if above-left MV can be merged with NEAREST */
           if (mv->as_int == near_mvs[CNT_NEAREST].as_int)
               cnt[CNT_NEAREST] += 1;
       }

       cnt[CNT_SPLITMV] = ((above->mbmi.mode == SPLITMV)
                            + (left->mbmi.mode == SPLITMV)) * 2
                           + (aboveleft->mbmi.mode == SPLITMV);

       /* Swap near and nearest if necessary */
       if (cnt[CNT_NEAR] > cnt[CNT_NEAREST]) {
           int tmp;
           tmp = cnt[CNT_NEAREST];
           cnt[CNT_NEAREST] = cnt[CNT_NEAR];
           cnt[CNT_NEAR] = tmp;
           tmp = near_mvs[CNT_NEAREST].as_int;
           near_mvs[CNT_NEAREST].as_int = near_mvs[CNT_NEAR].as_int;
           near_mvs[CNT_NEAR].as_int = tmp;
       }

       /* Use near_mvs[0] to store the "best" MV */
       if (cnt[CNT_NEAREST] >= cnt[CNT_ZERO])
           near_mvs[CNT_ZERO] = near_mvs[CNT_NEAREST];

       /* Set up return values */
       *best_mv = near_mvs[0].as_mv;
       *nearest = near_mvs[CNT_NEAREST].as_mv;
       *near = near_mvs[CNT_NEAR].as_mv;

       vp8_clamp_mv(nearest, xd);
       vp8_clamp_mv(near, xd);
       vp8_clamp_mv(best_mv, xd); //TODO: Move this up before
                                    the copy
   }

   ---- End code block ----------------------------------------

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   The mv_ref probability table (mv_ref_p) is then derived from the
   census as follows.

   ---- Begin code block --------------------------------------

   const int vp8_mode_contexts[6][4] =
   {
     {   7,     1,     1,   143,   },
     {  14,    18,    14,   107,   },
     { 135,    64,    57,    68,   },
     {  60,    56,   128,    65,   },
     { 159,   134,   128,    34,   },
     { 234,   188,   128,    28,   },
   }

   ---- End code block ----------------------------------------

   ---- Begin code block --------------------------------------

   vp8_prob *vp8_mv_ref_probs(vp8_prob mv_ref_p[VP8_MVREFS-1],
     int cnt[4])
   {
       mv_ref_p[0] = vp8_mode_contexts [cnt[0]] [0];
       mv_ref_p[1] = vp8_mode_contexts [cnt[1]] [1];
       mv_ref_p[2] = vp8_mode_contexts [cnt[2]] [2];
       mv_ref_p[3] = vp8_mode_contexts [cnt[3]] [3];
       return p;
   }

   ---- End code block ----------------------------------------

   Once mv_ref_p is established, the mv_ref is decoded as usual.

   ---- Begin code block --------------------------------------

     mvr = (mv_ref) treed_read(d, mv_ref_tree, mv_ref_p);

   ---- End code block ----------------------------------------

   For the first four inter-coding modes, the same motion vector is used
   for all the Y subblocks.  The first three modes use an implicit
   motion vector.

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   +------------+------------------------------------------------------+
   | Mode       | Instruction                                          |
   +------------+------------------------------------------------------+
   | mv_nearest | Use the nearest vector returned by                   |
   |            | vp8_find_near_mvs.                                   |
   |            |                                                      |
   | mv_near    | Use the near vector returned by vp8_find_near_mvs.   |
   |            |                                                      |
   | mv_zero    | Use a zero vector; that is, predict the current      |
   |            | macroblock from the corresponding macroblock in the  |
   |            | prediction frame.                                    |
   |            |                                                      |
   | NEWMV      | This mode is followed by an explicitly coded motion  |
   |            | vector (the format of which is described in the next |
   |            | section) that is added (component-wise) to the       |
   |            | best_mv reference vector returned by find_near_mvs   |
   |            | and applied to all 16 subblocks.                     |
   +------------+------------------------------------------------------+

16.4.  Split Prediction

   The remaining mode (SPLITMV) causes multiple vectors to be applied to
   the Y subblocks.  It is immediately followed by a partition
   specification that determines how many vectors will be specified and
   how they will be assigned to the subblocks.  The possible partitions,
   with indicated subdivisions and coding tree, are as follows.

   ---- Begin code block --------------------------------------

   typedef enum
   {
       mv_top_bottom,   /* two pieces {0...7} and {8...15} */
       mv_left_right,   /* {0,1,4,5,8,9,12,13} and
                           {2,3,6,7,10,11,14,15} */
       mv_quarters,    /* {0,1,4,5}, {2,3,6,7}, {8,9,12,13},
                          {10,11,14,15} */
       MV_16,          /* every subblock gets its own vector
                          {0} ... {15} */

       mv_num_partitions
   }
   MVpartition;

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   const tree_index mvpartition_tree [2 * (mvnum_partition - 1)] =
   {
    -MV_16, 2,                         /* MV_16 = "0" */
     -mv_quarters, 4,                  /* mv_quarters = "10" */
      -mv_top_bottom, -mv_left_right   /* top_bottom = "110",
                                          left_right = "111" */
   };

   ---- End code block ----------------------------------------

   The partition is decoded using a fixed, constant probability table:

   ---- Begin code block --------------------------------------

   const Prob mvpartition_probs [mvnum_partition - 1] =
     { 110, 111, 150};
   part = (MVpartition) treed_read(d, mvpartition_tree,
     mvpartition_probs);

   ---- End code block ----------------------------------------

   After the partition come two (for mv_top_bottom or mv_left_right),
   four (for mv_quarters), or sixteen (for MV_16) subblock
   inter-prediction modes.  These modes occur in the order indicated by
   the partition layouts (given as comments to the MVpartition enum) and
   are coded as follows.  (As was done for the macroblock-level modes,
   we offset the mode enumeration so that a single variable may
   unambiguously hold either an intra- or inter-subblock mode.)

   Prior to decoding each subblock, a decoding tree context is chosen as
   illustrated in the code snippet below.  The context is based on the
   immediate left and above subblock neighbors, and whether they are
   equal, are zero, or a combination of those.

   ---- Begin code block --------------------------------------

   typedef enum
   {
       LEFT4x4 = num_intra_bmodes,   /* use already-coded MV to
                                        my left */
       ABOVE4x4,             /* use already-coded MV above me */
       ZERO4x4,              /* use zero MV */
       NEW4x4,               /* explicit offset from "best" */

       num_sub_mv_ref
   };
   sub_mv_ref;

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   const tree_index sub_mv_ref_tree [2 * (num_sub_mv_ref - 1)] =
   {
    -LEFT4X4, 2,           /* LEFT = "0" */
     -ABOVE4X4, 4,         /* ABOVE = "10" */
      -ZERO4X4, -NEW4X4    /* ZERO = "110", NEW = "111" */
   };

   /* Choose correct decoding tree context
    * Function parameters are left subblock neighbor MV and above
    * subblock neighbor MV */
   int vp8_mvCont(MV *l, MV*a)
   {
       int lez = (l->row == 0 && l->col == 0);   /* left neighbor
                                                    is zero */
       int aez = (a->row == 0 && a->col == 0);   /* above neighbor
                                                    is zero */
       int lea = (l->row == a->row && l->col == a->col);  /* left
                                neighbor equals above neighbor */

       if (lea && lez)
           return SUBMVREF_LEFT_ABOVE_ZED; /* =4 */

       if (lea)
           return SUBMVREF_LEFT_ABOVE_SAME; /* =3 */

       if (aez)
           return SUBMVREF_ABOVE_ZED; /* =2 */

       if (lez)
           return SUBMVREF_LEFT_ZED; /* =1*/

       return SUBMVREF_NORMAL; /* =0 */
   }

   /* Constant probabilities and decoding procedure. */

   const Prob sub_mv_ref_prob [5][num_sub_mv_ref - 1] = {
       { 147,136,18 },
       { 106,145,1  },
       { 179,121,1  },
       { 223,1  ,34 },
       { 208,1  ,1  }
   };

       sub_ref = (sub_mv_ref) treed_read(d, sub_mv_ref_tree,
         sub_mv_ref_prob[context]);

   ---- End code block ----------------------------------------

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   The first two sub-prediction modes simply copy the already-coded
   motion vectors used by the blocks above and to the left of the
   subblock at the upper left corner of the current subset (i.e.,
   collection of subblocks being predicted).  These prediction blocks
   need not lie in the current macroblock and, if the current subset
   lies at the top or left edges of the frame, need not lie in the
   frame.  In this latter case, their motion vectors are taken to be
   zero, as are subblock motion vectors within an intra-predicted
   macroblock.  Also, to ensure the correctness of prediction within
   this macroblock, all subblocks lying in an already-decoded subset of
   the current macroblock must have their motion vectors set.

   ZERO4x4 uses a zero motion vector and predicts the current subset
   using the corresponding subset from the prediction frame.

   NEW4x4 is exactly like NEWMV except that NEW4x4 is applied only to
   the current subset.  It is followed by a two-dimensional motion
   vector offset (described in the next section) that is added to the
   best vector returned by the earlier call to find_near_mvs to form the
   motion vector in effect for the subset.

   Parsing of both inter-prediction modes and motion vectors (described
   next) can be found in the reference decoder file modemv.c
   (Section 20.11).

17.  Motion Vector Decoding

   As discussed above, motion vectors appear in two places in the VP8
   datastream: applied to whole macroblocks in NEWMV mode and applied to
   subsets of macroblocks in NEW4x4 mode.  The format of the vectors is
   identical in both cases.

   Each vector has two pieces: a vertical component (row) followed by a
   horizontal component (column).  The row and column use separate
   coding probabilities but are otherwise represented identically.

17.1.  Coding of Each Component

   Each component is a signed integer V representing a vertical or
   horizontal luma displacement of V quarter-pixels (and a chroma
   displacement of V eighth-pixels).  The absolute value of V, if
   non-zero, is followed by a boolean sign.  V may take any value
   between -1023 and +1023, inclusive.

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   The absolute value A is coded in one of two different ways according
   to its size.  For 0 <= A <= 7, A is tree-coded, and for 8 <= A <=
   1023, the bits in the binary expansion of A are coded using
   independent boolean probabilities.  The coding of A begins with a
   bool specifying which range is in effect.

   Decoding a motion vector component then requires a 19-position
   probability table, whose offsets, along with the procedure used to
   decode components, are as follows:

   ---- Begin code block --------------------------------------

   typedef enum
   {
       mvpis_short,         /* short (<= 7) vs long (>= 8) */
       MVPsign,             /* sign for non-zero */
       MVPshort,            /* 8 short values = 7-position tree */

       MVPbits = MVPshort + 7,      /* 8 long value bits
                                       w/independent probs */

       MVPcount = MVPbits + 10      /* 19 probabilities in total */
   }
   MVPindices;

   typedef Prob MV_CONTEXT [MVPcount];    /* Decoding spec for
                                             a single component */

   /* Tree used for small absolute values (has expected
      correspondence). */

   const tree_index small_mvtree [2 * (8 - 1)] =
   {
    2, 8,          /* "0" subtree, "1" subtree */
     4, 6,         /* "00" subtree, "01" subtree */
      -0, -1,      /* 0 = "000", 1 = "001" */
      -2, -3,      /* 2 = "010", 3 = "011" */
     10, 12,       /* "10" subtree, "11" subtree */
      -4, -5,      /* 4 = "100", 5 = "101" */
      -6, -7       /* 6 = "110", 7 = "111" */
   };

   /* Read MV component at current decoder position, using
      supplied probs. */

   int read_mvcomponent(bool_decoder *d, const MV_CONTEXT *mvc)
   {
       const Prob * const p = (const Prob *) mvc;

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       int A = 0;

       if (read_bool(d, p [mvpis_short]))    /* 8 <= A <= 1023 */
       {
           /* Read bits 0, 1, 2 */

           int i = 0;
           do { A += read_bool(d, p [MVPbits + i]) << i;}
             while (++i < 3);

           /* Read bits 9, 8, 7, 6, 5, 4 */

           i = 9;
           do { A += read_bool(d, p [MVPbits + i]) << i;}
             while (--i > 3);

           /* We know that A >= 8 because it is coded long,
              so if A <= 15, bit 3 is one and is not
              explicitly coded. */

           if (!(A & 0xfff0)  ||  read_bool(d, p [MVPbits + 3]))
               A += 8;
       }
       else    /* 0 <= A <= 7 */
           A = treed_read(d, small_mvtree, p + MVPshort);

       return A && read_bool(r, p [MVPsign]) ?  -A : A;
   }

   ---- End code block ----------------------------------------

17.2.  Probability Updates

   The decoder should maintain an array of two MV_CONTEXTs for decoding
   row and column components, respectively.  These MV_CONTEXTs should be
   set to their defaults every key frame.  Each individual probability
   may be updated every interframe (by field J of the frame header)
   using a constant table of update probabilities.  Each optional update
   is of the form B?  P(7), that is, a bool followed by a 7-bit
   probability specification if true.

   As with other dynamic probabilities used by VP8, the updates remain
   in effect until the next key frame or until replaced via another
   update.

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   In detail, the probabilities should then be managed as follows.

   ---- Begin code block --------------------------------------

   /* Never-changing table of update probabilities for each
      individual probability used in decoding motion vectors. */

   const MV_CONTEXT vp8_mv_update_probs[2] =
   {
     {
       237,
       246,
       253, 253, 254, 254, 254, 254, 254,
       254, 254, 254, 254, 254, 250, 250, 252, 254, 254
     },
     {
       231,
       243,
       245, 253, 254, 254, 254, 254, 254,
       254, 254, 254, 254, 254, 251, 251, 254, 254, 254
     }
   };

   /* Default MV decoding probabilities. */

   const MV_CONTEXT default_mv_context[2] =
   {
     {                       // row
       162,                    // is short
       128,                    // sign
         225, 146, 172, 147, 214,  39, 156,      // short tree
       128, 129, 132,  75, 145, 178, 206, 239, 254, 254 // long bits
     },

     {                       // same for column
       164,                    // is short
       128,
       204, 170, 119, 235, 140, 230, 228,
       128, 130, 130,  74, 148, 180, 203, 236, 254, 254 // long bits

     }
   };

   /* Current MV decoding probabilities, set to above defaults
      every key frame. */

   MV_CONTEXT mvc [2];     /* always row, then column */

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   /* Procedure for decoding a complete motion vector. */

   typedef struct { int16 row, col;}  MV;  /* as in previous section */

   MV read_mv(bool_decoder *d)
   {
       MV v;
       v.row = (int16) read_mvcomponent(d, mvc);
       v.col = (int16) read_mvcomponent(d, mvc + 1);
       return v;
   }

   /* Procedure for updating MV decoding probabilities, called
      every interframe with "d" at the appropriate position in
      the frame header. */

   void update_mvcontexts(bool_decoder *d)
   {
       int i = 0;
       do {                      /* component = row, then column */
           const Prob *up = mv_update_probs[i];    /* update probs
                                                      for component */
           Prob *p = mvc[i];                  /* start decode tbl "" */
           Prob * const pstop = p + MVPcount; /* end decode tbl "" */
           do {
               if (read_bool(d, *up++))     /* update this position */
               {
                   const Prob x = read_literal(d, 7);

                   *p = x? x<<1 : 1;
               }
           } while (++p < pstop);              /* next position */
       } while (++i < 2);                      /* next component */
   }

   ---- End code block ----------------------------------------

   This completes the description of the motion-vector decoding
   procedure and, with it, the procedure for decoding interframe
   macroblock prediction records.

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18.  Interframe Prediction

   Given an inter-prediction specification for the current macroblock,
   that is, a reference frame together with a motion vector for each of
   the sixteen Y subblocks, we describe the calculation of the
   prediction buffer for the macroblock.  Frame reconstruction is then
   completed via the previously described processes of residue summation
   (Section 14) and loop filtering (Section 15).

   The management of inter-predicted subblocks and sub-pixel
   interpolation may be found in the reference decoder file predict.c
   (Section 20.14).

18.1.  Bounds on, and Adjustment of, Motion Vectors

   Since each motion vector is differentially encoded from a neighboring
   block or macroblock and the only clamp is to ensure that the
   referenced motion vector represents a valid location inside a
   reference frame buffer, it is technically possible within the VP8
   format for a block or macroblock to have arbitrarily large motion
   vectors, up to the size of the input image plus the extended border
   areas.  For practical reasons, VP8 imposes a motion vector size range
   limit of -4096 to 4095 full pixels, regardless of image size (VP8
   defines 14 raw bits for width and height; 16383x16383 is the maximum
   possible image size).  Bitstream-compliant encoders and decoders
   shall enforce this limit.

   Because the motion vectors applied to the chroma subblocks have
   1/8-pixel resolution, the synthetic pixel calculation, outlined in
   Section 5 and detailed below, uses this resolution for the luma
   subblocks as well.  In accordance, the stored luma motion vectors are
   all doubled, each component of each luma vector becoming an even
   integer in the range -2046 to +2046, inclusive.

   The vector applied to each chroma subblock is calculated by averaging
   the vectors for the 4 luma subblocks occupying the same visible area
   as the chroma subblock in the usual correspondence; that is, the
   vector for U and V block 0 is the average of the vectors for the Y
   subblocks { 0, 1, 4, 5}, chroma block 1 corresponds to Y blocks { 2,
   3, 6, 7}, chroma block 2 to Y blocks { 8, 9, 12, 13}, and chroma
   block 3 to Y blocks { 10, 11, 14, 15}.

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   In detail, each of the two components of the vectors for each of the
   chroma subblocks is calculated from the corresponding luma vector
   components as follows:

   ---- Begin code block --------------------------------------

   int avg(int c1, int c2, int c3, int c4)
   {
       int s = c1 + c2 + c3 + c4;

       /* The shift divides by 8 (not 4) because chroma pixels
          have twice the diameter of luma pixels.  The handling
          of negative motion vector components is slightly
          cumbersome because, strictly speaking, right shifts
          of negative numbers are not well-defined in C. */

       return s >= 0 ?  (s + 4) >> 3 : -((-s + 4) >> 3);
   }

   ---- End code block ----------------------------------------

   Furthermore, if the version number in the frame tag specifies only
   full-pel chroma motion vectors, then the fractional parts of both
   components of the vector are truncated to zero, as illustrated in the
   following pseudocode (assuming 3 bits of fraction for both luma and
   chroma vectors):

   ---- Begin code block --------------------------------------

       x = x & (~7);
       y = y & (~7);

   ---- End code block ----------------------------------------

   Earlier in this document we described the vp8_clamp_mv() function to
   limit "nearest" and "near" motion vector predictors inside specified
   margins within the frame boundaries.  Additional clamping is
   performed for NEWMV macroblocks, for which the final motion vector is
   clamped again after combining the "best" predictor and the
   differential vector decoded from the stream.

   However, the secondary clamping is not performed for SPLITMV
   macroblocks, meaning that any subblock's motion vector within the
   SPLITMV macroblock may point outside the clamping zone.  These
   non-clamped vectors are also used when determining the decoding tree
   context for subsequent subblocks' modes in the vp8_mvCont() function.

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18.2.  Prediction Subblocks

   The prediction calculation for each subblock is then as follows.
   Temporarily disregarding the fractional part of the motion vector
   (that is, rounding "up" or "left" by right-shifting each component
   3 bits with sign propagation) and adding the origin (upper left
   position) of the (16x16 luma or 8x8 chroma) current macroblock gives
   us an origin in the Y, U, or V plane of the predictor frame (either
   the golden frame or previous frame).

   Considering that origin to be the upper left corner of a (luma or
   chroma) macroblock, we need to specify the relative positions of the
   pixels associated to that subblock, that is, any pixels that might be
   involved in the sub-pixel interpolation processes for the subblock.

18.3.  Sub-Pixel Interpolation

   The sub-pixel interpolation is effected via two one-dimensional
   convolutions.  These convolutions may be thought of as operating on a
   two-dimensional array of pixels whose origin is the subblock origin,
   that is the origin of the prediction macroblock described above plus
   the offset to the subblock.  Because motion vectors are arbitrary, so
   are these "prediction subblock origins".

   The integer part of the motion vector is subsumed in the origin of
   the prediction subblock; the 16 (synthetic) pixels we need to
   construct are given by 16 offsets from the origin.  The integer part
   of each of these offsets is the offset of the corresponding pixel
   from the subblock origin (using the vertical stride).  To these
   integer parts is added a constant fractional part, which is simply
   the difference between the actual motion vector and its integer
   truncation used to calculate the origins of the prediction macroblock
   and subblock.  Each component of this fractional part is an integer
   between 0 and 7, representing a forward displacement in eighths of a
   pixel.

   It is these fractional displacements that determine the filtering
   process.  If they both happen to be zero (that is, we had a "whole
   pixel" motion vector), the prediction subblock is simply copied into
   the corresponding piece of the current macroblock's prediction
   buffer.  As discussed in Section 14, the layout of the macroblock's
   prediction buffer can depend on the specifics of the reconstruction
   implementation chosen.  Of course, the vertical displacement between
   lines of the prediction subblock is given by the stride, as are all
   vertical displacements used here.

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   Otherwise, at least one of the fractional displacements is non-zero.
   We then synthesize the missing pixels via a horizontal, followed by a
   vertical, one-dimensional interpolation.

   The two interpolations are essentially identical.  Each uses a (at
   most) six-tap filter (the choice of which of course depends on the
   one-dimensional offset).  Thus, every calculated pixel references at
   most three pixels before (above or to the left of) it and at most
   three pixels after (below or to the right of) it.  The horizontal
   interpolation must calculate two extra rows above and three extra
   rows below the 4x4 block, to provide enough samples for the vertical
   interpolation to proceed.

   Depending on the reconstruction filter type given in the version
   number field in the frame tag, either a bicubic or a bilinear tap set
   is used.

   The exact implementation of subsampling is as follows.

   ---- Begin code block --------------------------------------

   /* Filter taps taken to 7-bit precision.
      Because DC is always passed, taps always sum to 128. */

   const int BilinearFilters[8][6] =
   {
       { 0, 0, 128,   0, 0, 0 },
       { 0, 0, 112,  16, 0, 0 },
       { 0, 0,  96,  32, 0, 0 },
       { 0, 0,  80,  48, 0, 0 },
       { 0, 0,  64,  64, 0, 0 },
       { 0, 0,  48,  80, 0, 0 },
       { 0, 0,  32,  96, 0, 0 },
       { 0, 0,  16, 112, 0, 0 }
   };

   const int filters [8] [6] = {        /* indexed by displacement */
       { 0,  0,  128,    0,   0,  0 },  /* degenerate whole-pixel */
       { 0, -6,  123,   12,  -1,  0 },  /* 1/8 */
       { 2, -11, 108,   36,  -8,  1 },  /* 1/4 */
       { 0, -9,   93,   50,  -6,  0 },  /* 3/8 */
       { 3, -16,  77,   77, -16,  3 },  /* 1/2 is symmetric */
       { 0, -6,   50,   93,  -9,  0 },  /* 5/8 = reverse of 3/8 */
       { 1, -8,   36,  108, -11,  2 },  /* 3/4 = reverse of 1/4 */
       { 0, -1,   12,  123,  -6,  0 }   /* 7/8 = reverse of 1/8 */
   };

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   /* One-dimensional synthesis of a single sample.
      Filter is determined by fractional displacement */

   Pixel interp(
       const int fil[6],   /* filter to apply */
       const Pixel *p,     /* origin (rounded "before") in
                              prediction area */
       const int s         /* size of one forward step "" */
   ) {
       int32 a = 0;
       int i = 0;
       p -= s + s;         /* move back two positions */

       do {
           a += *p * fil[i];
           p += s;
       }  while (++i < 6);

       return clamp255((a + 64) >> 7);    /* round to nearest
                                              8-bit value */
   }

   /* First do horizontal interpolation, producing intermediate
      buffer. */

   void Hinterp(
       Pixel temp[9][4],   /* 9 rows of 4 (intermediate)
                              destination values */
       const Pixel *p,     /* subblock origin in prediction
                              frame */
       int s,              /* vertical stride to be used in
                              prediction frame */
       uint hfrac,         /* 0 <= horizontal displacement <= 7 */
       uint bicubic        /* 1=bicubic filter, 0=bilinear */
   ) {
       const int * const fil = bicubic ? filters [hfrac] :
         BilinearFilters[hfrac];

       int r = 0;  do              /* for each row */
       {
           int c = 0;  do          /* for each destination sample */
           {
               /* Pixel separation = one horizontal step = 1 */

               temp[r][c] = interp(fil, p + c, 1);
           }

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           while (++c < 4);
       }
       while (p += s, ++r < 9);    /* advance p to next row */
   }

   /* Finish with vertical interpolation, producing final results.
      Input array "temp" is of course that computed above. */

   void Vinterp(
       Pixel final[4][4],  /* 4 rows of 4 (final) destination values */
       const Pixel temp[9][4],
       uint vfrac,         /* 0 <= vertical displacement <= 7 */
       uint bicubic        /* 1=bicubic filter, 0=bilinear */
   ) {
       const int * const fil = bicubic ? filters [vfrac] :
         BilinearFilters[vfrac];

       int r = 0;  do              /* for each row */
       {
           int c = 0;  do          /* for each destination sample */
           {
               /* Pixel separation = one vertical step = width
                  of array = 4 */

               final[r][c] = interp(fil, temp[r] + c, 4);
           }
           while (++c < 4);
       }
       while (++r < 4);
   }

   ---- End code block ----------------------------------------

18.4.  Filter Properties

   We discuss briefly the rationale behind the choice of filters.  Our
   approach is necessarily cursory; a genuinely accurate discussion
   would require a couple of books.  Readers unfamiliar with signal
   processing may or may not wish to skip this.

   All digital signals are of course sampled in some fashion.  The case
   where the inter-sample spacing (say in time for audio samples, or
   space for pixels) is uniform, that is, the same at all positions, is
   particularly common and amenable to analysis.  Many aspects of the
   treatment of such signals are best-understood in the frequency domain
   via Fourier Analysis, particularly those aspects of the signal that
   are not changed by shifts in position, especially when those
   positional shifts are not given by a whole number of samples.

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   Non-integral translates of a sampled signal are a textbook example of
   the foregoing.  In our case of non-integral motion vectors, we wish
   to say what the underlying image "really is" at these pixels;
   although we don't have values for them, we feel that it makes sense
   to talk about them.  The correctness of this feeling is predicated on
   the underlying signal being band-limited, that is, not containing any
   energy in spatial frequencies that cannot be faithfully rendered at
   the pixel resolution at our disposal.  In one dimension, this range
   of "OK" frequencies is called the Nyquist band; in our two-
   dimensional case of integer-grid samples, this range might be termed
   a Nyquist rectangle.  The finer the grid, the more we know about the
   image, and the wider the Nyquist rectangle.

   It turns out that, for such band-limited signals, there is indeed an
   exact mathematical formula to produce the correct sample value at an
   arbitrary point.  Unfortunately, this calculation requires the
   consideration of every single sample in the image, as well as needing
   to operate at infinite precision.  Also, strictly speaking, all band-
   limited signals have infinite spatial (or temporal) extent, so
   everything we are discussing is really some sort of approximation.

   It is true that the theoretically correct subsampling procedure, as
   well as any approximation thereof, is always given by a translation-
   invariant weighted sum (or filter) similar to that used by VP8.  It
   is also true that the reconstruction error made by such a filter can
   be simply represented as a multiplier in the frequency domain; that
   is, such filters simply multiply the Fourier transform of any signal
   to which they are applied by a fixed function associated to the
   filter.  This fixed function is usually called the frequency response
   (or transfer function); the ideal subsampling filter has a frequency
   response equal to one in the Nyquist rectangle and zero everywhere
   else.

   Another basic fact about approximations to "truly correct"
   subsampling is that the wider the subrectangle (within the Nyquist
   rectangle) of spatial frequencies one wishes to "pass" (that is,
   correctly render) or, put more accurately, the closer one wishes to
   approximate the ideal transfer function, the more samples of the
   original signal must be considered by the subsampling, and the wider
   the calculation precision necessitated.

   The filters chosen by VP8 were chosen, within the constraints of 4 or
   6 taps and 7-bit precision, to do the best possible job of handling
   the low spatial frequencies near the 0th DC frequency along with
   introducing no resonances (places where the absolute value of the
   frequency response exceeds one).

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   The justification for the foregoing has two parts.  First, resonances
   can produce extremely objectionable visible artifacts when, as often
   happens in actual compressed video streams, filters are applied
   repeatedly.  Second, the vast majority of energy in real-world images
   lies near DC and not at the high end.

   To get slightly more specific, the filters chosen by VP8 are the best
   resonance-free 4- or 6-tap filters possible, where "best" describes
   the frequency response near the origin: The response at 0 is required
   to be 1, and the graph of the response at 0 is as flat as possible.

   To provide an intuitively more obvious point of reference, the "best"
   2-tap filter is given by simple linear interpolation between the
   surrounding actual pixels.

   Finally, it should be noted that, because of the way motion vectors
   are calculated, the (shorter) 4-tap filters (used for odd fractional
   displacements) are applied in the chroma plane only.  Human color
   perception is notoriously poor, especially where higher spatial
   frequencies are involved.  The shorter filters are easier to
   understand mathematically, and the difference between them and a
   theoretically slightly better 6-tap filter is negligible where chroma
   is concerned.

19.  Annex A: Bitstream Syntax

   This annex presents the bitstream syntax in a tabular form.  All the
   information elements have been introduced and explained in the
   previous sections but are collected here for a quick reference.  Each
   syntax element is briefly described after the tabular representation
   along with a reference to the corresponding paragraph in the main
   document.  The meaning of each syntax element value is not repeated
   here.

   The top-level hierarchy of the bitstream is introduced in Section 4.

   Definition of syntax element coding types can be found in Section 8.
   The types used in the representation in this annex are:

   o  f(n), n-bit value from stream (n successive bits, not boolean
      encoded)

   o  L(n), n-bit number encoded as n booleans (with equal probability
      of being 0 or 1)

   o  B(p), bool with probability p of being 0

   o  T, tree-encoded value

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19.1.  Uncompressed Data Chunk

   | Frame Tag                                         | Type  |
   | ------------------------------------------------- | ----- |
   | frame_tag                                         | f(24) |
   | if (key_frame) {                                  |       |
   |     start_code                                    | f(24) |
   |     horizontal_size_code                          | f(16) |
   |     vertical_size_code                            | f(16) |
   | }                                                 |       |

   The 3-byte frame tag can be parsed as follows:

   ---- Begin code block --------------------------------------

   unsigned char *c = pbi->source;
   unsigned int tmp;

   tmp = (c[2] << 16) | (c[1] << 8) | c[0];

   key_frame = tmp & 0x1;
   version = (tmp >> 1) & 0x7;
   show_frame = (tmp >> 4) & 0x1;
   first_part_size = (tmp >> 5) & 0x7FFFF;

   ---- End code block ----------------------------------------

   Where:

   o  key_frame indicates whether the current frame is a key frame
      or not.

   o  version determines the bitstream version.

   o  show_frame indicates whether the current frame is meant to be
      displayed or not.

   o  first_part_size determines the size of the first partition
      (control partition), excluding the uncompressed data chunk.

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   The start_code is a constant 3-byte pattern having value 0x9d012a.
   The latter part of the uncompressed chunk (after the start_code) can
   be parsed as follows:

   ---- Begin code block --------------------------------------

   unsigned char *c = pbi->source + 6;
   unsigned int tmp;

   tmp = (c[1] << 8) | c[0];

   width = tmp & 0x3FFF;
   horizontal_scale = tmp >> 14;

   tmp = (c[3] << 8) | c[2];

   height = tmp & 0x3FFF;
   vertical_scale = tmp >> 14;

   ---- End code block ----------------------------------------

19.2.  Frame Header

   | Frame Header                                      | Type  |
   | ------------------------------------------------- | ----- |
   | if (key_frame) {                                  |       |
   |   color_space                                     | L(1)  |
   |   clamping_type                                   | L(1)  |
   | }                                                 |       |
   | segmentation_enabled                              | L(1)  |
   | if (segmentation_enabled)                         |       |
   |   update_segmentation()                           |       |
   | filter_type                                       | L(1)  |
   | loop_filter_level                                 | L(6)  |
   | sharpness_level                                   | L(3)  |
   | mb_lf_adjustments()                               |       |
   | log2_nbr_of_dct_partitions                        | L(2)  |
   | quant_indices()                                   |       |
   | if (key_frame)                                    |       |
   |   refresh_entropy_probs                           | L(1)  |

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   | else {                                            |       |
   |   refresh_golden_frame                            | L(1)  |
   |   refresh_alternate_frame                         | L(1)  |
   |   if (!refresh_golden_frame)                      |       |
   |     copy_buffer_to_golden                         | L(2)  |
   |   if (!refresh_alternate_frame)                   |       |
   |     copy_buffer_to_alternate                      | L(2)  |
   |   sign_bias_golden                                | L(1)  |
   |   sign_bias_alternate                             | L(1)  |
   |   refresh_entropy_probs                           | L(1)  |
   |   refresh_last                                    | L(1)  |
   | }                                                 |       |
   | token_prob_update()                               |       |
   | mb_no_skip_coeff                                  | L(1)  |
   | if (mb_no_skip_coeff)                             |       |
   |   prob_skip_false                                 | L(8)  |
   | if (!key_frame) {                                 |       |
   |   prob_intra                                      | L(8)  |
   |   prob_last                                       | L(8)  |
   |   prob_gf                                         | L(8)  |
   |   intra_16x16_prob_update_flag                    | L(1)  |
   |   if (intra_16x16_prob_update_flag) {             |       |
   |     for (i = 0; i < 4; i++)                       |       |
   |       intra_16x16_prob                            | L(8)  |
   |   }                                               |       |
   |   intra_chroma prob_update_flag                   | L(1)  |
   |   if (intra_chroma_prob_update_flag) {            |       |
   |     for (i = 0; i < 3; i++)                       |       |
   |       intra_chroma_prob                           | L(8)  |
   |   }                                               |       |
   |   mv_prob_update()                                |       |
   | }                                                 |       |

   o  color_space defines the YUV color space of the sequence
      (Section 9.2)

   o  clamping_type specifies if the decoder is required to clamp the
      reconstructed pixel values (Section 9.2)

   o  segmentation_enabled enables the segmentation feature for the
      current frame (Section 9.3)

   o  filter_type determines whether the normal or the simple loop
      filter is used (Sections 9.4, 15)

   o  loop_filter_level controls the deblocking filter
      (Sections 9.4, 15)

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   o  sharpness_level controls the deblocking filter (Sections 9.4, 15)

   o  log2_nbr_of_dct_partitions determines the number of separate
      partitions containing the DCT coefficients of the macroblocks
      (Section 9.5)

   o  refresh_entropy_probs determines whether updated token
      probabilities are used only for this frame or until further update

   o  refresh_golden_frame determines if the current decoded frame
      refreshes the golden frame (Section 9.7)

   o  refresh_alternate_frame determines if the current decoded frame
      refreshes the alternate reference frame (Section 9.7)

   o  copy_buffer_to_golden determines if the golden reference is
      replaced by another reference (Section 9.7)

   o  copy_buffer_to_alternate determines if the alternate reference is
      replaced by another reference (Section 9.7)

   o  sign_bias_golden controls the sign of motion vectors when the
      golden frame is referenced (Section 9.7)

   o  sign_bias_alternate controls the sign of motion vectors when the
      alternate frame is referenced (Section 9.7)

   o  refresh_last determines if the current decoded frame refreshes the
      last frame reference buffer (Section 9.8)

   o  mb_no_skip_coeff enables or disables the skipping of macroblocks
      containing no non-zero coefficients (Section 9.10)

   o  prob_skip_false indicates the probability that the macroblock is
      not skipped (flag indicating skipped macroblock is false)
      (Section 9.10)

   o  prob_intra indicates the probability of an intra macroblock
      (Section 9.10)

   o  prob_last indicates the probability that the last reference frame
      is used for inter-prediction (Section 9.10)

   o  prob_gf indicates the probability that the golden reference frame
      is used for inter-prediction (Section 9.10)

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   o  intra_16x16_prob_update_flag indicates if the branch probabilities
      used in the decoding of the luma intra-prediction mode are updated
      (Section 9.10)

   o  intra_16x16_prob indicates the branch probabilities of the luma
      intra-prediction mode decoding tree

   o  intra_chroma_prob_update_flag indicates if the branch
      probabilities used in the decoding of the chroma intra-prediction
      mode are updated (Section 9.10)

   o  intra_chroma_prob indicates the branch probabilities of the chroma
      intra-prediction mode decoding tree

   | update_segmentation()                             | Type  |
   | ------------------------------------------------- | ----- |
   | update_mb_segmentation_map                        | L(1)  |
   | update_segment_feature_data                       | L(1)  |
   | if (update_segment_feature_data) {                |       |
   |   segment_feature_mode                            | L(1)  |
   |   for (i = 0; i < 4; i++) {                       |       |
   |     quantizer_update                              | L(1)  |
   |     if (quantizer_update) {                       |       |
   |       quantizer_update_value                      | L(7)  |
   |       quantizer_update_sign                       | L(1)  |
   |     }                                             |       |
   |   }                                               |       |
   |   for (i = 0; i < 4; i++) {                       |       |
   |     loop_filter_update                            | L(1)  |
   |     if (loop_filter_update) {                     |       |
   |       lf_update_value                             | L(6)  |
   |       lf_update_sign                              | L(1)  |
   |     }                                             |       |
   |   }                                               |       |
   | }                                                 |       |
   | if (update_mb_segmentation_map) {                 |       |
   |   for (i = 0; i < 3; i++) {                       |       |
   |     segment_prob_update                           | L(1)  |
   |     if (segment_prob_update)                      |       |
   |       segment_prob                                | L(8)  |
   |   }                                               |       |
   | }                                                 |       |

   o  update_mb_segmentation_map determines if the MB segmentation map
      is updated in the current frame (Section 9.3)

   o  update_segment_feature_data indicates if the segment feature data
      is updated in the current frame (Section 9.3)

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   o  segment_feature_mode indicates the feature data update mode, 0 for
      delta and 1 for the absolute value (Section 9.3)

   o  quantizer_update indicates if the quantizer value is updated for
      the i^(th) segment (Section 9.3)

   o  quantizer_update_value indicates the update value for the segment
      quantizer (Section 9.3)

   o  quantizer_update_sign indicates the update sign for the segment
      quantizer (Section 9.3)

   o  loop_filter_update indicates if the loop filter level value is
      updated for the i^(th) segment (Section 9.3)

   o  lf_update_value indicates the update value for the loop filter
      level (Section 9.3)

   o  lf_update_sign indicates the update sign for the loop filter level
      (Section 9.3)

   o  segment_prob_update indicates whether the branch probabilities
      used to decode the segment_id in the MB header are decoded from
      the stream or use the default value of 255 (Section 9.3)

   o  segment_prob indicates the branch probabilities of the segment_id
      decoding tree (Section 9.3)

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   | mb_lf_adjustments()                               | Type  |
   | ------------------------------------------------- | ----- |
   | loop_filter_adj_enable                            | L(1)  |
   | if (loop_filter_adj_enable) {                     |       |
   |   mode_ref_lf_delta_update                        | L(1)  |
   |   if (mode_ref_lf_delta_update) {                 |       |
   |     for (i = 0; i < 4; i++) {                     |       |
   |       ref_frame_delta_update_flag                 | L(1)  |
   |       if (ref_frame_delta_update_flag) {          |       |
   |         delta_magnitude                           | L(6)  |
   |         delta_sign                                | L(1)  |
   |       }                                           |       |
   |     }                                             |       |
   |     for (i = 0; i < 4; i++) {                     |       |
   |       mb_mode_delta_update_flag                   | L(1)  |
   |       if (mb_mode_delta_update_flag) {            |       |
   |         delta_magnitude                           | L(6)  |
   |         delta_sign                                | L(1)  |
   |       }                                           |       |
   |     }                                             |       |
   |   }                                               |       |
   | }                                                 |       |

   o  loop_filter_adj_enable indicates if the MB-level loop filter
      adjustment (based on the used reference frame and coding mode) is
      on for the current frame (Section 9.4)

   o  mode_ref_lf_delta_update indicates if the delta values used in an
      adjustment are updated in the current frame (Section 9.4)

   o  ref_frame_delta_update_flag indicates if the adjustment delta
      value corresponding to a certain used reference frame is updated
      (Section 9.4)

   o  delta_magnitude is the absolute value of the delta value

   o  delta_sign is the sign of the delta value

   o  mb_mode_delta_update_flag indicates if the adjustment delta value
      corresponding to a certain MB prediction mode is updated
      (Section 9.4)

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   | quant_indices()                                   | Type  |
   | ------------------------------------------------- | ----- |
   | y_ac_qi                                           | L(7)  |
   | y_dc_delta_present                                | L(1)  |
   | if (y_dc_delta_present) {                         |       |
   |   y_dc_delta_magnitude                            | L(4)  |
   |   y_dc_delta_sign                                 | L(1)  |
   | }                                                 |       |
   | y2_dc_delta_present                               | L(1)  |
   | if (y2_dc_delta_present) {                        |       |
   |   y2_dc_delta_magnitude                           | L(4)  |
   |   y2_dc_delta_sign                                | L(1)  |
   | }                                                 |       |
   | y2_ac_delta_present                               | L(1)  |
   | if (y2_ac_delta_present) {                        |       |
   |   y2_ac_delta_magnitude                           | L(4)  |
   |   y2_ac_delta_sign                                | L(1)  |
   | }                                                 |       |
   | uv_dc_delta_present                               | L(1)  |
   | if (uv_dc_delta_present) {                        |       |
   |   uv_dc_delta_magnitude                           | L(4)  |
   |   uv_dc_delta_sign                                | L(1)  |
   | }                                                 |       |
   | uv_ac_delta_present                               | L(1)  |
   | if (uv_ac_delta_present) {                        |       |
   |   uv_ac_delta_magnitude                           | L(4)  |
   |   uv_ac_delta_sign                                | L(1)  |
   | }                                                 |       |

   o  y_ac_qi is the dequantization table index used for the luma AC
      coefficients (and other coefficient groups if no delta value is
      present) (Section 9.6)

   o  y_dc_delta_present indicates if the stream contains a delta value
      that is added to the baseline index to obtain the luma DC
      coefficient dequantization index (Section 9.6)

   o  y_dc_delta_magnitude is the magnitude of the delta value
      (Section 9.6)

   o  y_dc_delta_sign is the sign of the delta value (Section 9.6)

   o  y2_dc_delta_present indicates if the stream contains a delta value
      that is added to the baseline index to obtain the Y2 block DC
      coefficient dequantization index (Section 9.6)

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   o  y2_ac_delta_present indicates if the stream contains a delta value
      that is added to the baseline index to obtain the Y2 block AC
      coefficient dequantization index (Section 9.6)

   o  uv_dc_delta_present indicates if the stream contains a delta value
      that is added to the baseline index to obtain the chroma DC
      coefficient dequantization index (Section 9.6)

   o  uv_ac_delta_present indicates if the stream contains a delta value
      that is added to the baseline index to obtain the chroma AC
      coefficient dequantization index (Section 9.6)

   | token_prob_update()                               | Type  |
   | ------------------------------------------------- | ----- |
   | for (i = 0; i < 4; i++) {                         |       |
   |   for (j = 0; j < 8; j++) {                       |       |
   |     for (k = 0; k < 3; k++) {                     |       |
   |       for (l = 0; l < 11; l++) {                  |       |
   |         coeff_prob_update_flag                    | L(1)  |
   |         if (coeff_prob_update_flag)               |       |
   |           coeff_prob                              | L(8)  |
   |       }                                           |       |
   |     }                                             |       |
   |   }                                               |       |
   | }                                                 |       |

   o  coeff_prob_update_flag indicates if the corresponding branch
      probability is updated in the current frame (Section 13.4)

   o  coeff_prob is the new branch probability (Section 13.4)

   | mv_prob_update()                                  | Type  |
   | ------------------------------------------------- | ----- |
   | for (i = 0; i < 2; i++) {                         |       |
   |   for (j = 0; j < 19; j++) {                      |       |
   |     mv_prob_update_flag                           | L(1)  |
   |     if (mv_prob_update_flag)                      |       |
   |       prob                                        | L(7)  |
   |   }                                               |       |
   | }                                                 |       |

   o  mv_prob_update_flag indicates if the corresponding MV decoding
      probability is updated in the current frame (Section 17.2)

   o  prob is the updated probability (Section 17.2)

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19.3.  Macroblock Data

   | Macroblock Data                                   | Type  |
   | ------------------------------------------------- | ----- |
   | macroblock_header()                               |       |
   | residual_data()                                   |       |

   | macroblock_header()                               | Type  |
   | ------------------------------------------------- | ----- |
   | if (update_mb_segmentation_map)                   |       |
   |   segment_id                                      | T     |
   | if (mb_no_skip_coeff)                             |       |
   |   mb_skip_coeff                                   | B(p)  |
   | if (!key_frame)                                   |       |
   |   is_inter_mb                                     | B(p)  |
   | if (is_inter_mb) {                                |       |
   |   mb_ref_frame_sel1                               | B(p)  |
   |   if (mb_ref_frame_sel1)                          |       |
   |     mb_ref_frame_sel2                             | B(p)  |
   |   mv_mode                                         | T     |
   |   if (mv_mode == SPLITMV) {                       |       |
   |     mv_split_mode                                 | T     |
   |     for (i = 0; i < numMvs; i++) {                |       |
   |       sub_mv_mode                                 | T     |
   |       if (sub_mv_mode == NEWMV4x4) {              |       |
   |         read_mvcomponent()                        |       |
   |         read_mvcomponent()                        |       |
   |       }                                           |       |
   |     }                                             |       |
   |   } else if (mv_mode == NEWMV) {                  |       |
   |     read_mvcomponent()                            |       |
   |     read_mvcomponent()                            |       |
   |   }                                               |       |
   | } else { /* intra mb */                           |       |
   |   intra_y_mode                                    | T     |
   |   if (intra_y_mode == B_PRED) {                   |       |
   |     for (i = 0; i < 16; i++)                      |       |
   |       intra_b_mode                                | T     |
   |   }                                               |       |
   |   intra_uv_mode                                   | T     |
   | }                                                 |       |

   o  segment_id indicates to which segment the macroblock belongs
      (Section 10)

   o  mb_skip_coeff indicates whether the macroblock contains any coded
      coefficients or not (Section 11.1)

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   o  is_inter_mb indicates whether the macroblock is intra- or inter-
      coded (Section 16)

   o  mb_ref_frame_sel1 selects the reference frame to be used; last
      frame (0), golden/alternate (1) (Section 16.2)

   o  mb_ref_frame_sel2 selects whether the golden (0) or alternate
      reference frame (1) is used (Section 16.2)

   o  mv_mode determines the macroblock motion vector mode
      (Section 16.2)

   o  mv_split_mode gives the macroblock partitioning specification and
      determines the number of motion vectors used (numMvs)
      (Section 16.2)

   o  sub_mv_mode determines the sub-macroblock motion vector mode for
      macroblocks coded using the SPLITMV motion vector mode
      (Section 16.2)

   o  intra_y_mode selects the luminance intra-prediction mode
      (Section 16.1)

   o  intra_b_mode selects the sub-macroblock luminance prediction mode
      for macroblocks coded using B_PRED mode (Section 16.1)

   o  intra_uv_mode selects the chrominance intra-prediction mode
      (Section 16.1)

   | residual_data()                                   | Type  |
   | ------------------------------------------------- | ----- |
   | if (!mb_skip_coeff) {                             |       |
   |   if ( (is_inter_mb && mv_mode != SPLITMV) ||     |       |
   |        (!is_inter_mb && intra_y_mode != B_PRED) ) |       |
   |     residual_block() /* Y2 */                     |       |
   |   for (i = 0; i < 24; i++)                        |       |
   |     residual_block() /* 16 Y, 4 U, 4 V */         |       |
   | }                                                 |       |

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   | residual_block()                                  | Type  |
   | ------------------------------------------------- | ----- |
   | for (i = firstCoeff; i < 16; i++) {               |       |
   |   token                                           | T     |
   |   if (token == EOB) break;                        |       |
   |   if (token_has_extra_bits)                       |       |
   |     extra_bits                                    | L(n)  |
   |   if (coefficient != 0)                           |       |
   |     sign                                          | L(1)  |
   | }                                                 |       |

   o  firstCoeff is 1 for luma blocks of macroblocks containing Y2
      subblock; otherwise 0

   o  token defines the value of the coefficient, the value range of the
      coefficient, or the end of block (Section 13.2)

   o  extra_bits determines the value of the coefficient within the
      value range defined by the token (Section 13.2)

   o  sign indicates the sign of the coefficient (Section 13.2)

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20.  Attachment One: Reference Decoder Source Code

20.1.  bit_ops.h

   ---- Begin code block --------------------------------------

   /*
    *  Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    *  Use of this source code is governed by a BSD-style license
    *  that can be found in the LICENSE file in the root of the source
    *  tree.  An additional intellectual property rights grant can be
    *  found in the file PATENTS.  All contributing project authors may
    *  be found in the AUTHORS file in the root of the source tree.
    */
   #ifndef BIT_OPS_H
   #define BIT_OPS_H

   /* Evaluates to a mask with n bits set */
   #define BITS_MASK(n) ((1<<(n))-1)

   /* Returns len bits, with the LSB at position bit */
   #define BITS_GET(val, bit, len) (((val)>>(bit))&BITS_MASK(len))

   #endif

   ---- End code block ----------------------------------------

20.2.  bool_decoder.h

   ---- Begin code block --------------------------------------

   /*
    *  Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    *  Use of this source code is governed by a BSD-style license
    *  that can be found in the LICENSE file in the root of the source
    *  tree.  An additional intellectual property rights grant can be
    *  found in the file PATENTS.  All contributing project authors may
    *  be found in the AUTHORS file in the root of the source tree.
    */
   #ifndef BOOL_DECODER_H
   #define BOOL_DECODER_H
   #include <stddef.h>

Bankoski, et al.              Informational                   [Page 133]
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   struct bool_decoder
   {
       const unsigned char *input;      /* next compressed data byte */
       size_t               input_len;  /* length of the input buffer */
       unsigned int         range;      /* identical to encoder's
                                         * range */
       unsigned int         value;      /* contains at least 8
                                         * significant bits */
       int                  bit_count;  /* # of bits shifted out of
                                         * value, max 7 */
   };

   static void
   init_bool_decoder(struct bool_decoder *d,
                     const unsigned char *start_partition,
                     size_t               sz)
   {
       if (sz >= 2)
       {
           d->value = (start_partition[0] << 8) /* first 2 input
                                                 * bytes */
                      | start_partition[1];
           d->input = start_partition + 2;      /* ptr to next byte */
           d->input_len = sz - 2;
       }
       else
       {
           d->value = 0;
           d->input = NULL;
           d->input_len = 0;
       }

       d->range = 255;    /* initial range is full */
       d->bit_count = 0;  /* have not yet shifted out any bits */
   }

   static int bool_get(struct bool_decoder *d, int probability)
   {
       /* range and split are identical to the corresponding values
          used by the encoder when this bool was written */

       unsigned int  split = 1 + (((d->range - 1) * probability) >> 8);
       unsigned int  SPLIT = split << 8;
       int           retval;           /* will be 0 or 1 */

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       if (d->value >= SPLIT)    /* encoded a one */
       {
           retval = 1;
           d->range -= split;  /* reduce range */
           d->value -= SPLIT;  /* subtract off left endpoint of
                                * interval */
       }
       else                  /* encoded a zero */
       {
           retval = 0;
           d->range = split; /* reduce range, no change in left
                              * endpoint */
       }

       while (d->range < 128)    /* shift out irrelevant value bits */
       {
           d->value <<= 1;
           d->range <<= 1;

           if (++d->bit_count == 8)  /* shift in new bits 8 at a time */
           {
               d->bit_count = 0;

               if (d->input_len)
               {
                   d->value |= *d->input++;
                   d->input_len--;
               }
           }
       }

       return retval;
   }

   static int bool_get_bit(struct bool_decoder *br)
   {
       return bool_get(br, 128);
   }

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   static int bool_get_uint(struct bool_decoder *br, int bits)
   {
       int z = 0;
       int bit;

       for (bit = bits - 1; bit >= 0; bit--)
       {
           z |= (bool_get_bit(br) << bit);
       }

       return z;
   }

   static int bool_get_int(struct bool_decoder *br, int bits)
   {
       int z = 0;
       int bit;

       for (bit = bits - 1; bit >= 0; bit--)
       {
           z |= (bool_get_bit(br) << bit);
       }

       return bool_get_bit(br) ? -z : z;
   }

   static int bool_maybe_get_int(struct bool_decoder *br, int bits)
   {
       return bool_get_bit(br) ? bool_get_int(br, bits) : 0;
   }

   static int
   bool_read_tree(struct bool_decoder *bool,
                  const int           *t,
                  const unsigned char *p)
   {
       int i = 0;

       while ((i = t[ i + bool_get(bool, p[i>>1])]) > 0);

       return -i;
   }
   #endif

   ---- End code block ----------------------------------------

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RFC 6386           VP8 Data Format and Decoding Guide      November 2011

20.3.  dequant_data.h

   ---- Begin code block --------------------------------------

   /*
    *  Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    *  Use of this source code is governed by a BSD-style license
    *  that can be found in the LICENSE file in the root of the source
    *  tree.  An additional intellectual property rights grant can be
    *  found in the file PATENTS.  All contributing project authors may
    *  be found in the AUTHORS file in the root of the source tree.
    */

   static const int dc_q_lookup[128] =
   {
       4,    5,    6,    7,    8,    9,    10,   10,
       11,   12,   13,   14,   15,   16,   17,   17,
       18,   19,   20,   20,   21,   21,   22,   22,
       23,   23,   24,   25,   25,   26,   27,   28,
       29,   30,   31,   32,   33,   34,   35,   36,
       37,   37,   38,   39,   40,   41,   42,   43,
       44,   45,   46,   46,   47,   48,   49,   50,
       51,   52,   53,   54,   55,   56,   57,   58,
       59,   60,   61,   62,   63,   64,   65,   66,
       67,   68,   69,   70,   71,   72,   73,   74,
       75,   76,   76,   77,   78,   79,   80,   81,
       82,   83,   84,   85,   86,   87,   88,   89,
       91,   93,   95,   96,   98,   100,  101,  102,
       104,  106,  108,  110,  112,  114,  116,  118,
       122,  124,  126,  128,  130,  132,  134,  136,
       138,  140,  143,  145,  148,  151,  154,  157
   };

   static const int ac_q_lookup[128] =
   {
       4,    5,    6,    7,    8,    9,    10,   11,
       12,   13,   14,   15,   16,   17,   18,   19,
       20,   21,   22,   23,   24,   25,   26,   27,
       28,   29,   30,   31,   32,   33,   34,   35,
       36,   37,   38,   39,   40,   41,   42,   43,
       44,   45,   46,   47,   48,   49,   50,   51,
       52,   53,   54,   55,   56,   57,   58,   60,
       62,   64,   66,   68,   70,   72,   74,   76,
       78,   80,   82,   84,   86,   88,   90,   92,
       94,   96,   98,   100,  102,  104,  106,  108,
       110,  112,  114,  116,  119,  122,  125,  128,
       131,  134,  137,  140,  143,  146,  149,  152,

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       155,  158,  161,  164,  167,  170,  173,  177,
       181,  185,  189,  193,  197,  201,  205,  209,
       213,  217,  221,  225,  229,  234,  239,  245,
       249,  254,  259,  264,  269,  274,  279,  284
   };

   ---- End code block ----------------------------------------

20.4.  dixie.c

   ---- Begin code block --------------------------------------

   /*
    *  Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    *  Use of this source code is governed by a BSD-style license
    *  that can be found in the LICENSE file in the root of the source
    *  tree.  An additional intellectual property rights grant can be
    *  found in the file PATENTS.  All contributing project authors may
    *  be found in the AUTHORS file in the root of the source tree.
    */
   #include "vpx_codec_internal.h"
   #include "bit_ops.h"
   #include "dixie.h"
   #include "vp8_prob_data.h"
   #include "dequant_data.h"
   #include "modemv.h"
   #include "tokens.h"
   #include "predict.h"
   #include "dixie_loopfilter.h"
   #include <string.h>
   #include <assert.h>

   enum
   {
       FRAME_HEADER_SZ = 3,
       KEYFRAME_HEADER_SZ = 7
   };

   #define ARRAY_COPY(a,b) {\
       assert(sizeof(a)==sizeof(b));memcpy(a,b,sizeof(a));}
   static void
   decode_entropy_header(struct vp8_decoder_ctx    *ctx,
                         struct bool_decoder       *bool,
                         struct vp8_entropy_hdr    *hdr)

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RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   {
       int i, j, k, l;

       /* Read coefficient probability updates */
       for (i = 0; i < BLOCK_TYPES; i++)
           for (j = 0; j < COEFF_BANDS; j++)
               for (k = 0; k < PREV_COEFF_CONTEXTS; k++)
                   for (l = 0; l < ENTROPY_NODES; l++)
                       if (bool_get(bool,
                                    k_coeff_entropy_update_probs
                                        [i][j][k][l]))
                           hdr->coeff_probs[i][j][k][l] =
                               bool_get_uint(bool, 8);

       /* Read coefficient skip mode probability */
       hdr->coeff_skip_enabled = bool_get_bit(bool);

       if (hdr->coeff_skip_enabled)
           hdr->coeff_skip_prob = bool_get_uint(bool, 8);

       /* Parse interframe probability updates */
       if (!ctx->frame_hdr.is_keyframe)
       {
           hdr->prob_inter = bool_get_uint(bool, 8);
           hdr->prob_last  = bool_get_uint(bool, 8);
           hdr->prob_gf    = bool_get_uint(bool, 8);

           if (bool_get_bit(bool))
               for (i = 0; i < 4; i++)
                   hdr->y_mode_probs[i] = bool_get_uint(bool, 8);

           if (bool_get_bit(bool))
               for (i = 0; i < 3; i++)
                   hdr->uv_mode_probs[i] = bool_get_uint(bool, 8);

           for (i = 0; i < 2; i++)
               for (j = 0; j < MV_PROB_CNT; j++)
                   if (bool_get(bool, k_mv_entropy_update_probs[i][j]))
                   {
                       int x = bool_get_uint(bool, 7);
                       hdr->mv_probs[i][j] = x ? x << 1 : 1;
                   }
       }
   }

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   static void
   decode_reference_header(struct vp8_decoder_ctx    *ctx,
                           struct bool_decoder       *bool,
                           struct vp8_reference_hdr  *hdr)
   {
       unsigned int key = ctx->frame_hdr.is_keyframe;

       hdr->refresh_gf    = key ? 1 : bool_get_bit(bool);
       hdr->refresh_arf   = key ? 1 : bool_get_bit(bool);
       hdr->copy_gf       = key ? 0 : !hdr->refresh_gf
                            ? bool_get_uint(bool, 2) : 0;
       hdr->copy_arf      = key ? 0 : !hdr->refresh_arf
                            ? bool_get_uint(bool, 2) : 0;
       hdr->sign_bias[GOLDEN_FRAME] = key ? 0 : bool_get_bit(bool);
       hdr->sign_bias[ALTREF_FRAME] = key ? 0 : bool_get_bit(bool);
       hdr->refresh_entropy = bool_get_bit(bool);
       hdr->refresh_last  = key ? 1 : bool_get_bit(bool);
   }

   static void
   decode_quantizer_header(struct vp8_decoder_ctx    *ctx,
                           struct bool_decoder       *bool,
                           struct vp8_quant_hdr      *hdr)
   {
       int update;
       int last_q = hdr->q_index;

       hdr->q_index = bool_get_uint(bool, 7);
       update = last_q != hdr->q_index;
       update |= (hdr->y1_dc_delta_q = bool_maybe_get_int(bool, 4));
       update |= (hdr->y2_dc_delta_q = bool_maybe_get_int(bool, 4));
       update |= (hdr->y2_ac_delta_q = bool_maybe_get_int(bool, 4));
       update |= (hdr->uv_dc_delta_q = bool_maybe_get_int(bool, 4));
       update |= (hdr->uv_ac_delta_q = bool_maybe_get_int(bool, 4));
       hdr->delta_update = update;
   }

   static void
   decode_and_init_token_partitions(struct vp8_decoder_ctx    *ctx,
                                    struct bool_decoder       *bool,
                                    const unsigned char       *data,
                                    unsigned int               sz,
                                    struct vp8_token_hdr      *hdr)

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RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   {
       int i;

       hdr->partitions = 1 << bool_get_uint(bool, 2);

       if (sz < 3 *(hdr->partitions - 1))
           vpx_internal_error(&ctx->error, VPX_CODEC_CORRUPT_FRAME,
                              "Truncated packet found parsing partition"
                              " lengths.");

       sz -= 3 * (hdr->partitions - 1);

       for (i = 0; i < hdr->partitions; i++)
       {
           if (i < hdr->partitions - 1)
           {
               hdr->partition_sz[i] = (data[2] << 16)
                                      | (data[1] << 8) | data[0];
               data += 3;
           }
           else
               hdr->partition_sz[i] = sz;

           if (sz < hdr->partition_sz[i])
               vpx_internal_error(&ctx->error, VPX_CODEC_CORRUPT_FRAME,
                                  "Truncated partition %d", i);

           sz -= hdr->partition_sz[i];
       }

       for (i = 0; i < ctx->token_hdr.partitions; i++)
       {
           init_bool_decoder(&ctx->tokens[i].bool, data,
                             ctx->token_hdr.partition_sz[i]);
           data += ctx->token_hdr.partition_sz[i];
       }
   }

   static void
   decode_loopfilter_header(struct vp8_decoder_ctx    *ctx,
                            struct bool_decoder       *bool,
                            struct vp8_loopfilter_hdr *hdr)

Bankoski, et al.              Informational                   [Page 141]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   {
       if (ctx->frame_hdr.is_keyframe)
           memset(hdr, 0, sizeof(*hdr));

       hdr->use_simple    = bool_get_bit(bool);
       hdr->level         = bool_get_uint(bool, 6);
       hdr->sharpness     = bool_get_uint(bool, 3);
       hdr->delta_enabled = bool_get_bit(bool);

       if (hdr->delta_enabled && bool_get_bit(bool))
       {
           int i;

           for (i = 0; i < BLOCK_CONTEXTS; i++)
               hdr->ref_delta[i] = bool_maybe_get_int(bool, 6);

           for (i = 0; i < BLOCK_CONTEXTS; i++)
               hdr->mode_delta[i] = bool_maybe_get_int(bool, 6);
       }
   }

   static void
   decode_segmentation_header(struct vp8_decoder_ctx *ctx,
                              struct bool_decoder    *bool,
                              struct vp8_segment_hdr *hdr)
   {
       if (ctx->frame_hdr.is_keyframe)
           memset(hdr, 0, sizeof(*hdr));

       hdr->enabled = bool_get_bit(bool);

       if (hdr->enabled)
       {
           int i;

           hdr->update_map = bool_get_bit(bool);
           hdr->update_data = bool_get_bit(bool);

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           if (hdr->update_data)
           {
               hdr->abs = bool_get_bit(bool);

               for (i = 0; i < MAX_MB_SEGMENTS; i++)
                   hdr->quant_idx[i] = bool_maybe_get_int(bool, 7);

               for (i = 0; i < MAX_MB_SEGMENTS; i++)
                   hdr->lf_level[i] = bool_maybe_get_int(bool, 6);
           }

           if (hdr->update_map)
           {
               for (i = 0; i < MB_FEATURE_TREE_PROBS; i++)
                   hdr->tree_probs[i] = bool_get_bit(bool)
                                        ? bool_get_uint(bool, 8)
                                        : 255;
           }
       }
       else
       {
           hdr->update_map = 0;
           hdr->update_data = 0;
       }
   }

   static void
   dequant_global_init(struct dequant_factors dqf[MAX_MB_SEGMENTS])
   {
       int i;

       for (i = 0; i < MAX_MB_SEGMENTS; i++)
           dqf[i].quant_idx = -1;
   }

   static int
   clamp_q(int q)
   {
       if (q < 0) return 0;
       else if (q > 127) return 127;

       return q;
   }

Bankoski, et al.              Informational                   [Page 143]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static int
   dc_q(int q)
   {
       return dc_q_lookup[clamp_q(q)];
   }

   static int
   ac_q(int q)
   {
       return ac_q_lookup[clamp_q(q)];
   }

   static void
   dequant_init(struct dequant_factors        factors[MAX_MB_SEGMENTS],
                const struct vp8_segment_hdr *seg,
                const struct vp8_quant_hdr   *quant_hdr)
   {
       int i, q;
       struct dequant_factors *dqf = factors;

       for (i = 0; i < (seg->enabled ? MAX_MB_SEGMENTS : 1); i++)
       {
           q = quant_hdr->q_index;

           if (seg->enabled)
               q = (!seg->abs) ? q + seg->quant_idx[i]
                               : seg->quant_idx[i];

           if (dqf->quant_idx != q || quant_hdr->delta_update)
           {
               dqf->factor[TOKEN_BLOCK_Y1][0] =
                   dc_q(q + quant_hdr->y1_dc_delta_q);
               dqf->factor[TOKEN_BLOCK_Y1][1] =
                   ac_q(q);
               dqf->factor[TOKEN_BLOCK_UV][0] =
                   dc_q(q + quant_hdr->uv_dc_delta_q);
               dqf->factor[TOKEN_BLOCK_UV][1] =
                   ac_q(q + quant_hdr->uv_ac_delta_q);
               dqf->factor[TOKEN_BLOCK_Y2][0] =
                   dc_q(q + quant_hdr->y2_dc_delta_q) * 2;
               dqf->factor[TOKEN_BLOCK_Y2][1] =
                   ac_q(q + quant_hdr->y2_ac_delta_q) * 155 / 100;

               if (dqf->factor[TOKEN_BLOCK_Y2][1] < 8)
                   dqf->factor[TOKEN_BLOCK_Y2][1] = 8;

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RFC 6386           VP8 Data Format and Decoding Guide      November 2011

               if (dqf->factor[TOKEN_BLOCK_UV][0] > 132)
                   dqf->factor[TOKEN_BLOCK_UV][0] = 132;

               dqf->quant_idx = q;
           }

           dqf++;
       }
   }

   static void
   decode_frame(struct vp8_decoder_ctx *ctx,
                const unsigned char    *data,
                unsigned int            sz)
   {
       vpx_codec_err_t  res;
       struct bool_decoder  bool;
       int                  i, row, partition;

       ctx->saved_entropy_valid = 0;

       if ((res = vp8_parse_frame_header(data, sz, &ctx->frame_hdr)))
           vpx_internal_error(&ctx->error, res,
                              "Failed to parse frame header");

       if (ctx->frame_hdr.is_experimental)
           vpx_internal_error(&ctx->error, VPX_CODEC_UNSUP_BITSTREAM,
                              "Experimental bitstreams not supported.");

       data += FRAME_HEADER_SZ;
       sz -= FRAME_HEADER_SZ;

       if (ctx->frame_hdr.is_keyframe)
       {
           data += KEYFRAME_HEADER_SZ;
           sz -= KEYFRAME_HEADER_SZ;
           ctx->mb_cols = (ctx->frame_hdr.kf.w + 15) / 16;
           ctx->mb_rows = (ctx->frame_hdr.kf.h + 15) / 16;
       }

       /* Start the bitreader for the header/entropy partition */
       init_bool_decoder(&bool, data, ctx->frame_hdr.part0_sz);

Bankoski, et al.              Informational                   [Page 145]
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       /* Skip the colorspace and clamping bits */
       if (ctx->frame_hdr.is_keyframe)
           if (bool_get_uint(&bool, 2))
               vpx_internal_error(
                   &ctx->error, VPX_CODEC_UNSUP_BITSTREAM,
                   "Reserved bits not supported.");

       decode_segmentation_header(ctx, &bool, &ctx->segment_hdr);
       decode_loopfilter_header(ctx, &bool, &ctx->loopfilter_hdr);
       decode_and_init_token_partitions(ctx,
                                        &bool,
                                        data + ctx->frame_hdr.part0_sz,
                                        sz - ctx->frame_hdr.part0_sz,
                                        &ctx->token_hdr);
       decode_quantizer_header(ctx, &bool, &ctx->quant_hdr);
       decode_reference_header(ctx, &bool, &ctx->reference_hdr);

       /* Set keyframe entropy defaults.  These get updated on keyframes
        * regardless of the refresh_entropy setting.
        */
       if (ctx->frame_hdr.is_keyframe)
       {
           ARRAY_COPY(ctx->entropy_hdr.coeff_probs,
                      k_default_coeff_probs);
           ARRAY_COPY(ctx->entropy_hdr.mv_probs,
                      k_default_mv_probs);
           ARRAY_COPY(ctx->entropy_hdr.y_mode_probs,
                      k_default_y_mode_probs);
           ARRAY_COPY(ctx->entropy_hdr.uv_mode_probs,
                      k_default_uv_mode_probs);
       }

       if (!ctx->reference_hdr.refresh_entropy)
       {
           ctx->saved_entropy = ctx->entropy_hdr;
           ctx->saved_entropy_valid = 1;
       }

       decode_entropy_header(ctx, &bool, &ctx->entropy_hdr);

       vp8_dixie_modemv_init(ctx);
       vp8_dixie_tokens_init(ctx);
       vp8_dixie_predict_init(ctx);
       dequant_init(ctx->dequant_factors, &ctx->segment_hdr,
                    &ctx->quant_hdr);

Bankoski, et al.              Informational                   [Page 146]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

       for (row = 0, partition = 0; row < ctx->mb_rows; row++)
       {
           vp8_dixie_modemv_process_row(
               ctx, &bool, row, 0, ctx->mb_cols);
           vp8_dixie_tokens_process_row(ctx, partition, row, 0,
                                        ctx->mb_cols);
           vp8_dixie_predict_process_row(ctx, row, 0, ctx->mb_cols);

           if (ctx->loopfilter_hdr.level && row)
               vp8_dixie_loopfilter_process_row(ctx, row - 1, 0,
                                                ctx->mb_cols);

           if (++partition == ctx->token_hdr.partitions)
               partition = 0;
       }

       if (ctx->loopfilter_hdr.level)
           vp8_dixie_loopfilter_process_row(
               ctx, row - 1, 0, ctx->mb_cols);

       ctx->frame_cnt++;

       if (!ctx->reference_hdr.refresh_entropy)
       {
           ctx->entropy_hdr = ctx->saved_entropy;
           ctx->saved_entropy_valid = 0;
       }

       /* Handle reference frame updates */
       if (ctx->reference_hdr.copy_arf == 1)
       {
           vp8_dixie_release_ref_frame(ctx->ref_frames[ALTREF_FRAME]);
           ctx->ref_frames[ALTREF_FRAME] =
               vp8_dixie_ref_frame(ctx->ref_frames[LAST_FRAME]);
       }
       else if (ctx->reference_hdr.copy_arf == 2)
       {
           vp8_dixie_release_ref_frame(ctx->ref_frames[ALTREF_FRAME]);
           ctx->ref_frames[ALTREF_FRAME] =
               vp8_dixie_ref_frame(ctx->ref_frames[GOLDEN_FRAME]);
       }

       if (ctx->reference_hdr.copy_gf == 1)
       {
           vp8_dixie_release_ref_frame(ctx->ref_frames[GOLDEN_FRAME]);
           ctx->ref_frames[GOLDEN_FRAME] =
               vp8_dixie_ref_frame(ctx->ref_frames[LAST_FRAME]);
       }

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RFC 6386           VP8 Data Format and Decoding Guide      November 2011

       else if (ctx->reference_hdr.copy_gf == 2)
       {
           vp8_dixie_release_ref_frame(ctx->ref_frames[GOLDEN_FRAME]);
           ctx->ref_frames[GOLDEN_FRAME] =
               vp8_dixie_ref_frame(ctx->ref_frames[ALTREF_FRAME]);
       }

       if (ctx->reference_hdr.refresh_gf)
       {
           vp8_dixie_release_ref_frame(ctx->ref_frames[GOLDEN_FRAME]);
           ctx->ref_frames[GOLDEN_FRAME] =
               vp8_dixie_ref_frame(ctx->ref_frames[CURRENT_FRAME]);
       }

       if (ctx->reference_hdr.refresh_arf)
       {
           vp8_dixie_release_ref_frame(ctx->ref_frames[ALTREF_FRAME]);
           ctx->ref_frames[ALTREF_FRAME] =
               vp8_dixie_ref_frame(ctx->ref_frames[CURRENT_FRAME]);
       }

       if (ctx->reference_hdr.refresh_last)
       {
           vp8_dixie_release_ref_frame(ctx->ref_frames[LAST_FRAME]);
           ctx->ref_frames[LAST_FRAME] =
               vp8_dixie_ref_frame(ctx->ref_frames[CURRENT_FRAME]);
       }

   }

   void
   vp8_dixie_decode_init(struct vp8_decoder_ctx *ctx)
   {
       dequant_global_init(ctx->dequant_factors);
   }

Bankoski, et al.              Informational                   [Page 148]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   #define CHECK_FOR_UPDATE(lval,rval,update_flag) do {\
           unsigned int old = lval; \
           update_flag |= (old != (lval = rval)); \
       } while (0)

   vpx_codec_err_t
   vp8_parse_frame_header(const unsigned char   *data,
                          unsigned int           sz,
                          struct vp8_frame_hdr  *hdr)
   {
       unsigned long raw;

       if (sz < 10)
           return VPX_CODEC_CORRUPT_FRAME;

       /* The frame header is defined as a three-byte little endian
        * value
        */
       raw = data[0] | (data[1] << 8) | (data[2] << 16);
       hdr->is_keyframe     = !BITS_GET(raw, 0, 1);
       hdr->version         = BITS_GET(raw, 1, 2);
       hdr->is_experimental = BITS_GET(raw, 3, 1);
       hdr->is_shown        = BITS_GET(raw, 4, 1);
       hdr->part0_sz        = BITS_GET(raw, 5, 19);

       if (sz <= hdr->part0_sz + (hdr->is_keyframe ? 10 : 3))
           return VPX_CODEC_CORRUPT_FRAME;

       hdr->frame_size_updated = 0;

       if (hdr->is_keyframe)
       {
           unsigned int update = 0;

           /* Keyframe header consists of a three-byte sync code
            * followed by the width and height and associated scaling
            * factors.
            */
           if (data[3] != 0x9d || data[4] != 0x01 || data[5] != 0x2a)
               return VPX_CODEC_UNSUP_BITSTREAM;

Bankoski, et al.              Informational                   [Page 149]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

           raw = data[6] | (data[7] << 8)
                 | (data[8] << 16) | (data[9] << 24);
           CHECK_FOR_UPDATE(hdr->kf.w,       BITS_GET(raw,  0, 14),
                            update);
           CHECK_FOR_UPDATE(hdr->kf.scale_w, BITS_GET(raw, 14,  2),
                            update);
           CHECK_FOR_UPDATE(hdr->kf.h,       BITS_GET(raw, 16, 14),
                            update);
           CHECK_FOR_UPDATE(hdr->kf.scale_h, BITS_GET(raw, 30,  2),
                            update);

           hdr->frame_size_updated = update;

           if (!hdr->kf.w || !hdr->kf.h)
               return VPX_CODEC_UNSUP_BITSTREAM;
       }

       return VPX_CODEC_OK;
   }

   vpx_codec_err_t
   vp8_dixie_decode_frame(struct vp8_decoder_ctx *ctx,
                          const unsigned char    *data,
                          unsigned int            sz)
   {
       volatile struct vp8_decoder_ctx *ctx_ = ctx;

       ctx->error.error_code = VPX_CODEC_OK;
       ctx->error.has_detail = 0;

       if (!setjmp(ctx->error.jmp))
           decode_frame(ctx, data, sz);

       return ctx_->error.error_code;
   }

   void
   vp8_dixie_decode_destroy(struct vp8_decoder_ctx *ctx)
   {
       vp8_dixie_predict_destroy(ctx);
       vp8_dixie_tokens_destroy(ctx);
       vp8_dixie_modemv_destroy(ctx);
   }

   ---- End code block ----------------------------------------

Bankoski, et al.              Informational                   [Page 150]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

20.5.  dixie.h

   ---- Begin code block --------------------------------------

   /*
    *  Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    *  Use of this source code is governed by a BSD-style license
    *  that can be found in the LICENSE file in the root of the source
    *  tree.  An additional intellectual property rights grant can be
    *  found in the file PATENTS.  All contributing project authors may
    *  be found in the AUTHORS file in the root of the source tree.
    */
   #ifndef DIXIE_H
   #define DIXIE_H
   #include "vpx_codec_internal.h"
   #include "bool_decoder.h"

   struct vp8_frame_hdr
   {
       unsigned int is_keyframe;      /* Frame is a keyframe */
       unsigned int is_experimental;  /* Frame is a keyframe */
       unsigned int version;          /* Bitstream version */
       unsigned int is_shown;         /* Frame is to be displayed. */
       unsigned int part0_sz;         /* Partition 0 length, in bytes */

       struct vp8_kf_hdr
       {
           unsigned int w;        /* Width */
           unsigned int h;        /* Height */
           unsigned int scale_w;  /* Scaling factor, Width */
           unsigned int scale_h;  /* Scaling factor, Height */
       } kf;

       unsigned int frame_size_updated; /* Flag to indicate a resolution
                                         * update.
                                         */
   };

   enum
   {
       MB_FEATURE_TREE_PROBS = 3,
       MAX_MB_SEGMENTS = 4
   };

Bankoski, et al.              Informational                   [Page 151]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   struct vp8_segment_hdr
   {
       unsigned int         enabled;
       unsigned int         update_data;
       unsigned int         update_map;
       unsigned int         abs;    /* 0=deltas, 1=absolute values */
       unsigned int         tree_probs[MB_FEATURE_TREE_PROBS];
       int                  lf_level[MAX_MB_SEGMENTS];
       int                  quant_idx[MAX_MB_SEGMENTS];

   };

   enum
   {
       BLOCK_CONTEXTS = 4
   };

   struct vp8_loopfilter_hdr
   {
       unsigned int         use_simple;
       unsigned int         level;
       unsigned int         sharpness;
       unsigned int         delta_enabled;
       int                  ref_delta[BLOCK_CONTEXTS];
       int                  mode_delta[BLOCK_CONTEXTS];
   };

   enum
   {
       MAX_PARTITIONS = 8
   };

   struct vp8_token_hdr
   {
       unsigned int        partitions;
       unsigned int        partition_sz[MAX_PARTITIONS];
   };

Bankoski, et al.              Informational                   [Page 152]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   struct vp8_quant_hdr
   {
       unsigned int       q_index;
       int                delta_update;
       int                y1_dc_delta_q;
       int                y2_dc_delta_q;
       int                y2_ac_delta_q;
       int                uv_dc_delta_q;
       int                uv_ac_delta_q;
   };

   struct vp8_reference_hdr
   {
       unsigned int refresh_last;
       unsigned int refresh_gf;
       unsigned int refresh_arf;
       unsigned int copy_gf;
       unsigned int copy_arf;
       unsigned int sign_bias[4];
       unsigned int refresh_entropy;
   };

   enum
   {
       BLOCK_TYPES        = 4,
       PREV_COEFF_CONTEXTS = 3,
       COEFF_BANDS         = 8,
       ENTROPY_NODES      = 11,
   };
   typedef unsigned char coeff_probs_table_t[BLOCK_TYPES][COEFF_BANDS]
   [PREV_COEFF_CONTEXTS]
   [ENTROPY_NODES];

   enum
   {
       MV_PROB_CNT = 2 + 8 - 1 + 10 /* from entropymv.h */
   };
   typedef unsigned char mv_component_probs_t[MV_PROB_CNT];

Bankoski, et al.              Informational                   [Page 153]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   struct vp8_entropy_hdr
   {
       coeff_probs_table_t   coeff_probs;
       mv_component_probs_t  mv_probs[2];
       unsigned int          coeff_skip_enabled;
       unsigned char         coeff_skip_prob;
       unsigned char         y_mode_probs[4];
       unsigned char         uv_mode_probs[3];
       unsigned char         prob_inter;
       unsigned char         prob_last;
       unsigned char         prob_gf;
   };

   enum reference_frame
   {
       CURRENT_FRAME,
       LAST_FRAME,
       GOLDEN_FRAME,
       ALTREF_FRAME,
       NUM_REF_FRAMES
   };

   enum prediction_mode
   {
       /* 16x16 intra modes */
       DC_PRED, V_PRED, H_PRED, TM_PRED, B_PRED,

       /* 16x16 inter modes */
       NEARESTMV, NEARMV, ZEROMV, NEWMV, SPLITMV,

       MB_MODE_COUNT,

       /* 4x4 intra modes */
       B_DC_PRED = 0, B_TM_PRED, B_VE_PRED, B_HE_PRED, B_LD_PRED,
       B_RD_PRED, B_VR_PRED, B_VL_PRED, B_HD_PRED, B_HU_PRED,

       /* 4x4 inter modes */
       LEFT4X4, ABOVE4X4, ZERO4X4, NEW4X4,

       B_MODE_COUNT
   };

Bankoski, et al.              Informational                   [Page 154]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   enum splitmv_partitioning
   {
       SPLITMV_16X8,
       SPLITMV_8X16,
       SPLITMV_8X8,
       SPLITMV_4X4
   };

   typedef short filter_t[6];

   typedef union mv
   {
       struct
       {
           int16_t x, y;
       }  d;
       uint32_t               raw;
   } mv_t;

   struct mb_base_info
   {
       unsigned char y_mode     : 4;
       unsigned char uv_mode    : 4;
       unsigned char segment_id : 2;
       unsigned char ref_frame  : 2;
       unsigned char skip_coeff : 1;
       unsigned char need_mc_border : 1;
       enum splitmv_partitioning  partitioning : 2;
       union mv      mv;
       unsigned int  eob_mask;
   };

   struct mb_info
   {
       struct mb_base_info base;
       union
       {
           union mv              mvs[16];
           enum prediction_mode  modes[16];
       } split;
   };

Bankoski, et al.              Informational                   [Page 155]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   /* A "token entropy context" has 4 Y values, 2 U, 2 V, and 1 Y2 */
   typedef int token_entropy_ctx_t[4 + 2 + 2 + 1];

   struct token_decoder
   {
       struct bool_decoder  bool;
       token_entropy_ctx_t  left_token_entropy_ctx;
       short               *coeffs;
   };

   enum token_block_type
   {
       TOKEN_BLOCK_Y1,
       TOKEN_BLOCK_UV,
       TOKEN_BLOCK_Y2,
       TOKEN_BLOCK_TYPES,
   };

   struct dequant_factors
   {
       int   quant_idx;
       short factor[TOKEN_BLOCK_TYPES][2]; /* [ Y1, UV, Y2 ]
                                            * [ DC, AC ] */
   };

   struct ref_cnt_img
   {
       vpx_image_t  img;
       unsigned int ref_cnt;
   };

   struct vp8_decoder_ctx
   {
       struct vpx_internal_error_info  error;
       unsigned int                    frame_cnt;

       struct vp8_frame_hdr            frame_hdr;
       struct vp8_segment_hdr          segment_hdr;
       struct vp8_loopfilter_hdr       loopfilter_hdr;
       struct vp8_token_hdr            token_hdr;
       struct vp8_quant_hdr            quant_hdr;
       struct vp8_reference_hdr        reference_hdr;
       struct vp8_entropy_hdr          entropy_hdr;

       struct vp8_entropy_hdr          saved_entropy;
       unsigned int                    saved_entropy_valid;

Bankoski, et al.              Informational                   [Page 156]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

       unsigned int                    mb_rows;
       unsigned int                    mb_cols;
       struct mb_info                 *mb_info_storage;
       struct mb_info                **mb_info_rows_storage;
       struct mb_info                **mb_info_rows;

       token_entropy_ctx_t            *above_token_entropy_ctx;
       struct token_decoder            tokens[MAX_PARTITIONS];
       struct dequant_factors          dequant_factors[MAX_MB_SEGMENTS];

       struct ref_cnt_img              frame_strg[NUM_REF_FRAMES];
       struct ref_cnt_img             *ref_frames[NUM_REF_FRAMES];
       ptrdiff_t                       ref_frame_offsets[4];

       const filter_t                 *subpixel_filters;
   };

   void
   vp8_dixie_decode_init(struct vp8_decoder_ctx *ctx);

   void
   vp8_dixie_decode_destroy(struct vp8_decoder_ctx *ctx);

   vpx_codec_err_t
   vp8_parse_frame_header(const unsigned char   *data,
                          unsigned int           sz,
                          struct vp8_frame_hdr  *hdr);

   vpx_codec_err_t
   vp8_dixie_decode_frame(struct vp8_decoder_ctx *ctx,
                          const unsigned char    *data,
                          unsigned int            sz);

   #define CLAMP_255(x) ((x)<0?0:((x)>255?255:(x)))

   #endif

   ---- End code block ----------------------------------------

Bankoski, et al.              Informational                   [Page 157]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

20.6.  dixie_loopfilter.c

   ---- Begin code block --------------------------------------

   /*
    *  Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    *  Use of this source code is governed by a BSD-style license
    *  that can be found in the LICENSE file in the root of the source
    *  tree.  An additional intellectual property rights grant can be
    *  found in the file PATENTS.  All contributing project authors may
    *  be found in the AUTHORS file in the root of the source tree.
    */
   #include "dixie.h"
   #include "dixie_loopfilter.h"

   #define ABS(x) ((x) >= 0 ? (x) : -(x))

   #define p3 pixels[-4*stride]
   #define p2 pixels[-3*stride]
   #define p1 pixels[-2*stride]
   #define p0 pixels[-1*stride]
   #define q0 pixels[ 0*stride]
   #define q1 pixels[ 1*stride]
   #define q2 pixels[ 2*stride]
   #define q3 pixels[ 3*stride]

   #define static
   static int

   saturate_int8(int x)
   {
       if (x < -128)
           return -128;

       if (x > 127)
           return 127;

       return x;
   }

Bankoski, et al.              Informational                   [Page 158]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static int
   saturate_uint8(int x)
   {
       if (x < 0)
           return 0;

       if (x > 255)
           return 255;

       return x;
   }

   static int
   high_edge_variance(unsigned char *pixels,
                      int            stride,
                      int            hev_threshold)
   {
       return ABS(p1 - p0) > hev_threshold ||
              ABS(q1 - q0) > hev_threshold;
   }

   static int
   simple_threshold(unsigned char *pixels,
                    int            stride,
                    int            filter_limit)
   {
       return (ABS(p0 - q0) * 2 + (ABS(p1 - q1) >> 1)) <= filter_limit;
   }

   static int
   normal_threshold(unsigned char *pixels,
                    int            stride,
                    int            edge_limit,
                    int            interior_limit)
   {
       int E = edge_limit;
       int I = interior_limit;

       return simple_threshold(pixels, stride, 2 * E + I)
              && ABS(p3 - p2) <= I && ABS(p2 - p1) <= I
              && ABS(p1 - p0) <= I && ABS(q3 - q2) <= I
              && ABS(q2 - q1) <= I && ABS(q1 - q0) <= I;
   }

Bankoski, et al.              Informational                   [Page 159]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static void
   filter_common(unsigned char *pixels,
                 int            stride,
                 int            use_outer_taps)
   {
       int a, f1, f2;

       a = 3 * (q0 - p0);

       if (use_outer_taps)
           a += saturate_int8(p1 - q1);

       a = saturate_int8(a);

       f1 = ((a + 4 > 127) ? 127 : a + 4) >> 3;
       f2 = ((a + 3 > 127) ? 127 : a + 3) >> 3;

       p0 = saturate_uint8(p0 + f2);
       q0 = saturate_uint8(q0 - f1);

       if (!use_outer_taps)
       {
           /* This handles the case of subblock_filter()
            * (from the bitstream guide.
            */
           a = (f1 + 1) >> 1;
           p1 = saturate_uint8(p1 + a);
           q1 = saturate_uint8(q1 - a);
       }
   }

Bankoski, et al.              Informational                   [Page 160]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static void
   filter_mb_edge(unsigned char *pixels,
                  int            stride)
   {
       int w, a;

       w = saturate_int8(saturate_int8(p1 - q1) + 3 * (q0 - p0));

       a = (27 * w + 63) >> 7;
       p0 = saturate_uint8(p0 + a);
       q0 = saturate_uint8(q0 - a);

       a = (18 * w + 63) >> 7;
       p1 = saturate_uint8(p1 + a);
       q1 = saturate_uint8(q1 - a);

       a = (9 * w + 63) >> 7;
       p2 = saturate_uint8(p2 + a);
       q2 = saturate_uint8(q2 - a);
   }

   static void
   filter_mb_v_edge(unsigned char *src,
                    int            stride,
                    int            edge_limit,
                    int            interior_limit,
                    int            hev_threshold,
                    int            size)
   {
       int i;

       for (i = 0; i < 8 * size; i++)
       {
           if (normal_threshold(src, 1, edge_limit, interior_limit))
           {
               if (high_edge_variance(src, 1, hev_threshold))
                   filter_common(src, 1, 1);
               else
                   filter_mb_edge(src, 1);
           }

           src += stride;
       }
   }

Bankoski, et al.              Informational                   [Page 161]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static void
   filter_subblock_v_edge(unsigned char *src,
                          int            stride,
                          int            edge_limit,
                          int            interior_limit,
                          int            hev_threshold,
                          int            size)
   {
       int i;

       for (i = 0; i < 8 * size; i++)
       {
           if (normal_threshold(src, 1, edge_limit, interior_limit))
               filter_common(src, 1,
                             high_edge_variance(src, 1, hev_threshold));

           src += stride;
       }
   }

   static void
   filter_mb_h_edge(unsigned char *src,
                    int            stride,
                    int            edge_limit,
                    int            interior_limit,
                    int            hev_threshold,
                    int            size)
   {
       int i;

       for (i = 0; i < 8 * size; i++)
       {
           if (normal_threshold(src, stride, edge_limit,
                                interior_limit))
           {
               if (high_edge_variance(src, stride, hev_threshold))
                   filter_common(src, stride, 1);
               else
                   filter_mb_edge(src, stride);
           }

           src += 1;
       }
   }

Bankoski, et al.              Informational                   [Page 162]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static void
   filter_subblock_h_edge(unsigned char *src,
                          int            stride,
                          int            edge_limit,
                          int            interior_limit,
                          int            hev_threshold,
                          int            size)
   {
       int i;

       for (i = 0; i < 8 * size; i++)
       {
           if (normal_threshold(src, stride, edge_limit,
                                interior_limit))
               filter_common(src, stride,
                             high_edge_variance(src, stride,
                                                hev_threshold));

           src += 1;
       }
   }

   static void
   filter_v_edge_simple(unsigned char *src,
                        int            stride,
                        int            filter_limit)
   {
       int i;

       for (i = 0; i < 16; i++)
       {
           if (simple_threshold(src, 1, filter_limit))
               filter_common(src, 1, 1);

           src += stride;
       }
   }

Bankoski, et al.              Informational                   [Page 163]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static void
   filter_h_edge_simple(unsigned char *src,
                        int            stride,
                        int            filter_limit)
   {
       int i;

       for (i = 0; i < 16; i++)
       {
           if (simple_threshold(src, stride, filter_limit))
               filter_common(src, stride, 1);

           src += 1;
       }
   }

   static void
   calculate_filter_parameters(struct vp8_decoder_ctx *ctx,
                               struct mb_info         *mbi,
                               int                    *edge_limit_,
                               int                    *interior_limit_,
                               int                    *hev_threshold_)
   {
       int filter_level, interior_limit, hev_threshold;

       /* Reference code/spec seems to conflate filter_level and
        * edge_limit
        */

       filter_level = ctx->loopfilter_hdr.level;

       if (ctx->segment_hdr.enabled)
       {
           if (!ctx->segment_hdr.abs)
               filter_level +=
                   ctx->segment_hdr.lf_level[mbi->base.segment_id];
           else
               filter_level =
                   ctx->segment_hdr.lf_level[mbi->base.segment_id];
       }

       if (filter_level > 63)
           filter_level = 63;
       else if (filter_level < 0)
           filter_level = 0;

Bankoski, et al.              Informational                   [Page 164]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

       if (ctx->loopfilter_hdr.delta_enabled)
       {
           filter_level +=
               ctx->loopfilter_hdr.ref_delta[mbi->base.ref_frame];

           if (mbi->base.ref_frame == CURRENT_FRAME)
           {
               if (mbi->base.y_mode == B_PRED)
                   filter_level += ctx->loopfilter_hdr.mode_delta[0];
           }
           else if (mbi->base.y_mode == ZEROMV)
               filter_level += ctx->loopfilter_hdr.mode_delta[1];
           else if (mbi->base.y_mode == SPLITMV)
               filter_level += ctx->loopfilter_hdr.mode_delta[3];
           else
               filter_level += ctx->loopfilter_hdr.mode_delta[2];
       }

       if (filter_level > 63)
           filter_level = 63;
       else if (filter_level < 0)
           filter_level = 0;

       interior_limit = filter_level;

       if (ctx->loopfilter_hdr.sharpness)
       {
           interior_limit >>= ctx->loopfilter_hdr.sharpness > 4 ? 2 : 1;

           if (interior_limit > 9 - ctx->loopfilter_hdr.sharpness)
               interior_limit = 9 - ctx->loopfilter_hdr.sharpness;
       }

       if (interior_limit < 1)
           interior_limit = 1;

       hev_threshold = (filter_level >= 15);

       if (filter_level >= 40)
           hev_threshold++;

       if (filter_level >= 20 && !ctx->frame_hdr.is_keyframe)
           hev_threshold++;

       *edge_limit_ = filter_level;
       *interior_limit_ = interior_limit;
       *hev_threshold_ = hev_threshold;
   }

Bankoski, et al.              Informational                   [Page 165]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static void
   filter_row_normal(struct vp8_decoder_ctx *ctx,
                     unsigned int            row,
                     unsigned int            start_col,
                     unsigned int            num_cols)
   {
       unsigned char  *y, *u, *v;
       int             stride, uv_stride;
       struct mb_info *mbi;
       unsigned int    col;

       /* Adjust pointers based on row, start_col */

       stride    = ctx->ref_frames[CURRENT_FRAME]->img.stride[PLANE_Y];
       uv_stride = ctx->ref_frames[CURRENT_FRAME]->img.stride[PLANE_U];
       y = ctx->ref_frames[CURRENT_FRAME]->img.planes[PLANE_Y];
       u = ctx->ref_frames[CURRENT_FRAME]->img.planes[PLANE_U];
       v = ctx->ref_frames[CURRENT_FRAME]->img.planes[PLANE_V];
       y += (stride * row + start_col) * 16;
       u += (uv_stride * row + start_col) * 8;
       v += (uv_stride * row + start_col) * 8;
       mbi = ctx->mb_info_rows[row] + start_col;

       for (col = start_col; col < start_col + num_cols; col++)
       {
           int edge_limit, interior_limit, hev_threshold;

           /* TODO: Only need to recalculate every MB if segmentation is
            * enabled.
            */
           calculate_filter_parameters(ctx, mbi, &edge_limit,
                                       &interior_limit, &hev_threshold);

           if (edge_limit)
           {
               if (col)
               {
                   filter_mb_v_edge(y, stride, edge_limit + 2,
                                    interior_limit, hev_threshold, 2);
                   filter_mb_v_edge(u, uv_stride, edge_limit + 2,
                                    interior_limit, hev_threshold, 1);
                   filter_mb_v_edge(v, uv_stride, edge_limit + 2,
                                    interior_limit, hev_threshold, 1);
               }

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               /* NOTE: This conditional is actually dependent on the
                * number of coefficients decoded, not the skip flag as
                * coded in the bitstream.  The tokens task is expected
                * to set 31 if there is *any* non-zero data.
                */
               if (mbi->base.eob_mask
                   || mbi->base.y_mode == SPLITMV
                   || mbi->base.y_mode == B_PRED)
               {
                   filter_subblock_v_edge(y + 4, stride, edge_limit,
                                          interior_limit, hev_threshold,
                                          2);
                   filter_subblock_v_edge(y + 8, stride, edge_limit,
                                          interior_limit, hev_threshold,
                                          2);
                   filter_subblock_v_edge(y + 12, stride, edge_limit,
                                          interior_limit, hev_threshold,
                                          2);
                   filter_subblock_v_edge(u + 4, uv_stride, edge_limit,
                                          interior_limit, hev_threshold,
                                          1);
                   filter_subblock_v_edge(v + 4, uv_stride, edge_limit,
                                          interior_limit, hev_threshold,
                                          1);
               }

               if (row)
               {
                   filter_mb_h_edge(y, stride, edge_limit + 2,
                                    interior_limit, hev_threshold, 2);
                   filter_mb_h_edge(u, uv_stride, edge_limit + 2,
                                    interior_limit, hev_threshold, 1);
                   filter_mb_h_edge(v, uv_stride, edge_limit + 2,
                                    interior_limit, hev_threshold, 1);
               }

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               if (mbi->base.eob_mask
                   || mbi->base.y_mode == SPLITMV
                   || mbi->base.y_mode == B_PRED)
               {
                   filter_subblock_h_edge(y + 4 * stride, stride,
                                          edge_limit, interior_limit,
                                          hev_threshold, 2);
                   filter_subblock_h_edge(y + 8 * stride, stride,
                                          edge_limit, interior_limit,
                                          hev_threshold, 2);
                   filter_subblock_h_edge(y + 12 * stride, stride,
                                          edge_limit, interior_limit,
                                          hev_threshold, 2);
                   filter_subblock_h_edge(u + 4 * uv_stride, uv_stride,
                                          edge_limit, interior_limit,
                                          hev_threshold, 1);
                   filter_subblock_h_edge(v + 4 * uv_stride, uv_stride,
                                          edge_limit, interior_limit,
                                          hev_threshold, 1);
               }
           }

           y += 16;
           u += 8;
           v += 8;
           mbi++;
       }
   }

   static void
   filter_row_simple(struct vp8_decoder_ctx *ctx,
                     unsigned int            row,
                     unsigned int            start_col,
                     unsigned int            num_cols)
   {
       unsigned char  *y;
       int             stride;
       struct mb_info *mbi;
       unsigned int    col;

       /* Adjust pointers based on row, start_col */
       stride    = ctx->ref_frames[CURRENT_FRAME]->img.stride[PLANE_Y];
       y = ctx->ref_frames[CURRENT_FRAME]->img.planes[PLANE_Y];
       y += (stride * row + start_col) * 16;
       mbi = ctx->mb_info_rows[row] + start_col;

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       for (col = start_col; col < start_col + num_cols; col++)
       {
           int edge_limit, interior_limit, hev_threshold;

           /* TODO: Only need to recalculate every MB if segmentation is
            * enabled.
            */
           calculate_filter_parameters(ctx, mbi, &edge_limit,
                                       &interior_limit, &hev_threshold);

           if (edge_limit)
           {

               /* NOTE: This conditional is actually dependent on the
                * number of coefficients decoded, not the skip flag as
                * coded in the bitstream.  The tokens task is expected
                * to set 31 if there is *any* non-zero data.
                */
               int filter_subblocks = (mbi->base.eob_mask
                                       || mbi->base.y_mode == SPLITMV
                                       || mbi->base.y_mode == B_PRED);
               int mb_limit = (edge_limit + 2) * 2 + interior_limit;
               int b_limit = edge_limit * 2 + interior_limit;

               if (col)
                   filter_v_edge_simple(y, stride, mb_limit);

               if (filter_subblocks)
               {
                   filter_v_edge_simple(y + 4, stride, b_limit);
                   filter_v_edge_simple(y + 8, stride, b_limit);
                   filter_v_edge_simple(y + 12, stride, b_limit);
               }

               if (row)
                   filter_h_edge_simple(y, stride, mb_limit);

               if (filter_subblocks)
               {
                   filter_h_edge_simple(y + 4 * stride, stride,
                                        b_limit);
                   filter_h_edge_simple(y + 8 * stride, stride,
                                        b_limit);
                   filter_h_edge_simple(y + 12 * stride, stride,
                                        b_limit);
               }
           }

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           y += 16;
           mbi++;
       }
   }

   void
   vp8_dixie_loopfilter_process_row(struct vp8_decoder_ctx *ctx,
                                    unsigned int            row,
                                    unsigned int            start_col,
                                    unsigned int            num_cols)
   {
       if (ctx->loopfilter_hdr.use_simple)
           filter_row_simple(ctx, row, start_col, num_cols);
       else
           filter_row_normal(ctx, row, start_col, num_cols);
   }

   ---- End code block ----------------------------------------

20.7.  dixie_loopfilter.h

   ---- Begin code block --------------------------------------

   /*
    *  Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    *  Use of this source code is governed by a BSD-style license
    *  that can be found in the LICENSE file in the root of the source
    *  tree.  An additional intellectual property rights grant can be
    *  found in the file PATENTS.  All contributing project authors may
    *  be found in the AUTHORS file in the root of the source tree.
    */
   #ifndef DIXIE_LOOPFILTER_H
   #define DIXIE_LOOPFILTER_H

   void
   vp8_dixie_loopfilter_process_row(struct vp8_decoder_ctx *ctx,
                                    unsigned int            row,
                                    unsigned int            start_col,
                                    unsigned int            num_cols);

   #endif

   ---- End code block ----------------------------------------

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20.8.  idct_add.c

   ---- Begin code block --------------------------------------

   /*
    *  Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    *  Use of this source code is governed by a BSD-style license
    *  that can be found in the LICENSE file in the root of the source
    *  tree.  An additional intellectual property rights grant can be
    *  found in the file PATENTS.  All contributing project authors may
    *  be found in the AUTHORS file in the root of the source tree.
    */
   #include "dixie.h"
   #include "idct_add.h"
   #include <assert.h>

   void
   vp8_dixie_walsh(const short *input, short *output)
   {
       int i;
       int a1, b1, c1, d1;
       int a2, b2, c2, d2;
       const short *ip = input;
       short *op = output;

       for (i = 0; i < 4; i++)
       {
           a1 = ip[0] + ip[12];
           b1 = ip[4] + ip[8];
           c1 = ip[4] - ip[8];
           d1 = ip[0] - ip[12];

           op[0] = a1 + b1;
           op[4] = c1 + d1;
           op[8] = a1 - b1;
           op[12] = d1 - c1;
           ip++;
           op++;
       }

       ip = output;
       op = output;

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       for (i = 0; i < 4; i++)
       {
           a1 = ip[0] + ip[3];
           b1 = ip[1] + ip[2];
           c1 = ip[1] - ip[2];
           d1 = ip[0] - ip[3];

           a2 = a1 + b1;
           b2 = c1 + d1;
           c2 = a1 - b1;
           d2 = d1 - c1;

           op[0] = (a2 + 3) >> 3;
           op[1] = (b2 + 3) >> 3;
           op[2] = (c2 + 3) >> 3;
           op[3] = (d2 + 3) >> 3;

           ip += 4;
           op += 4;
       }
   }

   #define cospi8sqrt2minus1 20091
   #define sinpi8sqrt2       35468
   #define rounding          0
   static void
   idct_columns(const short *input, short *output)
   {
       int i;
       int a1, b1, c1, d1;

       const short *ip = input;
       short *op = output;
       int temp1, temp2;
       int shortpitch = 4;

       for (i = 0; i < 4; i++)
       {
           a1 = ip[0] + ip[8];
           b1 = ip[0] - ip[8];

           temp1 = (ip[4] * sinpi8sqrt2 + rounding) >> 16;
           temp2 = ip[12] +
               ((ip[12] * cospi8sqrt2minus1 + rounding) >> 16);
           c1 = temp1 - temp2;

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           temp1 = ip[4] +
               ((ip[4] * cospi8sqrt2minus1 + rounding) >> 16);
           temp2 = (ip[12] * sinpi8sqrt2 + rounding) >> 16;
           d1 = temp1 + temp2;

           op[shortpitch*0] = a1 + d1;
           op[shortpitch*3] = a1 - d1;

           op[shortpitch*1] = b1 + c1;
           op[shortpitch*2] = b1 - c1;

           ip++;
           op++;
       }
   }

   void
   vp8_dixie_idct_add(unsigned char        *recon,
                      const unsigned char  *predict,
                      int                   stride,
                      const short          *coeffs)
   {
       int i;
       int a1, b1, c1, d1, temp1, temp2;
       short tmp[16];
       idct_columns(coeffs, tmp);
       coeffs = tmp;

       for (i = 0; i < 4; i++)
       {
           a1 = coeffs[0] + coeffs[2];
           b1 = coeffs[0] - coeffs[2];

           temp1 = (coeffs[1] * sinpi8sqrt2 + rounding) >> 16;
           temp2 = coeffs[3] +
               ((coeffs[3] * cospi8sqrt2minus1 + rounding) >> 16);
           c1 = temp1 - temp2;

           temp1 = coeffs[1] +
               ((coeffs[1] * cospi8sqrt2minus1 + rounding) >> 16);
           temp2 = (coeffs[3] * sinpi8sqrt2 + rounding) >> 16;
           d1 = temp1 + temp2;

           recon[0] = CLAMP_255(predict[0] + ((a1 + d1 + 4) >> 3));
           recon[3] = CLAMP_255(predict[3] + ((a1 - d1 + 4) >> 3));
           recon[1] = CLAMP_255(predict[1] + ((b1 + c1 + 4) >> 3));
           recon[2] = CLAMP_255(predict[2] + ((b1 - c1 + 4) >> 3));

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           coeffs += 4;
           recon += stride;
           predict += stride;
       }
   }

   ---- End code block ----------------------------------------

20.9.  idct_add.h

   ---- Begin code block --------------------------------------

   /*
    *  Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    *  Use of this source code is governed by a BSD-style license
    *  that can be found in the LICENSE file in the root of the source
    *  tree.  An additional intellectual property rights grant can be
    *  found in the file PATENTS.  All contributing project authors may
    *  be found in the AUTHORS file in the root of the source tree.
    */
   #ifndef IDCT_ADD_H
   #define IDCT_ADD_H

   void
   vp8_dixie_idct_add_init(struct vp8_decoder_ctx *ctx);

   void
   vp8_dixie_idct_add(unsigned char        *recon,
                      const unsigned char  *predict,
                      int                   stride,
                      const short          *coeffs);

   void
   vp8_dixie_walsh(const short *in, short *out);

   void
   vp8_dixie_idct_add_process_row(struct vp8_decoder_ctx *ctx,
                                  short                  *coeffs,
                                  unsigned int            row,
                                  unsigned int            start_col,
                                  unsigned int            num_cols);
   #endif

   ---- End code block ----------------------------------------

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20.10.  mem.h

   ---- Begin code block --------------------------------------

   /*
    *  Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    *  Use of this source code is governed by a BSD-style license
    *  that can be found in the LICENSE file in the root of the source
    *  tree.  An additional intellectual property rights grant can be
    *  found in the file PATENTS.  All contributing project authors may
    *  be found in the AUTHORS file in the root of the source tree.
    */
   #ifndef VPX_PORTS_MEM_H
   #define VPX_PORTS_MEM_H
   #include "vpx_config.h"
   #include "vpx_integer.h"

   #if defined(__GNUC__) && __GNUC__
   #define DECLARE_ALIGNED(n,typ,val)  typ val __attribute__ \
       ((aligned (n)))
   #elif defined(_MSC_VER)
   #define DECLARE_ALIGNED(n,typ,val)  __declspec(align(n)) typ val
   #else
   #warning No alignment directives known for this compiler.
   #define DECLARE_ALIGNED(n,typ,val)  typ val
   #endif
   #endif

   /* Declare an aligned array on the stack, for situations where the
    * stack pointer may not have the alignment we expect.  Creates an
    * array with a modified name, then defines val to be a pointer, and
    * aligns that pointer within the array.
    */
   #define DECLARE_ALIGNED_ARRAY(a,typ,val,n)\
   typ val##_[(n)+(a)/sizeof(typ)+1];\
   typ *val = (typ*)((((intptr_t)val##_)+(a)-1)&((intptr_t)-(a)))

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   /* Indicates that the usage of the specified variable has been
    * audited to assure that it's safe to use uninitialized.  Silences
    * 'may be used uninitialized' warnings on gcc.
    */
   #if defined(__GNUC__) && __GNUC__
   #define UNINITIALIZED_IS_SAFE(x) x=x
   #else
   #define UNINITIALIZED_IS_SAFE(x) x
   #endif

   ---- End code block ----------------------------------------

20.11.  modemv.c

   ---- Begin code block --------------------------------------

   /*
    *  Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    *  Use of this source code is governed by a BSD-style license
    *  that can be found in the LICENSE file in the root of the source
    *  tree.  An additional intellectual property rights grant can be
    *  found in the file PATENTS.  All contributing project authors may
    *  be found in the AUTHORS file in the root of the source tree.
    */
   #include "dixie.h"
   #include "modemv_data.h"
   #include <stdlib.h>
   #include <assert.h>

   struct mv_clamp_rect
   {
       int to_left, to_right, to_top, to_bottom;
   };

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RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static union mv
           clamp_mv(union mv raw, const struct mv_clamp_rect *bounds)
   {
       union mv newmv;

       newmv.d.x = (raw.d.x < bounds->to_left)
                   ? bounds->to_left : raw.d.x;
       newmv.d.x = (raw.d.x > bounds->to_right)
                   ? bounds->to_right : newmv.d.x;
       newmv.d.y = (raw.d.y < bounds->to_top)
                   ? bounds->to_top : raw.d.y;
       newmv.d.y = (raw.d.y > bounds->to_bottom)
                   ? bounds->to_bottom : newmv.d.y;
       return newmv;
   }

   static int
   read_segment_id(struct bool_decoder *bool,
                   struct vp8_segment_hdr *seg)
   {
       return bool_get(bool, seg->tree_probs[0])
              ? 2 + bool_get(bool, seg->tree_probs[2])
              : bool_get(bool, seg->tree_probs[1]);
   }

   static enum prediction_mode
   above_block_mode(const struct mb_info *this,
                    const struct mb_info *above,
                    unsigned int b)
   {
       if (b < 4)
       {
           switch (above->base.y_mode)
           {
           case DC_PRED:
               return B_DC_PRED;
           case V_PRED:
               return B_VE_PRED;
           case H_PRED:
               return B_HE_PRED;

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           case TM_PRED:
               return B_TM_PRED;
           case B_PRED:
               return above->split.modes[b+12];
           default:
               assert(0);
           }
       }

       return this->split.modes[b-4];
   }

   static enum prediction_mode
   left_block_mode(const struct mb_info *this,
                   const struct mb_info *left,
                   unsigned int b)
   {
       if (!(b & 3))
       {
           switch (left->base.y_mode)
           {
           case DC_PRED:
               return B_DC_PRED;
           case V_PRED:
               return B_VE_PRED;
           case H_PRED:
               return B_HE_PRED;
           case TM_PRED:
               return B_TM_PRED;
           case B_PRED:
               return left->split.modes[b+3];
           default:
               assert(0);
           }
       }

       return this->split.modes[b-1];
   }

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   static void
   decode_kf_mb_mode(struct mb_info      *this,
                     struct mb_info      *left,
                     struct mb_info      *above,
                     struct bool_decoder *bool)
   {
       int y_mode, uv_mode;

       y_mode = bool_read_tree(bool, kf_y_mode_tree, kf_y_mode_probs);

       if (y_mode == B_PRED)
       {
           unsigned int i;

           for (i = 0; i < 16; i++)
           {
               enum prediction_mode a = above_block_mode(this, above,
                                                         i);
               enum prediction_mode l = left_block_mode(this, left, i);
               enum prediction_mode b;

               b = bool_read_tree(bool, b_mode_tree,
                                  kf_b_mode_probs[a][l]);
               this->split.modes[i] = b;
           }
       }

       uv_mode = bool_read_tree(bool, uv_mode_tree, kf_uv_mode_probs);

       this->base.y_mode = y_mode;
       this->base.uv_mode = uv_mode;
       this->base.mv.raw = 0;
       this->base.ref_frame = 0;
   }

   static void
   decode_intra_mb_mode(struct mb_info         *this,
                        struct vp8_entropy_hdr *hdr,
                        struct bool_decoder    *bool)
   {
       /* Like decode_kf_mb_mode, but with probabilities transmitted in
        * the bitstream and no context on the above/left block mode.
        */
       int y_mode, uv_mode;

       y_mode = bool_read_tree(bool, y_mode_tree, hdr->y_mode_probs);

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       if (y_mode == B_PRED)
       {
           unsigned int i;

           for (i = 0; i < 16; i++)
           {
               enum prediction_mode b;

               b = bool_read_tree(bool, b_mode_tree,
                                  default_b_mode_probs);
               this->split.modes[i] = b;
           }
       }

       uv_mode = bool_read_tree(bool, uv_mode_tree, hdr->uv_mode_probs);

       this->base.y_mode = y_mode;
       this->base.uv_mode = uv_mode;
       this->base.mv.raw = 0;
       this->base.ref_frame = CURRENT_FRAME;
   }

   static int
   read_mv_component(struct bool_decoder *bool,
                     const unsigned char  mvc[MV_PROB_CNT])
   {
       enum {IS_SHORT, SIGN, SHORT, BITS = SHORT + 8 - 1,
             LONG_WIDTH = 10};
       int x = 0;

       if (bool_get(bool, mvc[IS_SHORT])) /* Large */
       {
           int i = 0;

           for (i = 0; i < 3; i++)
               x += bool_get(bool, mvc[BITS + i]) << i;

           /* Skip bit 3, which is sometimes implicit */
           for (i = LONG_WIDTH - 1; i > 3; i--)
               x += bool_get(bool, mvc[BITS + i]) << i;

           if (!(x & 0xFFF0)  ||  bool_get(bool, mvc[BITS + 3]))
               x += 8;
       }
       else   /* small */
           x = bool_read_tree(bool, small_mv_tree, mvc + SHORT);

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       if (x && bool_get(bool, mvc[SIGN]))
           x = -x;

       return x << 1;
   }

   static mv_t
   above_block_mv(const struct mb_info *this,
                  const struct mb_info *above,
                  unsigned int          b)
   {
       if (b < 4)
       {
           if (above->base.y_mode == SPLITMV)
               return above->split.mvs[b+12];

           return above->base.mv;
       }

       return this->split.mvs[b-4];
   }

   static mv_t
   left_block_mv(const struct mb_info *this,
                 const struct mb_info *left,
                 unsigned int          b)
   {
       if (!(b & 3))
       {
           if (left->base.y_mode == SPLITMV)
               return left->split.mvs[b+3];

           return left->base.mv;
       }

       return this->split.mvs[b-1];
   }

Bankoski, et al.              Informational                   [Page 181]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static enum prediction_mode
   submv_ref(struct bool_decoder *bool, union mv l, union mv a)
   {
       enum subblock_mv_ref
       {
           SUBMVREF_NORMAL,
           SUBMVREF_LEFT_ZED,
           SUBMVREF_ABOVE_ZED,
           SUBMVREF_LEFT_ABOVE_SAME,
           SUBMVREF_LEFT_ABOVE_ZED
       };

       int lez = !(l.raw);
       int aez = !(a.raw);
       int lea = l.raw == a.raw;
       enum subblock_mv_ref ctx = SUBMVREF_NORMAL;

       if (lea && lez)
           ctx = SUBMVREF_LEFT_ABOVE_ZED;
       else if (lea)
           ctx = SUBMVREF_LEFT_ABOVE_SAME;
       else if (aez)
           ctx = SUBMVREF_ABOVE_ZED;
       else if (lez)
           ctx = SUBMVREF_LEFT_ZED;

       return bool_read_tree(bool, submv_ref_tree,
                             submv_ref_probs2[ctx]);
   }

   static void
   read_mv(struct bool_decoder  *bool,
           union mv             *mv,
           mv_component_probs_t  mvc[2])
   {
       mv->d.y = read_mv_component(bool, mvc[0]);
       mv->d.x = read_mv_component(bool, mvc[1]);
   }

Bankoski, et al.              Informational                   [Page 182]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static void
   mv_bias(const struct mb_info *mb,
           const unsigned int   sign_bias[3],
           enum reference_frame ref_frame,
           union mv             *mv)
   {
       if (sign_bias[mb->base.ref_frame] ^ sign_bias[ref_frame])
       {
           mv->d.x *= -1;
           mv->d.y *= -1;
       }
   }

   enum near_mv_v
   {
       CNT_BEST = 0,
       CNT_ZEROZERO = 0,
       CNT_NEAREST,
       CNT_NEAR,
       CNT_SPLITMV
   };

   static void
   find_near_mvs(const struct mb_info   *this,
                 const struct mb_info   *left,
                 const struct mb_info   *above,
                 const unsigned int      sign_bias[3],
                 union  mv               near_mvs[4],
                 int                     cnt[4])
   {
       const struct mb_info *aboveleft = above - 1;
       union  mv             *mv = near_mvs;
       int                   *cntx = cnt;

       /* Zero accumulators */
       mv[0].raw = mv[1].raw = mv[2].raw = 0;
       cnt[0] = cnt[1] = cnt[2] = cnt[3] = 0;

Bankoski, et al.              Informational                   [Page 183]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

       /* Process above */
       if (above->base.ref_frame != CURRENT_FRAME)
       {
           if (above->base.mv.raw)
           {
               (++mv)->raw = above->base.mv.raw;
               mv_bias(above, sign_bias, this->base.ref_frame, mv);
               ++cntx;
           }

           *cntx += 2;
       }

       /* Process left */
       if (left->base.ref_frame != CURRENT_FRAME)
       {
           if (left->base.mv.raw)
           {
               union mv this_mv;

               this_mv.raw = left->base.mv.raw;
               mv_bias(left, sign_bias, this->base.ref_frame, &this_mv);

               if (this_mv.raw != mv->raw)
               {
                   (++mv)->raw = this_mv.raw;
                   ++cntx;
               }

               *cntx += 2;
           }
           else
               cnt[CNT_ZEROZERO] += 2;
       }

       /* Process above left */
       if (aboveleft->base.ref_frame != CURRENT_FRAME)
       {
           if (aboveleft->base.mv.raw)
           {
               union mv this_mv;

               this_mv.raw = aboveleft->base.mv.raw;
               mv_bias(aboveleft, sign_bias, this->base.ref_frame,
                       &this_mv);

Bankoski, et al.              Informational                   [Page 184]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

               if (this_mv.raw != mv->raw)
               {
                   (++mv)->raw = this_mv.raw;
                   ++cntx;
               }

               *cntx += 1;
           }
           else
               cnt[CNT_ZEROZERO] += 1;
       }

       /* If we have three distinct MVs ... */

       if (cnt[CNT_SPLITMV])
       {
           /* See if above-left MV can be merged with NEAREST */
           if (mv->raw == near_mvs[CNT_NEAREST].raw)
               cnt[CNT_NEAREST] += 1;
       }

       cnt[CNT_SPLITMV] = ((above->base.y_mode == SPLITMV)
                           + (left->base.y_mode == SPLITMV)) * 2
                          + (aboveleft->base.y_mode == SPLITMV);

       /* Swap near and nearest if necessary */
       if (cnt[CNT_NEAR] > cnt[CNT_NEAREST])
       {
           int tmp;
           tmp = cnt[CNT_NEAREST];
           cnt[CNT_NEAREST] = cnt[CNT_NEAR];
           cnt[CNT_NEAR] = tmp;
           tmp = near_mvs[CNT_NEAREST].raw;
           near_mvs[CNT_NEAREST].raw = near_mvs[CNT_NEAR].raw;
           near_mvs[CNT_NEAR].raw = tmp;
       }

       /* Use near_mvs[CNT_BEST] to store the "best" MV.  Note that this
        * storage shares the same address as near_mvs[CNT_ZEROZERO].
        */
       if (cnt[CNT_NEAREST] >= cnt[CNT_BEST])
           near_mvs[CNT_BEST] = near_mvs[CNT_NEAREST];
   }

Bankoski, et al.              Informational                   [Page 185]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static void
   decode_split_mv(struct mb_info         *this,
                   const struct mb_info   *left,
                   const struct mb_info   *above,
                   struct vp8_entropy_hdr *hdr,
                   union  mv              *best_mv,
                   struct bool_decoder    *bool)
   {
       const int *partition;
       int        j, k, mask, partition_id;

       partition_id = bool_read_tree(bool, split_mv_tree,
                                     split_mv_probs);
       partition = mv_partitions[partition_id];
       this->base.partitioning = partition_id;

       for (j = 0, mask = 0; mask < 65535; j++)
       {
           union mv mv, left_mv, above_mv;
           enum prediction_mode subblock_mode;

           /* Find the first subblock in this partition. */
           for (k = 0; j != partition[k]; k++);

           /* Decode the next MV */
           left_mv = left_block_mv(this, left, k);
           above_mv = above_block_mv(this, above, k);
           subblock_mode = submv_ref(bool, left_mv,  above_mv);

           switch (subblock_mode)
           {
           case LEFT4X4:
               mv = left_mv;
               break;
           case ABOVE4X4:
               mv = above_mv;
               break;
           case ZERO4X4:
               mv.raw = 0;
               break;
           case NEW4X4:
               read_mv(bool, &mv, hdr->mv_probs);
               mv.d.x += best_mv->d.x;
               mv.d.y += best_mv->d.y;
               break;
           default:
               assert(0);
           }

Bankoski, et al.              Informational                   [Page 186]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

           /* Fill the MVs for this partition */
           for (; k < 16; k++)
               if (j == partition[k])
               {
                   this->split.mvs[k] = mv;
                   mask |= 1 << k;
               }
       }
   }

   static int
   need_mc_border(union mv mv, int l, int t, int b_w, int w, int h)
   {
       int b, r;

       /* Get distance to edge for top-left pixel */
       l += (mv.d.x >> 3);
       t += (mv.d.y >> 3);

       /* Get distance to edge for bottom-right pixel */
       r = w - (l + b_w);
       b = h - (t + b_w);

       return (l >> 1 < 2 || r >> 1 < 3 || t >> 1 < 2 || b >> 1 < 3);
   }

   static void
   decode_mvs(struct vp8_decoder_ctx       *ctx,
              struct mb_info               *this,
              const struct mb_info         *left,
              const struct mb_info         *above,
              const struct mv_clamp_rect   *bounds,
              struct bool_decoder          *bool)
   {
       struct vp8_entropy_hdr *hdr = &ctx->entropy_hdr;
       union mv          near_mvs[4];
       union mv          clamped_best_mv;
       int               mv_cnts[4];
       unsigned char     probs[4];
       enum {BEST, NEAREST, NEAR};
       int x, y, w, h, b;

       this->base.ref_frame = bool_get(bool, hdr->prob_last)
                              ? 2 + bool_get(bool, hdr->prob_gf)
                              : 1;

Bankoski, et al.              Informational                   [Page 187]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

       find_near_mvs(this, this - 1, above,
                     ctx->reference_hdr.sign_bias, near_mvs, mv_cnts);
       probs[0] = mv_counts_to_probs[mv_cnts[0]][0];
       probs[1] = mv_counts_to_probs[mv_cnts[1]][1];
       probs[2] = mv_counts_to_probs[mv_cnts[2]][2];
       probs[3] = mv_counts_to_probs[mv_cnts[3]][3];

       this->base.y_mode = bool_read_tree(bool, mv_ref_tree, probs);
       this->base.uv_mode = this->base.y_mode;

       this->base.need_mc_border = 0;
       x = (-bounds->to_left - 128) >> 3;
       y = (-bounds->to_top - 128) >> 3;
       w = ctx->mb_cols * 16;
       h = ctx->mb_rows * 16;

       switch (this->base.y_mode)
       {
       case NEARESTMV:
           this->base.mv = clamp_mv(near_mvs[NEAREST], bounds);
           break;
       case NEARMV:
           this->base.mv = clamp_mv(near_mvs[NEAR], bounds);
           break;
       case ZEROMV:
           this->base.mv.raw = 0;
           return; //skip need_mc_border check
       case NEWMV:
           clamped_best_mv = clamp_mv(near_mvs[BEST], bounds);
           read_mv(bool, &this->base.mv, hdr->mv_probs);
           this->base.mv.d.x += clamped_best_mv.d.x;
           this->base.mv.d.y += clamped_best_mv.d.y;
           break;
       case SPLITMV:
       {
           union mv          chroma_mv[4] = {{{0}}};

           clamped_best_mv = clamp_mv(near_mvs[BEST], bounds);
           decode_split_mv(this, left, above, hdr, &clamped_best_mv,
                           bool);
           this->base.mv = this->split.mvs[15];

Bankoski, et al.              Informational                   [Page 188]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

           for (b = 0; b < 16; b++)
           {
               chroma_mv[(b>>1&1) + (b>>2&2)].d.x +=
                   this->split.mvs[b].d.x;
               chroma_mv[(b>>1&1) + (b>>2&2)].d.y +=
                   this->split.mvs[b].d.y;

               if (need_mc_border(this->split.mvs[b],
               x + (b & 3) * 4, y + (b & ~3), 4, w, h))
               {
                   this->base.need_mc_border = 1;
                   break;
               }
           }

           for (b = 0; b < 4; b++)
           {
               chroma_mv[b].d.x += 4 + 8 * (chroma_mv[b].d.x >> 31);
               chroma_mv[b].d.y += 4 + 8 * (chroma_mv[b].d.y >> 31);
               chroma_mv[b].d.x /= 4;
               chroma_mv[b].d.y /= 4;

               //note we're passing in non-subsampled coordinates

               if (need_mc_border(chroma_mv[b],
               x + (b & 1) * 8, y + (b >> 1) * 8, 16, w, h))
               {
                   this->base.need_mc_border = 1;
                   break;
               }
           }

           return; //skip need_mc_border check
       }
       default:
           assert(0);
       }

       if (need_mc_border(this->base.mv, x, y, 16, w, h))
           this->base.need_mc_border = 1;
   }

Bankoski, et al.              Informational                   [Page 189]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   void
   vp8_dixie_modemv_process_row(struct vp8_decoder_ctx *ctx,
   struct bool_decoder    *bool,
   int                     row,
   int                     start_col,
   int                     num_cols)
   {
       struct mb_info       *above, *this;
       unsigned int          col;
       struct mv_clamp_rect  bounds;

       this = ctx->mb_info_rows[row] + start_col;
       above = ctx->mb_info_rows[row - 1] + start_col;

       /* Calculate the eighth-pel MV bounds using a 1 MB border. */
       bounds.to_left   = -((start_col + 1) << 7);
       bounds.to_right  = (ctx->mb_cols - start_col) << 7;
       bounds.to_top    = -((row + 1) << 7);
       bounds.to_bottom = (ctx->mb_rows - row) << 7;

       for (col = start_col; col < start_col + num_cols; col++)
       {
           if (ctx->segment_hdr.update_map)
               this->base.segment_id = read_segment_id(bool,
               &ctx->segment_hdr);

           if (ctx->entropy_hdr.coeff_skip_enabled)
               this->base.skip_coeff = bool_get(bool,
               ctx->entropy_hdr.coeff_skip_prob);

           if (ctx->frame_hdr.is_keyframe)
           {
               if (!ctx->segment_hdr.update_map)
                   this->base.segment_id = 0;

               decode_kf_mb_mode(this, this - 1, above, bool);
           }
           else
           {
               if (bool_get(bool, ctx->entropy_hdr.prob_inter))
                   decode_mvs(ctx, this, this - 1, above, &bounds,
                              bool);
               else
                   decode_intra_mb_mode(this, &ctx->entropy_hdr, bool);

               bounds.to_left -= 16 << 3;
               bounds.to_right -= 16 << 3;
           }

Bankoski, et al.              Informational                   [Page 190]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

           /* Advance to next mb */
           this++;
           above++;
       }
   }

   void
   vp8_dixie_modemv_init(struct vp8_decoder_ctx *ctx)
   {
       unsigned int    mbi_w, mbi_h, i;
       struct mb_info *mbi;

       mbi_w = ctx->mb_cols + 1; /* For left border col */
       mbi_h = ctx->mb_rows + 1; /* For above border row */

       if (ctx->frame_hdr.frame_size_updated)
       {
           free(ctx->mb_info_storage);
           ctx->mb_info_storage = NULL;
           free(ctx->mb_info_rows_storage);
           ctx->mb_info_rows_storage = NULL;
       }

       if (!ctx->mb_info_storage)
           ctx->mb_info_storage = calloc(mbi_w * mbi_h,
           sizeof(*ctx->mb_info_storage));

       if (!ctx->mb_info_rows_storage)

           ctx->mb_info_rows_storage = calloc(mbi_h,
           sizeof(*ctx->mb_info_rows_storage));

       /* Set up row pointers */
       mbi = ctx->mb_info_storage + 1;

       for (i = 0; i < mbi_h; i++)
       {
           ctx->mb_info_rows_storage[i] = mbi;
           mbi += mbi_w;
       }

       ctx->mb_info_rows = ctx->mb_info_rows_storage + 1;
   }

Bankoski, et al.              Informational                   [Page 191]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   void
   vp8_dixie_modemv_destroy(struct vp8_decoder_ctx *ctx)
   {
       free(ctx->mb_info_storage);
       ctx->mb_info_storage = NULL;
       free(ctx->mb_info_rows_storage);
       ctx->mb_info_rows_storage = NULL;
   }

   ---- End code block ----------------------------------------

20.12.  modemv.h

   ---- Begin code block --------------------------------------

   /*
    *  Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    *  Use of this source code is governed by a BSD-style license
    *  that can be found in the LICENSE file in the root of the source
    *  tree.  An additional intellectual property rights grant can be
    *  found in the file PATENTS.  All contributing project authors may
    *  be found in the AUTHORS file in the root of the source tree.
    */
   #ifndef MODEMV_H
   #define MODEMV_H

   void
   vp8_dixie_modemv_init(struct vp8_decoder_ctx *ctx);

   void
   vp8_dixie_modemv_destroy(struct vp8_decoder_ctx *ctx);

   void
   vp8_dixie_modemv_process_row(struct vp8_decoder_ctx *ctx,
                                struct bool_decoder    *bool,
                                int                     row,
                                int                     start_col,
                                int                     num_cols);

   #endif

   ---- End code block ----------------------------------------

Bankoski, et al.              Informational                   [Page 192]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

20.13.  modemv_data.h

   ---- Begin code block --------------------------------------

   /*
    *  Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    *  Use of this source code is governed by a BSD-style license
    *  that can be found in the LICENSE file in the root of the source
    *  tree.  An additional intellectual property rights grant can be
    *  found in the file PATENTS.  All contributing project authors may
    *  be found in the AUTHORS file in the root of the source tree.
    */

   static const unsigned char kf_y_mode_probs[] = { 145, 156, 163, 128};
   static const unsigned char kf_uv_mode_probs[] = { 142, 114, 183};
   static const unsigned char kf_b_mode_probs[10][10][9] =
   {
     { /* above mode 0 */
       { /* left mode 0 */ 231, 120,  48,  89, 115, 113, 120, 152, 112},
       { /* left mode 1 */ 152, 179,  64, 126, 170, 118,  46,  70,  95},
       { /* left mode 2 */ 175,  69, 143,  80,  85,  82,  72, 155, 103},
       { /* left mode 3 */  56,  58,  10, 171, 218, 189,  17,  13, 152},
       { /* left mode 4 */ 144,  71,  10,  38, 171, 213, 144,  34,  26},
       { /* left mode 5 */ 114,  26,  17, 163,  44, 195,  21,  10, 173},
       { /* left mode 6 */ 121,  24,  80, 195,  26,  62,  44,  64,  85},
       { /* left mode 7 */ 170,  46,  55,  19, 136, 160,  33, 206,  71},
       { /* left mode 8 */  63,  20,   8, 114, 114, 208,  12,   9, 226},
       { /* left mode 9 */  81,  40,  11,  96, 182,  84,  29,  16,  36}
     },
     { /* above mode 1 */
       { /* left mode 0 */ 134, 183,  89, 137,  98, 101, 106, 165, 148},
       { /* left mode 1 */  72, 187, 100, 130, 157, 111,  32,  75,  80},
       { /* left mode 2 */  66, 102, 167,  99,  74,  62,  40, 234, 128},
       { /* left mode 3 */  41,  53,   9, 178, 241, 141,  26,   8, 107},
       { /* left mode 4 */ 104,  79,  12,  27, 217, 255,  87,  17,   7},
       { /* left mode 5 */  74,  43,  26, 146,  73, 166,  49,  23, 157},
       { /* left mode 6 */  65,  38, 105, 160,  51,  52,  31, 115, 128},
       { /* left mode 7 */  87,  68,  71,  44, 114,  51,  15, 186,  23},
       { /* left mode 8 */  47,  41,  14, 110, 182, 183,  21,  17, 194},
       { /* left mode 9 */  66,  45,  25, 102, 197, 189,  23,  18,  22}
     },
     { /* above mode 2 */
       { /* left mode 0 */  88,  88, 147, 150,  42,  46,  45, 196, 205},
       { /* left mode 1 */  43,  97, 183, 117,  85,  38,  35, 179,  61},
       { /* left mode 2 */  39,  53, 200,  87,  26,  21,  43, 232, 171},
       { /* left mode 3 */  56,  34,  51, 104, 114, 102,  29,  93,  77},
       { /* left mode 4 */ 107,  54,  32,  26,  51,   1,  81,  43,  31},

Bankoski, et al.              Informational                   [Page 193]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

       { /* left mode 5 */  39,  28,  85, 171,  58, 165,  90,  98,  64},
       { /* left mode 6 */  34,  22, 116, 206,  23,  34,  43, 166,  73},
       { /* left mode 7 */  68,  25, 106,  22,  64, 171,  36, 225, 114},
       { /* left mode 8 */  34,  19,  21, 102, 132, 188,  16,  76, 124},
       { /* left mode 9 */  62,  18,  78,  95,  85,  57,  50,  48,  51}
     },
     { /* above mode 3 */
       { /* left mode 0 */ 193, 101,  35, 159, 215, 111,  89,  46, 111},
       { /* left mode 1 */  60, 148,  31, 172, 219, 228,  21,  18, 111},
       { /* left mode 2 */ 112, 113,  77,  85, 179, 255,  38, 120, 114},
       { /* left mode 3 */  40,  42,   1, 196, 245, 209,  10,  25, 109},
       { /* left mode 4 */ 100,  80,   8,  43, 154,   1,  51,  26,  71},
       { /* left mode 5 */  88,  43,  29, 140, 166, 213,  37,  43, 154},
       { /* left mode 6 */  61,  63,  30, 155,  67,  45,  68,   1, 209},
       { /* left mode 7 */ 142,  78,  78,  16, 255, 128,  34, 197, 171},
       { /* left mode 8 */  41,  40,   5, 102, 211, 183,   4,   1, 221},
       { /* left mode 9 */  51,  50,  17, 168, 209, 192,  23,  25,  82}
     },
     { /* above mode 4 */
       { /* left mode 0 */ 125,  98,  42,  88, 104,  85, 117, 175,  82},
       { /* left mode 1 */  95,  84,  53,  89, 128, 100, 113, 101,  45},
       { /* left mode 2 */  75,  79, 123,  47,  51, 128,  81, 171,   1},
       { /* left mode 3 */  57,  17,   5,  71, 102,  57,  53,  41,  49},
       { /* left mode 4 */ 115,  21,   2,  10, 102, 255, 166,  23,   6},
       { /* left mode 5 */  38,  33,  13, 121,  57,  73,  26,   1,  85},
       { /* left mode 6 */  41,  10,  67, 138,  77, 110,  90,  47, 114},
       { /* left mode 7 */ 101,  29,  16,  10,  85, 128, 101, 196,  26},
       { /* left mode 8 */  57,  18,  10, 102, 102, 213,  34,  20,  43},
       { /* left mode 9 */ 117,  20,  15,  36, 163, 128,  68,   1,  26}
     },
     { /* above mode 5 */
       { /* left mode 0 */ 138,  31,  36, 171,  27, 166,  38,  44, 229},
       { /* left mode 1 */  67,  87,  58, 169,  82, 115,  26,  59, 179},
       { /* left mode 2 */  63,  59,  90, 180,  59, 166,  93,  73, 154},
       { /* left mode 3 */  40,  40,  21, 116, 143, 209,  34,  39, 175},
       { /* left mode 4 */  57,  46,  22,  24, 128,   1,  54,  17,  37},
       { /* left mode 5 */  47,  15,  16, 183,  34, 223,  49,  45, 183},
       { /* left mode 6 */  46,  17,  33, 183,   6,  98,  15,  32, 183},
       { /* left mode 7 */  65,  32,  73, 115,  28, 128,  23, 128, 205},
       { /* left mode 8 */  40,   3,   9, 115,  51, 192,  18,   6, 223},
       { /* left mode 9 */  87,  37,   9, 115,  59,  77,  64,  21,  47}
     },
     { /* above mode 6 */
       { /* left mode 0 */ 104,  55,  44, 218,   9,  54,  53, 130, 226},
       { /* left mode 1 */  64,  90,  70, 205,  40,  41,  23,  26,  57},
       { /* left mode 2 */  54,  57, 112, 184,   5,  41,  38, 166, 213},
       { /* left mode 3 */  30,  34,  26, 133, 152, 116,  10,  32, 134},
       { /* left mode 4 */  75,  32,  12,  51, 192, 255, 160,  43,  51},

Bankoski, et al.              Informational                   [Page 194]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

       { /* left mode 5 */  39,  19,  53, 221,  26, 114,  32,  73, 255},
       { /* left mode 6 */  31,   9,  65, 234,   2,  15,   1, 118,  73},
       { /* left mode 7 */  88,  31,  35,  67, 102,  85,  55, 186,  85},
       { /* left mode 8 */  56,  21,  23, 111,  59, 205,  45,  37, 192},
       { /* left mode 9 */  55,  38,  70, 124,  73, 102,   1,  34,  98}
     },
     { /* above mode 7 */
       { /* left mode 0 */ 102,  61,  71,  37,  34,  53,  31, 243, 192},
       { /* left mode 1 */  69,  60,  71,  38,  73, 119,  28, 222,  37},
       { /* left mode 2 */  68,  45, 128,  34,   1,  47,  11, 245, 171},
       { /* left mode 3 */  62,  17,  19,  70, 146,  85,  55,  62,  70},
       { /* left mode 4 */  75,  15,   9,   9,  64, 255, 184, 119,  16},
       { /* left mode 5 */  37,  43,  37, 154, 100, 163,  85, 160,   1},
       { /* left mode 6 */  63,   9,  92, 136,  28,  64,  32, 201,  85},
       { /* left mode 7 */  86,   6,  28,   5,  64, 255,  25, 248,   1},
       { /* left mode 8 */  56,   8,  17, 132, 137, 255,  55, 116, 128},
       { /* left mode 9 */  58,  15,  20,  82, 135,  57,  26, 121,  40}
     },
     { /* above mode 8 */
       { /* left mode 0 */ 164,  50,  31, 137, 154, 133,  25,  35, 218},
       { /* left mode 1 */  51, 103,  44, 131, 131, 123,  31,   6, 158},
       { /* left mode 2 */  86,  40,  64, 135, 148, 224,  45, 183, 128},
       { /* left mode 3 */  22,  26,  17, 131, 240, 154,  14,   1, 209},
       { /* left mode 4 */  83,  12,  13,  54, 192, 255,  68,  47,  28},
       { /* left mode 5 */  45,  16,  21,  91,  64, 222,   7,   1, 197},
       { /* left mode 6 */  56,  21,  39, 155,  60, 138,  23, 102, 213},
       { /* left mode 7 */  85,  26,  85,  85, 128, 128,  32, 146, 171},
       { /* left mode 8 */  18,  11,   7,  63, 144, 171,   4,   4, 246},
       { /* left mode 9 */  35,  27,  10, 146, 174, 171,  12,  26, 128}
     },
     { /* above mode 9 */
       { /* left mode 0 */ 190,  80,  35,  99, 180,  80, 126,  54,  45},
       { /* left mode 1 */  85, 126,  47,  87, 176,  51,  41,  20,  32},
       { /* left mode 2 */ 101,  75, 128, 139, 118, 146, 116, 128,  85},
       { /* left mode 3 */  56,  41,  15, 176, 236,  85,  37,   9,  62},
       { /* left mode 4 */ 146,  36,  19,  30, 171, 255,  97,  27,  20},
       { /* left mode 5 */  71,  30,  17, 119, 118, 255,  17,  18, 138},
       { /* left mode 6 */ 101,  38,  60, 138,  55,  70,  43,  26, 142},
       { /* left mode 7 */ 138,  45,  61,  62, 219,   1,  81, 188,  64},
       { /* left mode 8 */  32,  41,  20, 117, 151, 142,  20,  21, 163},
       { /* left mode 9 */ 112,  19,  12,  61, 195, 128,  48,   4,  24}
     }
   };

Bankoski, et al.              Informational                   [Page 195]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static const int kf_y_mode_tree[] =
   {
     -B_PRED, 2,
     4, 6,
     -DC_PRED, -V_PRED,
     -H_PRED, -TM_PRED
   };
   static const int y_mode_tree[] =
   {
     -DC_PRED, 2,
     4, 6,
     -V_PRED, -H_PRED,
     -TM_PRED, -B_PRED
   };
   static const int uv_mode_tree[6] =
   {
     -DC_PRED, 2,
     -V_PRED, 4,
     -H_PRED, -TM_PRED
   };
   static const int b_mode_tree[18] =
   {
     -B_DC_PRED, 2,               /* 0 = DC_NODE */
     -B_TM_PRED, 4,               /* 1 = TM_NODE */
     -B_VE_PRED, 6,               /* 2 = VE_NODE */
     8, 12,                       /* 3 = COM_NODE */
     -B_HE_PRED, 10,              /* 4 = HE_NODE */
     -B_RD_PRED, -B_VR_PRED,      /* 5 = RD_NODE */
     -B_LD_PRED, 14,              /* 6 = LD_NODE */
     -B_VL_PRED, 16,              /* 7 = VL_NODE */
     -B_HD_PRED, -B_HU_PRED       /* 8 = HD_NODE */
   };
   static const int small_mv_tree[14] =
   {
     2, 8,
     4, 6,
     -0, -1,
     -2, -3,
     10, 12,
     -4, -5,
     -6, -7
   };

Bankoski, et al.              Informational                   [Page 196]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static const int mv_ref_tree[8] =
   {
     -ZEROMV, 2,
     -NEARESTMV, 4,
     -NEARMV, 6,
     -NEWMV, -SPLITMV
   };
   static const int submv_ref_tree[6] =
   {
     -LEFT4X4, 2,
     -ABOVE4X4, 4,
     -ZERO4X4, -NEW4X4
   };
   static const int split_mv_tree[6] =
   {
     -3, 2,
     -2, 4,
     -0, -1
   };
   static const unsigned char default_b_mode_probs[] =
   { 120,  90,  79, 133,  87,  85,  80, 111, 151};
   static const unsigned char mv_counts_to_probs[6][4] =
   {
     {   7,   1,   1, 143 },
     {  14,  18,  14, 107 },
     { 135,  64,  57,  68 },
     {  60,  56, 128,  65 },
     { 159, 134, 128,  34 },
     { 234, 188, 128,  28 }

   };
   static const unsigned char split_mv_probs[3] =
   { 110, 111, 150};

   static const unsigned char submv_ref_probs2[5][3] =
   {
     { 147, 136, 18 },
     { 106, 145,  1 },
     { 179, 121,  1 },
     { 223,   1, 34 },
     { 208,   1,  1 }
   };

Bankoski, et al.              Informational                   [Page 197]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   const static int mv_partitions[4][16] =
   {
     {0, 0, 0, 0, 0, 0, 0, 0, 1, 1,  1,  1,  1,  1,  1,  1 },
     {0, 0, 1, 1, 0, 0, 1, 1, 0, 0,  1,  1,  0,  0,  1,  1 },
     {0, 0, 1, 1, 0, 0, 1, 1, 2, 2,  3,  3,  2,  2,  3,  3 },
     {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 }
   };

   ---- End code block ----------------------------------------

20.14.  predict.c

   ---- Begin code block --------------------------------------

   /*
    *  Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    *  Use of this source code is governed by a BSD-style license
    *  that can be found in the LICENSE file in the root of the source
    *  tree.  An additional intellectual property rights grant can be
    *  found in the file PATENTS.  All contributing project authors may
    *  be found in the AUTHORS file in the root of the source tree.
    */
   #include "dixie.h"
   #include "predict.h"
   #include "idct_add.h"
   #include "mem.h"
   #include <assert.h>
   #include <string.h>

   enum
   {
       BORDER_PIXELS     = 16,
   };

   static const filter_t sixtap_filters[8] =
   {
       { 0,   0, 128,    0,   0,  0 },
       { 0,  -6, 123,   12,  -1,  0 },
       { 2, -11, 108,   36,  -8,  1 },
       { 0,  -9,  93,   50,  -6,  0 },
       { 3, -16,  77,   77, -16,  3 },
       { 0,  -6,  50,   93,  -9,  0 },
       { 1,  -8,  36,  108, -11,  2 },
       { 0,  -1,  12,  123,  -6,  0 }
   };

Bankoski, et al.              Informational                   [Page 198]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static const filter_t bilinear_filters[8] =
   {
       { 0,  0,  128,    0,   0,  0 },
       { 0,  0,  112,   16,   0,  0 },
       { 0,  0,   96,   32,   0,  0 },
       { 0,  0,   80,   48,   0,  0 },
       { 0,  0,   64,   64,   0,  0 },
       { 0,  0,   48,   80,   0,  0 },
       { 0,  0,   32,   96,   0,  0 },
       { 0,  0,   16,  112,   0,  0 }
   };

   static void
   predict_h_nxn(unsigned char *predict,
                 int            stride,
                 int            n)
   {
       unsigned char *left = predict - 1;
       int            i, j;

       for (i = 0; i < n; i++)
           for (j = 0; j < n; j++)
               predict[i *stride + j] = left[i * stride];
   }

   static void
   predict_v_nxn(unsigned char *predict,
                 int            stride,
                 int            n)
   {
       unsigned char *above = predict - stride;
       int            i, j;

       for (i = 0; i < n; i++)
           for (j = 0; j < n; j++)
               predict[i *stride + j] = above[j];
   }

Bankoski, et al.              Informational                   [Page 199]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static void
   predict_tm_nxn(unsigned char *predict,
                  int            stride,
                  int            n)
   {
       /* Transposes the left column to the top row for later
        * consumption by the idct/recon stage
        */
       unsigned char *left = predict - 1;
       unsigned char *above = predict - stride;
       unsigned char  p = above[-1];
       int            i, j;

       for (j = 0; j < n; j++)
       {
           for (i = 0; i < n; i++)
               predict[i] = CLAMP_255(*left + above[i] - p);

           predict += stride;
           left += stride;
       }
   }

   static void
   predict_dc_nxn(unsigned char *predict,
                  int            stride,
                  int            n)
   {
       unsigned char *left = predict - 1;
       unsigned char *above = predict - stride;
       int            i, j, dc = 0;

       for (i = 0; i < n; i++)
       {
           dc += *left + above[i];
           left += stride;
       }

Bankoski, et al.              Informational                   [Page 200]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

       switch (n)
       {
       case 16:
           dc = (dc + 16) >> 5;
           break;
       case  8:
           dc = (dc + 8) >> 4;
           break;
       case  4:
           dc = (dc + 4) >> 3;
           break;
       }

       for (i = 0; i < n; i++)
           for (j = 0; j < n; j++)
               predict[i *stride + j] = dc;
   }

   static void
   predict_ve_4x4(unsigned char *predict,
                  int            stride)
   {
       unsigned char *above = predict - stride;
       int            i, j;

       predict[0] = (above[-1] + 2 * above[0] + above[1] + 2) >> 2;
       predict[1] = (above[ 0] + 2 * above[1] + above[2] + 2) >> 2;
       predict[2] = (above[ 1] + 2 * above[2] + above[3] + 2) >> 2;
       predict[3] = (above[ 2] + 2 * above[3] + above[4] + 2) >> 2;

       for (i = 1; i < 4; i++)
           for (j = 0; j < 4; j++)
               predict[i *stride + j] = predict[j];
   }

Bankoski, et al.              Informational                   [Page 201]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static void
   predict_he_4x4(unsigned char *predict,
                  int            stride)
   {
       unsigned char *left = predict - 1;

       predict[0] =
       predict[1] =
       predict[2] =
       predict[3] = (left[-stride] + 2 * left[0] +
                     left[stride] + 2) >> 2;
       predict += stride;
       left += stride;

       predict[0] =
       predict[1] =
       predict[2] =
       predict[3] = (left[-stride] + 2 * left[0] +
                     left[stride] + 2) >> 2;
       predict += stride;
       left += stride;

       predict[0] =
       predict[1] =
       predict[2] =
       predict[3] = (left[-stride] + 2 * left[0] +
                     left[stride] + 2) >> 2;
       predict += stride;
       left += stride;

       predict[0] =
       predict[1] =
       predict[2] =
       predict[3] = (left[-stride] + 2 * left[0] + left[0] + 2) >> 2;
   }

Bankoski, et al.              Informational                   [Page 202]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static void
   predict_ld_4x4(unsigned char *predict,
                  int            stride)
   {
       unsigned char *above = predict - stride;
       int            pred0, pred1, pred2, pred3, pred4, pred5, pred6;

       predict[0] = pred0 = (above[0] + 2 * above[1] +
                             above[2] + 2) >> 2;
       predict[1] = pred1 = (above[1] + 2 * above[2] +
                             above[3] + 2) >> 2;
       predict[2] = pred2 = (above[2] + 2 * above[3] +
                             above[4] + 2) >> 2;
       predict[3] = pred3 = (above[3] + 2 * above[4] +
                             above[5] + 2) >> 2;
       predict += stride;

       predict[0] = pred1;
       predict[1] = pred2;
       predict[2] = pred3;
       predict[3] = pred4 = (above[4] + 2 * above[5] +
                             above[6] + 2) >> 2;
       predict += stride;

       predict[0] = pred2;
       predict[1] = pred3;
       predict[2] = pred4;
       predict[3] = pred5 = (above[5] + 2 * above[6] +
                             above[7] + 2) >> 2;
       predict += stride;

       predict[0] = pred3;
       predict[1] = pred4;
       predict[2] = pred5;
       predict[3] = pred6 = (above[6] + 2 * above[7] +
                             above[7] + 2) >> 2;
   }

Bankoski, et al.              Informational                   [Page 203]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static void
   predict_rd_4x4(unsigned char *predict,
                  int            stride)
   {
       unsigned char *left = predict - 1;
       unsigned char *above = predict - stride;
       int            pred0, pred1, pred2, pred3, pred4, pred5, pred6;

       predict[0] = pred0 =
           (left[ 0] + 2 * above[-1] + above[0] + 2) >> 2;
       predict[1] = pred1 =
           (above[-1] + 2 * above[ 0] + above[1] + 2) >> 2;
       predict[2] = pred2 =
           (above[ 0] + 2 * above[ 1] + above[2] + 2) >> 2;
       predict[3] = pred3 =
           (above[ 1] + 2 * above[ 2] + above[3] + 2) >> 2;
       predict += stride;

       predict[0] = pred4 =
           (left[stride] + 2 * left[0] + above[-1] + 2) >> 2;
       predict[1] = pred0;
       predict[2] = pred1;
       predict[3] = pred2;
       predict += stride;

       predict[0] = pred5 =
           (left[stride*2] + 2 * left[stride] + left[0] + 2) >> 2;
       predict[1] = pred4;
       predict[2] = pred0;
       predict[3] = pred1;
       predict += stride;

       predict[0] = pred6 = (left[stride*3] + 2 * left[stride*2] +
                             left[stride] + 2) >> 2;
       predict[1] = pred5;
       predict[2] = pred4;
       predict[3] = pred0;
   }

Bankoski, et al.              Informational                   [Page 204]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static void
   predict_vr_4x4(unsigned char *predict,
                  int            stride)
   {
       unsigned char *left = predict - 1;
       unsigned char *above = predict - stride;
       int            pred0, pred1, pred2, pred3, pred4, pred5, pred6,
                      pred7, pred8, pred9;

       predict[0] = pred0 = (above[-1] + above[0] + 1) >> 1;
       predict[1] = pred1 = (above[ 0] + above[1] + 1) >> 1;
       predict[2] = pred2 = (above[ 1] + above[2] + 1) >> 1;
       predict[3] = pred3 = (above[ 2] + above[3] + 1) >> 1;
       predict += stride;

       predict[0] = pred4 = (left[ 0] + 2 * above[-1] +
                             above[0] + 2) >> 2;
       predict[1] = pred5 = (above[-1] + 2 * above[ 0] +
                             above[1] + 2) >> 2;
       predict[2] = pred6 = (above[ 0] + 2 * above[ 1] +
                             above[2] + 2) >> 2;
       predict[3] = pred7 = (above[ 1] + 2 * above[ 2] +
                             above[3] + 2) >> 2;
       predict += stride;

       predict[0] = pred8 =
           (left[stride] + 2 * left[0] + above[-1] + 2) >> 2;
       predict[1] = pred0;
       predict[2] = pred1;
       predict[3] = pred2;
       predict += stride;

       predict[0] = pred9 =
           (left[stride*2] + 2 * left[stride] + left[0] + 2) >> 2;
       predict[1] = pred4;
       predict[2] = pred5;
       predict[3] = pred6;
   }

Bankoski, et al.              Informational                   [Page 205]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static void
   predict_vl_4x4(unsigned char *predict,
                  int            stride)
   {
       unsigned char *above = predict - stride;
       int            pred0, pred1, pred2, pred3, pred4, pred5, pred6,
                      pred7, pred8, pred9;

       predict[0] = pred0 = (above[0] + above[1] + 1) >> 1;
       predict[1] = pred1 = (above[1] + above[2] + 1) >> 1;
       predict[2] = pred2 = (above[2] + above[3] + 1) >> 1;
       predict[3] = pred3 = (above[3] + above[4] + 1) >> 1;
       predict += stride;

       predict[0] = pred4 = (above[0] + 2 * above[1] +
                             above[2] + 2) >> 2;
       predict[1] = pred5 = (above[1] + 2 * above[2] +
                             above[3] + 2) >> 2;
       predict[2] = pred6 = (above[2] + 2 * above[3] +
                             above[4] + 2) >> 2;
       predict[3] = pred7 = (above[3] + 2 * above[4] +
                             above[5] + 2) >> 2;
       predict += stride;

       predict[0] = pred1;
       predict[1] = pred2;
       predict[2] = pred3;
       predict[3] = pred8 = (above[4] + 2 * above[5] +
                             above[6] + 2) >> 2;
       predict += stride;

       predict[0] = pred5;
       predict[1] = pred6;
       predict[2] = pred7;
       predict[3] = pred9 = (above[5] + 2 * above[6] +
                             above[7] + 2) >> 2;
   }

Bankoski, et al.              Informational                   [Page 206]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static void
   predict_hd_4x4(unsigned char *predict,
                  int            stride)
   {
       unsigned char *left = predict - 1;
       unsigned char *above = predict - stride;
       int            pred0, pred1, pred2, pred3, pred4, pred5, pred6,
                      pred7, pred8, pred9;

       predict[0] = pred0 = (left[ 0] + above[-1] + 1) >> 1;
       predict[1] = pred1 = (left[ 0] + 2 * above[-1] +
                             above[0] + 2) >> 2;
       predict[2] = pred2 = (above[-1] + 2 * above[ 0] +
                             above[1] + 2) >> 2;
       predict[3] = pred3 = (above[ 0] + 2 * above[ 1] +
                             above[2] + 2) >> 2;
       predict += stride;

       predict[0] = pred4 = (left[stride] + left[0] + 1) >> 1;
       predict[1] = pred5 = (left[stride] + 2 * left[0] +
                             above[-1] + 2) >> 2;
       predict[2] = pred0;
       predict[3] = pred1;
       predict += stride;

       predict[0] = pred6 = (left[stride*2] + left[stride] + 1) >> 1;
       predict[1] = pred7 = (left[stride*2] + 2 * left[stride] +
                             left[0] + 2) >> 2;
       predict[2] = pred4;
       predict[3] = pred5;
       predict += stride;

       predict[0] = pred8 = (left[stride*3] + left[stride*2] + 1) >> 1;
       predict[1] = pred9 = (left[stride*3] + 2 * left[stride*2] +
                             left[stride] + 2) >> 2;
       predict[2] = pred6;
       predict[3] = pred7;
   }

Bankoski, et al.              Informational                   [Page 207]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static void
   predict_hu_4x4(unsigned char *predict,
                  int            stride)
   {
       unsigned char *left = predict - 1;
       int            pred0, pred1, pred2, pred3, pred4, pred5, pred6;

       predict[0] = pred0 = (left[stride*0] +
                             left[stride*1] + 1) >> 1;
       predict[1] = pred1 = (left[stride*0] + 2 * left[stride*1] +
                             left[stride*2] + 2) >> 2;
       predict[2] = pred2 = (left[stride*1] + left[stride*2] + 1) >> 1;
       predict[3] = pred3 = (left[stride*1] + 2 * left[stride*2] +
                             left[stride*3] + 2) >> 2;
       predict += stride;

       predict[0] = pred2;
       predict[1] = pred3;
       predict[2] = pred4 = (left[stride*2] + left[stride*3] + 1) >> 1;
       predict[3] = pred5 = (left[stride*2] + 2 * left[stride*3] +
                             left[stride*3] + 2) >> 2;
       predict += stride;

       predict[0] = pred4;
       predict[1] = pred5;
       predict[2] = pred6 = left[stride*3];
       predict[3] = pred6;
       predict += stride;

       predict[0] = pred6;
       predict[1] = pred6;
       predict[2] = pred6;
       predict[3] = pred6;
   }

   static void
   predict_h_16x16(unsigned char *predict, int stride)
   {
       predict_h_nxn(predict, stride, 16);
   }

   static void
   predict_v_16x16(unsigned char *predict, int stride)
   {
       predict_v_nxn(predict, stride, 16);
   }

Bankoski, et al.              Informational                   [Page 208]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static void
   predict_tm_16x16(unsigned char *predict, int stride)
   {
       predict_tm_nxn(predict, stride, 16);
   }

   static void
   predict_h_8x8(unsigned char *predict, int stride)
   {
       predict_h_nxn(predict, stride, 8);
   }

   static void
   predict_v_8x8(unsigned char *predict, int stride)
   {
       predict_v_nxn(predict, stride, 8);
   }

   static void
   predict_tm_8x8(unsigned char *predict, int stride)
   {
       predict_tm_nxn(predict, stride, 8);
   }

   static void
   predict_tm_4x4(unsigned char *predict, int stride)
   {
       predict_tm_nxn(predict, stride, 4);
   }

   static void
   copy_down(unsigned char           *recon,
             int                      stride)
   {
       /* Copy the four pixels above-right of subblock 3 to
        * above-right of subblocks 7, 11, and 15
        */
       uint32_t tmp, *copy = (void *)(recon + 16 - stride);

Bankoski, et al.              Informational                   [Page 209]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

       stride = stride / sizeof(unsigned int);
       tmp = *copy;
       copy += stride * 4;
       *copy = tmp;
       copy += stride * 4;
       *copy = tmp;
       copy += stride * 4;
       *copy = tmp;
   }

   static void
   b_pred(unsigned char  *predict,
          int             stride,
          struct mb_info *mbi,
          short          *coeffs)
   {
       int i;

       copy_down(predict, stride);

       for (i = 0; i < 16; i++)
       {
           unsigned char *b_predict = predict + (i & 3) * 4;

           switch (mbi->split.modes[i])
           {
           case B_DC_PRED:
               predict_dc_nxn(b_predict, stride, 4);
               break;
           case B_TM_PRED:
               predict_tm_4x4(b_predict, stride);
               break;
           case B_VE_PRED:
               predict_ve_4x4(b_predict, stride);
               break;
           case B_HE_PRED:
               predict_he_4x4(b_predict, stride);
               break;
           case B_LD_PRED:
               predict_ld_4x4(b_predict, stride);
               break;
           case B_RD_PRED:
               predict_rd_4x4(b_predict, stride);
               break;
           case B_VR_PRED:
               predict_vr_4x4(b_predict, stride);
               break;

Bankoski, et al.              Informational                   [Page 210]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

           case B_VL_PRED:
               predict_vl_4x4(b_predict, stride);
               break;
           case B_HD_PRED:
               predict_hd_4x4(b_predict, stride);
               break;
           case B_HU_PRED:
               predict_hu_4x4(b_predict, stride);
               break;
           default:
               assert(0);
           }

           vp8_dixie_idct_add(b_predict, b_predict, stride, coeffs);
           coeffs += 16;

           if ((i & 3) == 3)
           {
               predict += stride * 4;
           }
       }
   }

   static void

   fixup_dc_coeffs(struct mb_info *mbi,
                   short          *coeffs)
   {
       short y2[16];
       int   i;

       vp8_dixie_walsh(coeffs + 24 * 16, y2);

       for (i = 0; i < 16; i++)
           coeffs[i*16] = y2[i];
   }

   static void
   predict_intra_luma(unsigned char   *predict,
                      int              stride,
                      struct mb_info  *mbi,
                      short           *coeffs)
   {
       if (mbi->base.y_mode == B_PRED)
           b_pred(predict, stride, mbi, coeffs);
       else

Bankoski, et al.              Informational                   [Page 211]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

       {
           int i;

           switch (mbi->base.y_mode)
           {
           case DC_PRED:
               predict_dc_nxn(predict, stride, 16);
               break;
           case V_PRED:
               predict_v_16x16(predict, stride);
               break;
           case H_PRED:
               predict_h_16x16(predict, stride);
               break;
           case TM_PRED:
               predict_tm_16x16(predict, stride);
               break;
           default:
               assert(0);
           }

           fixup_dc_coeffs(mbi, coeffs);

           for (i = 0; i < 16; i++)
           {
               vp8_dixie_idct_add(predict, predict, stride, coeffs);
               coeffs += 16;
               predict += 4;

               if ((i & 3) == 3)
                   predict += stride * 4 - 16;
           }
       }
   }

   static void
   predict_intra_chroma(unsigned char   *predict_u,
                        unsigned char   *predict_v,
                        int              stride,
                        struct mb_info  *mbi,
                        short           *coeffs)
   {
       int i;

Bankoski, et al.              Informational                   [Page 212]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

       switch (mbi->base.uv_mode)
       {
       case DC_PRED:
           predict_dc_nxn(predict_u, stride, 8);
           predict_dc_nxn(predict_v, stride, 8);
           break;
       case V_PRED:
           predict_v_8x8(predict_u, stride);
           predict_v_8x8(predict_v, stride);
           break;
       case H_PRED:
           predict_h_8x8(predict_u, stride);
           predict_h_8x8(predict_v, stride);
           break;
       case TM_PRED:
           predict_tm_8x8(predict_u, stride);
           predict_tm_8x8(predict_v, stride);
           break;
       default:
           assert(0);
       }

       coeffs += 16 * 16;

       for (i = 16; i < 20; i++)
       {
           vp8_dixie_idct_add(predict_u, predict_u, stride, coeffs);
           coeffs += 16;
           predict_u += 4;

           if (i & 1)
               predict_u += stride * 4 - 8;
       }

       for (i = 20; i < 24; i++)
       {
           vp8_dixie_idct_add(predict_v, predict_v, stride, coeffs);
           coeffs += 16;
           predict_v += 4;

           if (i & 1)
               predict_v += stride * 4 - 8;
       }
   }

Bankoski, et al.              Informational                   [Page 213]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static void
   sixtap_horiz(unsigned char       *output,
                int                  output_stride,
                const unsigned char *reference,
                int                  reference_stride,
                int                  cols,
                int                  rows,
                const filter_t       filter
               )
   {
       int r, c, temp;

       for (r = 0; r < rows; r++)
       {
           for (c = 0; c < cols; c++)
           {
               temp = (reference[-2] * filter[0]) +
                      (reference[-1] * filter[1]) +
                      (reference[ 0] * filter[2]) +
                      (reference[ 1] * filter[3]) +
                      (reference[ 2] * filter[4]) +
                      (reference[ 3] * filter[5]) +
                      64;
               temp >>= 7;
               output[c] = CLAMP_255(temp);
               reference++;
           }

           reference += reference_stride - cols;
           output += output_stride;
       }
   }

   static void
   sixtap_vert(unsigned char       *output,
               int                  output_stride,
               const unsigned char *reference,
               int                  reference_stride,
               int                  cols,
               int                  rows,
               const filter_t       filter
              )
   {
       int r, c, temp;

       for (r = 0; r < rows; r++)
       {

Bankoski, et al.              Informational                   [Page 214]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

           for (c = 0; c < cols; c++)
           {
               temp = (reference[-2*reference_stride] * filter[0]) +
                      (reference[-1*reference_stride] * filter[1]) +
                      (reference[ 0*reference_stride] * filter[2]) +
                      (reference[ 1*reference_stride] * filter[3]) +
                      (reference[ 2*reference_stride] * filter[4]) +
                      (reference[ 3*reference_stride] * filter[5]) +
                      64;
               temp >>= 7;
               output[c] = CLAMP_255(temp);
               reference++;
           }

           reference += reference_stride - cols;
           output += output_stride;
       }
   }

   static void
   sixtap_2d(unsigned char       *output,
             int                  output_stride,
             const unsigned char *reference,
             int                  reference_stride,
             int                  cols,
             int                  rows,
             int                  mx,
             int                  my,
             const filter_t       filters[8]
            )
   {
       DECLARE_ALIGNED(16, unsigned char, temp[16*(16+5)]);

       sixtap_horiz(temp, 16,
                    reference - 2 * reference_stride, reference_stride,
                    cols, rows + 5, filters[mx]);
       sixtap_vert(output, output_stride,
                   temp + 2 * 16, 16,
                   cols, rows, filters[my]);
   }

   struct img_index
   {
       unsigned char *y, *u, *v;
       int            stride, uv_stride;
   };

Bankoski, et al.              Informational                   [Page 215]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static const unsigned char *
   filter_block(unsigned char        *output,
                const unsigned char  *reference,
                int                   stride,
                const union mv       *mv,
                const filter_t        filters[8])
   {
       int mx, my;

       /* Handle 0,0 as a special case.  TODO: Does this make it any
        * faster?
        */
       if (!mv->raw)
           return reference;

       mx = mv->d.x & 7;
       my = mv->d.y & 7;
       reference += ((mv->d.y >> 3) * stride) + (mv->d.x >> 3);

       if (mx | my)
       {
           sixtap_2d(output, stride, reference, stride, 4, 4, mx, my,
                     filters);
           reference = output;
       }

       return reference;
   }

   static void
   recon_1_block(unsigned char        *output,
                 const unsigned char  *reference,
                 int                   stride,
                 const union mv       *mv,
                 const filter_t        filters[8],
                 short                *coeffs,
                 struct mb_info       *mbi,
                 int                   b
                )
   {
       const unsigned char *predict;

       predict = filter_block(output, reference, stride, mv, filters);
       vp8_dixie_idct_add(output, predict, stride, coeffs + 16 * b);
   }

Bankoski, et al.              Informational                   [Page 216]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static mv_t
   calculate_chroma_splitmv(struct mb_info *mbi,
                            int             b,
                            int             full_pixel)
   {
       int temp;
       union mv mv;

       temp = mbi->split.mvs[b].d.x +
              mbi->split.mvs[b+1].d.x +
              mbi->split.mvs[b+4].d.x +
              mbi->split.mvs[b+5].d.x;

       if (temp < 0)
           temp -= 4;
       else
           temp += 4;

       mv.d.x = temp / 8;

       temp = mbi->split.mvs[b].d.y +
              mbi->split.mvs[b+1].d.y +
              mbi->split.mvs[b+4].d.y +
              mbi->split.mvs[b+5].d.y;

       if (temp < 0)
           temp -= 4;
       else
           temp += 4;

       mv.d.y = temp / 8;

       if (full_pixel)
       {
           mv.d.x &= ~7;
           mv.d.y &= ~7;
       }

       return mv;
   }

Bankoski, et al.              Informational                   [Page 217]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   /* Note: We rely on the reconstructed border having the same stride
    * as the reference buffer because the filter_block can't adjust the
    * stride with its return value, only the reference pointer.
    */
   static void
   build_mc_border(unsigned char       *dst,
                   const unsigned char *src,
                   int                  stride,
                   int                  x,
                   int                  y,
                   int                  b_w,
                   int                  b_h,
                   int                  w,
                   int                  h
                  )
   {
       const unsigned char *ref_row;

       /* Get a pointer to the start of the real data for this row */
       ref_row = src - x - y * stride;

       if (y >= h)
           ref_row += (h - 1) * stride;
       else if (y > 0)
           ref_row += y * stride;

       do
       {
           int left, right = 0, copy;

           left = x < 0 ? -x : 0;

           if (left > b_w)
               left = b_w;

           if (x + b_w > w)
               right = x + b_w - w;

           if (right > b_w)
               right = b_w;

Bankoski, et al.              Informational                   [Page 218]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

           copy = b_w - left - right;

           if (left)
               memset(dst, ref_row[0], left);

           if (copy)
               memcpy(dst + left, ref_row + x + left, copy);

           if (right)
               memset(dst + left + copy, ref_row[w-1], right);

           dst += stride;
           y++;

           if (y < h && y > 0)
               ref_row += stride;
       }
       while (--b_h);
   }

   static void
   recon_1_edge_block(unsigned char        *output,
                      unsigned char        *emul_block,
                      const unsigned char  *reference,
                      int                   stride,
                      const union mv       *mv,
                      const filter_t        filters[8],
                      short                *coeffs,
                      struct mb_info       *mbi,
                      int                   x,
                      int                   y,
                      int                   w,
                      int                   h,
                      int                   start_b
                     )
   {
       const unsigned char *predict;
       int                  b = start_b;
       const int            b_w = 4;
       const int            b_h = 4;

       x += mv->d.x >> 3;
       y += mv->d.y >> 3;

Bankoski, et al.              Informational                   [Page 219]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

       /* Need two pixels left/above, 3 right/below for 6-tap */
       if (x < 2 || x + b_w - 1 + 3 >= w || y < 2 ||
           y + b_h - 1 + 3 >= h)
       {
           reference += (mv->d.x >> 3) + (mv->d.y >> 3) * stride;
           build_mc_border(emul_block,
                           reference - 2 - 2 * stride, stride,
                           x - 2, y - 2, b_w + 5, b_h + 5, w, h);
           reference = emul_block + 2 * stride + 2;
           reference -= (mv->d.x >> 3) + (mv->d.y >> 3) * stride;
       }

       predict = filter_block(output, reference, stride, mv, filters);
       vp8_dixie_idct_add(output, predict, stride, coeffs + 16 * b);
   }

   static void
   predict_inter_emulated_edge(struct vp8_decoder_ctx  *ctx,
                               struct img_index        *img,
                               short                   *coeffs,
                               struct mb_info          *mbi,
                               int                      mb_col,
                               int                      mb_row)
   {
       /* TODO: Move this into its own buffer.  This only works because
        * we still have a border allocated.
        */
       unsigned char *emul_block = ctx->frame_strg[0].img.img_data;
       unsigned char *reference;
       unsigned char *output;
       ptrdiff_t      reference_offset;
       int            w, h, x, y, b;
       union mv       chroma_mv[4];
       unsigned char *u = img->u, *v = img->v;
       int            full_pixel = ctx->frame_hdr.version == 3;

       x = mb_col * 16;
       y = mb_row * 16;
       w = ctx->mb_cols * 16;
       h = ctx->mb_rows * 16;

       output = img->y;
       reference_offset = ctx->ref_frame_offsets[mbi->base.ref_frame];
       reference = output + reference_offset;

Bankoski, et al.              Informational                   [Page 220]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

       if (mbi->base.y_mode != SPLITMV)
       {
           union mv uvmv;

           uvmv = mbi->base.mv;
           uvmv.d.x = (uvmv.d.x + 1 + (uvmv.d.x >> 31) * 2) / 2;
           uvmv.d.y = (uvmv.d.y + 1 + (uvmv.d.y >> 31) * 2) / 2;

           if (full_pixel)
           {
               uvmv.d.x &= ~7;
               uvmv.d.y &= ~7;
           }

           chroma_mv[0] = uvmv;
           chroma_mv[1] = uvmv;
           chroma_mv[2] = uvmv;
           chroma_mv[3] = uvmv;
       }
       else
       {
           chroma_mv[0] = calculate_chroma_splitmv(mbi,  0, full_pixel);
           chroma_mv[1] = calculate_chroma_splitmv(mbi,  2, full_pixel);
           chroma_mv[2] = calculate_chroma_splitmv(mbi,  8, full_pixel);
           chroma_mv[3] = calculate_chroma_splitmv(mbi, 10, full_pixel);
       }

       /* Luma */
       for (b = 0; b < 16; b++)
       {
           union mv *ymv;

           if (mbi->base.y_mode != SPLITMV)
               ymv = &mbi->base.mv;
           else
               ymv = mbi->split.mvs + b;

           recon_1_edge_block(output, emul_block, reference,
               img->stride, ymv, ctx->subpixel_filters, coeffs,
               mbi, x, y, w, h, b);

           x += 4;
           output += 4;
           reference += 4;

Bankoski, et al.              Informational                   [Page 221]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

           if ((b & 3) == 3)
           {
               x -= 16;
               y += 4;
               output += 4 * img->stride - 16;
               reference += 4 * img->stride - 16;
           }
       }

       x = mb_col * 16;
       y = mb_row * 16;

       /* Chroma */
       x >>= 1;
       y >>= 1;
       w >>= 1;
       h >>= 1;

       for (b = 0; b < 4; b++)
       {
           recon_1_edge_block(u, emul_block, u + reference_offset,
                              img->uv_stride,
                              &chroma_mv[b], ctx->subpixel_filters,
                              coeffs, mbi, x, y, w, h, b + 16);
           recon_1_edge_block(v, emul_block, v + reference_offset,
                              img->uv_stride,
                              &chroma_mv[b], ctx->subpixel_filters,
                              coeffs, mbi, x, y, w, h, b + 20);
           u += 4;
           v += 4;
           x += 4;

           if (b & 1)
           {
               x -= 8;
               y += 4;
               u += 4 * img->uv_stride - 8;
               v += 4 * img->uv_stride - 8;
           }
       }

   }

   static void
   predict_inter(struct vp8_decoder_ctx  *ctx,
                 struct img_index        *img,
                 short                   *coeffs,
                 struct mb_info          *mbi)

Bankoski, et al.              Informational                   [Page 222]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   {
       unsigned char *y = img->y;
       unsigned char *u = img->u;
       unsigned char *v = img->v;
       ptrdiff_t      reference_offset;
       union mv       chroma_mv[4];
       int            full_pixel = ctx->frame_hdr.version == 3;
       int b;

       if (mbi->base.y_mode != SPLITMV)
       {
           union mv             uvmv;

           uvmv = mbi->base.mv;
           uvmv.d.x = (uvmv.d.x + 1 + (uvmv.d.x >> 31) * 2) / 2;
           uvmv.d.y = (uvmv.d.y + 1 + (uvmv.d.y >> 31) * 2) / 2;

           if (full_pixel)
           {
               uvmv.d.x &= ~7;
               uvmv.d.y &= ~7;
           }

           chroma_mv[0] =
               chroma_mv[1] =
                   chroma_mv[2] =
                       chroma_mv[3] = uvmv;
       }
       else
       {
           chroma_mv[0] = calculate_chroma_splitmv(mbi,  0, full_pixel);
           chroma_mv[1] = calculate_chroma_splitmv(mbi,  2, full_pixel);
           chroma_mv[2] = calculate_chroma_splitmv(mbi,  8, full_pixel);
           chroma_mv[3] = calculate_chroma_splitmv(mbi, 10, full_pixel);
       }

       reference_offset = ctx->ref_frame_offsets[mbi->base.ref_frame];

       for (b = 0; b < 16; b++)
       {
           union mv *ymv;

           if (mbi->base.y_mode != SPLITMV)
               ymv = &mbi->base.mv;
           else
               ymv = mbi->split.mvs + b;
           recon_1_block(y, y + reference_offset, img->stride,
                         ymv, ctx->subpixel_filters, coeffs, mbi, b);

Bankoski, et al.              Informational                   [Page 223]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

           y += 4;

           if ((b & 3) == 3)
               y += 4 * img->stride - 16;
       }

       for (b = 0; b < 4; b++)
       {
           recon_1_block(u, u + reference_offset,
                         img->uv_stride, &chroma_mv[b],
                         ctx->subpixel_filters, coeffs, mbi, b + 16);
           recon_1_block(v, v + reference_offset,
                         img->uv_stride, &chroma_mv[b],
                         ctx->subpixel_filters, coeffs, mbi, b + 20);
           u += 4;
           v += 4;

           if (b & 1)
           {
               u += 4 * img->uv_stride - 8;
               v += 4 * img->uv_stride - 8;
           }
       }
   }

   void
   vp8_dixie_release_ref_frame(struct ref_cnt_img *rcimg)
   {
       if (rcimg)
       {
           assert(rcimg->ref_cnt);
           rcimg->ref_cnt--;
       }
   }

   struct ref_cnt_img *
   vp8_dixie_ref_frame(struct ref_cnt_img *rcimg)
   {
       rcimg->ref_cnt++;
       return rcimg;
   }

Bankoski, et al.              Informational                   [Page 224]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   struct ref_cnt_img *
   vp8_dixie_find_free_ref_frame(struct ref_cnt_img *frames)
   {
       int i;

       for (i = 0; i < NUM_REF_FRAMES; i++)
           if (frames[i].ref_cnt == 0)
           {
               frames[i].ref_cnt = 1;
               return &frames[i];
           }

       assert(0);
       return NULL;
   }

   static void
   fixup_left(unsigned char        *predict,
              int                   width,
              int                   stride,
              unsigned int          row,
              enum prediction_mode  mode)
   {
       /* The left column of out-of-frame pixels is taken to be 129,
        * unless we're doing DC_PRED, in which case we duplicate the
        * above row, unless this is also row 0, in which case we use
        * 129.
        */
       unsigned char *left = predict - 1;
       int i;

       if (mode == DC_PRED && row)
       {
           unsigned char *above = predict - stride;

           for (i = 0; i < width; i++)
           {
               *left = above[i];
               left += stride;
           }
       }
       else
       {
           /* Need to re-set the above row, in case the above MB was
            * DC_PRED.
            */
           left -= stride;

Bankoski, et al.              Informational                   [Page 225]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

           for (i = -1; i < width; i++)
           {
               *left = 129;
               left += stride;
           }
       }
   }

   static void
   fixup_above(unsigned char        *predict,
               int                   width,
               int                   stride,
               unsigned int          col,
               enum prediction_mode  mode)
   {
       /* The above row of out-of-frame pixels is taken to be 127,
        * unless we're doing DC_PRED, in which case we duplicate the
        * left col, unless this is also col 0, in which case we use
        * 127.
        */
       unsigned char *above = predict - stride;
       int i;

       if (mode == DC_PRED && col)
       {
           unsigned char *left = predict - 1;

           for (i = 0; i < width; i++)
           {
               above[i] = *left;
               left += stride;
           }
       }
       else
           /* Need to re-set the left col, in case the last MB was
            * DC_PRED.
            */
           memset(above - 1, 127, width + 1);

       memset(above + width, 127, 4); // for above-right subblock modes
   }

Bankoski, et al.              Informational                   [Page 226]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   void
   vp8_dixie_predict_init(struct vp8_decoder_ctx *ctx)
   {

       int i;
       unsigned char *this_frame_base;

       if (ctx->frame_hdr.frame_size_updated)
       {
           for (i = 0; i < NUM_REF_FRAMES; i++)
           {
               unsigned int w = ctx->mb_cols * 16 + BORDER_PIXELS * 2;
               unsigned int h = ctx->mb_rows * 16 + BORDER_PIXELS * 2;

               vpx_img_free(&ctx->frame_strg[i].img);
               ctx->frame_strg[i].ref_cnt = 0;
               ctx->ref_frames[i] = NULL;

               if (!vpx_img_alloc(&ctx->frame_strg[i].img,
                                  IMG_FMT_I420, w, h, 16))
                   vpx_internal_error(&ctx->error, VPX_CODEC_MEM_ERROR,
                                      "Failed to allocate %dx%d"
                                      " framebuffer",
                                      w, h);

               vpx_img_set_rect(&ctx->frame_strg[i].img, BORDER_PIXELS,
                   BORDER_PIXELS, ctx->frame_hdr.kf.w,
                   ctx->frame_hdr.kf.h);

           }

           if (ctx->frame_hdr.version)
               ctx->subpixel_filters = bilinear_filters;
           else
               ctx->subpixel_filters = sixtap_filters;
       }

       /* Find a free framebuffer to predict into */
       if (ctx->ref_frames[CURRENT_FRAME])
           vp8_dixie_release_ref_frame(ctx->ref_frames[CURRENT_FRAME]);

       ctx->ref_frames[CURRENT_FRAME] =
           vp8_dixie_find_free_ref_frame(ctx->frame_strg);
       this_frame_base = ctx->ref_frames[CURRENT_FRAME]->img.img_data;

Bankoski, et al.              Informational                   [Page 227]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

       /* Calculate offsets to the other reference frames */
       for (i = 0; i < NUM_REF_FRAMES; i++)
       {
           struct ref_cnt_img  *ref = ctx->ref_frames[i];

           ctx->ref_frame_offsets[i] =
               ref ? ref->img.img_data - this_frame_base : 0;
       }

       /* TODO: No need to do this on every frame... */
   }

   void
   vp8_dixie_predict_destroy(struct vp8_decoder_ctx *ctx)
   {
       int i;

       for (i = 0; i < NUM_REF_FRAMES; i++)
       {
           vpx_img_free(&ctx->frame_strg[i].img);
           ctx->frame_strg[i].ref_cnt = 0;
           ctx->ref_frames[i] = NULL;
       }
   }

   void
   vp8_dixie_predict_process_row(struct vp8_decoder_ctx *ctx,
                                 unsigned int            row,
                                 unsigned int            start_col,
                                 unsigned int            num_cols)
   {
       struct img_index img;
       struct mb_info *mbi;
       unsigned int    col;
       short          *coeffs;

Bankoski, et al.              Informational                   [Page 228]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

       /* Adjust pointers based on row, start_col */
       img.stride =
           ctx->ref_frames[CURRENT_FRAME]->img.stride[PLANE_Y];
       img.uv_stride =
           ctx->ref_frames[CURRENT_FRAME]->img.stride[PLANE_U];
       img.y = ctx->ref_frames[CURRENT_FRAME]->img.planes[PLANE_Y];
       img.u = ctx->ref_frames[CURRENT_FRAME]->img.planes[PLANE_U];
       img.v = ctx->ref_frames[CURRENT_FRAME]->img.planes[PLANE_V];
       img.y += (img.stride * row + start_col) * 16;
       img.u += (img.uv_stride * row + start_col) * 8;
       img.v += (img.uv_stride * row + start_col) * 8;
       mbi = ctx->mb_info_rows[row] + start_col;
       coeffs = ctx->tokens[row &
           (ctx->token_hdr.partitions - 1)].coeffs +
           25 * 16 * start_col;

       /* Fix up the out-of-frame pixels */

       if (start_col == 0)
       {
           fixup_left(img.y, 16, img.stride, row, mbi->base.y_mode);
           fixup_left(img.u, 8, img.uv_stride, row, mbi->base.uv_mode);
           fixup_left(img.v, 8, img.uv_stride, row, mbi->base.uv_mode);

           if (row == 0)
               *(img.y - img.stride - 1) = 127;
       }

       for (col = start_col; col < start_col + num_cols; col++)
       {
           if (row == 0)
           {
               fixup_above(img.y, 16, img.stride, col,
                           mbi->base.y_mode);
               fixup_above(img.u, 8, img.uv_stride, col,
                           mbi->base.uv_mode);
               fixup_above(img.v, 8, img.uv_stride, col,
                           mbi->base.uv_mode);
           }

           if (mbi->base.y_mode <= B_PRED)
           {
               predict_intra_luma(img.y, img.stride, mbi, coeffs);
               predict_intra_chroma(img.u, img.v, img.uv_stride, mbi,
                                    coeffs);
           }

Bankoski, et al.              Informational                   [Page 229]
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           else
           {
               if (mbi->base.y_mode != SPLITMV) // && != BPRED
                   fixup_dc_coeffs(mbi, coeffs);

               if (mbi->base.need_mc_border)
                   predict_inter_emulated_edge(ctx, &img, coeffs, mbi,
                                               col, row);
               else
                   predict_inter(ctx, &img, coeffs, mbi);
           }

           /* Advance to the next macroblock */
           mbi++;
           img.y += 16;
           img.u += 8;
           img.v += 8;
           coeffs += 25 * 16;
       }

       if (col == ctx->mb_cols)
       {
           /* Extend the last row by four pixels for intra-prediction.
            * This will be propagated later by copy_down.
            */
           uint32_t *extend = (uint32_t *)(img.y + 15 * img.stride);
           uint32_t  val = 0x01010101 * img.y[-1 + 15 * img.stride];
           *extend = val;
       }
   }

   ---- End code block ----------------------------------------

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20.15.  predict.h

   ---- Begin code block --------------------------------------

   /*
    *  Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    *  Use of this source code is governed by a BSD-style license
    *  that can be found in the LICENSE file in the root of the source
    *  tree.  An additional intellectual property rights grant can be
    *  found in the file PATENTS.  All contributing project authors may
    *  be found in the AUTHORS file in the root of the source tree.
    */
   #ifndef PREDICT_H
   #define PREDICT_H

   void
   vp8_dixie_predict_init(struct vp8_decoder_ctx *ctx);

   void
   vp8_dixie_predict_destroy(struct vp8_decoder_ctx *ctx);

   void
   vp8_dixie_predict_process_row(struct vp8_decoder_ctx *ctx,
                                 unsigned int            row,
                                 unsigned int            start_col,
                                 unsigned int            num_cols);

   void
   vp8_dixie_release_ref_frame(struct ref_cnt_img *rcimg);

   struct ref_cnt_img *
   vp8_dixie_ref_frame(struct ref_cnt_img *rcimg);

   struct ref_cnt_img *
   vp8_dixie_find_free_ref_frame(struct ref_cnt_img *frames);

   #endif

   ---- End code block ----------------------------------------

Bankoski, et al.              Informational                   [Page 231]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

20.16.  tokens.c

   ---- Begin code block --------------------------------------

   /*
    *  Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    *  Use of this source code is governed by a BSD-style license
    *  that can be found in the LICENSE file in the root of the source
    *  tree.  An additional intellectual property rights grant can be
    *  found in the file PATENTS.  All contributing project authors may
    *  be found in the AUTHORS file in the root of the source tree.
    */
   #include "vpx_codec_internal.h"
   #include "dixie.h"
   #include "tokens.h"
   #include <stdlib.h>
   #include <string.h>
   #include <malloc.h>

   enum
   {
       EOB_CONTEXT_NODE,
       ZERO_CONTEXT_NODE,
       ONE_CONTEXT_NODE,
       LOW_VAL_CONTEXT_NODE,
       TWO_CONTEXT_NODE,
       THREE_CONTEXT_NODE,
       HIGH_LOW_CONTEXT_NODE,
       CAT_ONE_CONTEXT_NODE,
       CAT_THREEFOUR_CONTEXT_NODE,
       CAT_THREE_CONTEXT_NODE,
       CAT_FIVE_CONTEXT_NODE
   };
   enum
   {
       ZERO_TOKEN,
       ONE_TOKEN,
       TWO_TOKEN,
       THREE_TOKEN,
       FOUR_TOKEN,
       DCT_VAL_CATEGORY1,
       DCT_VAL_CATEGORY2,
       DCT_VAL_CATEGORY3,

Bankoski, et al.              Informational                   [Page 232]
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       DCT_VAL_CATEGORY4,
       DCT_VAL_CATEGORY5,
       DCT_VAL_CATEGORY6,
       DCT_EOB_TOKEN,
       MAX_ENTROPY_TOKENS
   };
   struct extrabits
   {
       short         min_val;
       short         length;
       unsigned char probs[12];
   };
   static const unsigned int left_context_index[25] =
   {
       0, 0, 0, 0, 1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3,
       4, 4, 5, 5, 6, 6, 7, 7, 8
   };
   static const unsigned int above_context_index[25] =
   {
       0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3,
       4, 5, 4, 5, 6, 7, 6, 7, 8
   };
   #define X(n) ((n) * PREV_COEFF_CONTEXTS * ENTROPY_NODES)
   static const unsigned int bands_x[16] =
   {
       X(0), X(1), X(2), X(3), X(6), X(4), X(5), X(6),
       X(6), X(6), X(6), X(6), X(6), X(6), X(6), X(7)
   };
   #undef X
   static const struct extrabits extrabits[MAX_ENTROPY_TOKENS] =
   {
       { 0, -1, {  0,   0,   0,   0,   0,   0,
                   0,   0,   0,   0,   0,   0 } }, //ZERO_TOKEN
       { 1, 0,  {  0,   0,   0,   0,   0,   0,
                   0,   0,   0,   0,   0,   0 } }, //ONE_TOKEN
       { 2, 0,  {  0,   0,   0,   0,   0,   0,
                   0,   0,   0,   0,   0,   0 } }, //TWO_TOKEN
       { 3, 0,  {  0,   0,   0,   0,   0,   0,
                   0,   0,   0,   0,   0,   0 } }, //THREE_TOKEN
       { 4, 0,  {  0,   0,   0,   0,   0,   0,
                   0,   0,   0,   0,   0,   0 } }, //FOUR_TOKEN
       { 5, 0,  {159,   0,   0,   0,   0,   0,
                   0,   0,   0,   0,   0,   0 } }, //DCT_VAL_CATEGORY1
       { 7, 1,  {145, 165,   0,   0,   0,   0,
                   0,   0,   0,   0,   0,   0 } }, //DCT_VAL_CATEGORY2
       {11, 2,  {140, 148, 173,   0,   0,   0,
                   0,   0,   0,   0,   0,   0 } }, //DCT_VAL_CATEGORY3

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       {19, 3,  {135, 140, 155, 176,   0,   0,
                   0,   0,   0,   0,   0,   0 } }, //DCT_VAL_CATEGORY4
       {35, 4,  {130, 134, 141, 157, 180,   0,
                   0,   0,   0,   0,   0,   0 } }, //DCT_VAL_CATEGORY5
       {67, 10, {129, 130, 133, 140, 153, 177,
                 196, 230, 243, 254, 254,   0 } }, //DCT_VAL_CATEGORY6
       { 0, -1, {  0,   0,   0,   0,   0,   0,
                   0,   0,   0,   0,   0,   0 } }, // EOB TOKEN
   };

   static const unsigned int zigzag[16] =
   {
       0,  1,  4,  8,  5,  2,  3,  6,  9, 12, 13, 10,  7, 11, 14, 15
   };

   #define DECODE_AND_APPLYSIGN(value_to_sign) \
       v = (bool_get_bit(bool) ? -value_to_sign \
                               : value_to_sign) * dqf[!!c];

   #define DECODE_AND_BRANCH_IF_ZERO(probability,branch) \
       if (!bool_get(bool, probability)) goto branch;

   #define DECODE_AND_LOOP_IF_ZERO(probability,branch) \
       if (!bool_get(bool, probability)) \
       { \
           prob = type_probs; \
           if (c<15) {\
               ++c; \
               prob += bands_x[c]; \
               goto branch; \
           }\
           else \
               goto BLOCK_FINISHED; /* for malformed input */\
       }

   #define DECODE_SIGN_WRITE_COEFF_AND_CHECK_EXIT(val) \
       DECODE_AND_APPLYSIGN(val) \
       prob = type_probs + (ENTROPY_NODES*2); \
       if (c < 15){\
           b_tokens[zigzag[c]] = v; \
           ++c; \
           goto DO_WHILE; }\
       b_tokens[zigzag[15]] = v; \
       goto BLOCK_FINISHED;

Bankoski, et al.              Informational                   [Page 234]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   #define DECODE_EXTRABIT_AND_ADJUST_VAL(t,bits_count)\
       val += bool_get(bool, extrabits[t].probs[bits_count]) << \
       bits_count;

   static int
   decode_mb_tokens(struct bool_decoder  *bool,
                    token_entropy_ctx_t   left,
                    token_entropy_ctx_t   above,
                    short                *tokens,
                    enum prediction_mode  mode,
                    coeff_probs_table_t   probs,
                    short                 factor[TOKEN_BLOCK_TYPES][2])
   {
       int            i, stop, type;
       int            c, t, v;
       int            val, bits_count;
       int            eob_mask;
       short         *b_tokens;   // tokens for this block
       unsigned char *type_probs; // probabilities for this block type
       unsigned char *prob;
       short         *dqf;

       eob_mask = 0;

       if (mode != B_PRED && mode != SPLITMV)
       {
           i = 24;
           stop = 24;
           type = 1;
           b_tokens = tokens + 24 * 16;
           dqf = factor[TOKEN_BLOCK_Y2];
       }
       else
       {
           i = 0;
           stop = 16;
           type = 3;
           b_tokens = tokens;
           dqf = factor[TOKEN_BLOCK_Y1];
       }

       /* Save a pointer to the coefficient probs for the current type.
        * Need to repeat this whenever type changes.
        */
       type_probs = probs[type][0][0];

Bankoski, et al.              Informational                   [Page 235]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   BLOCK_LOOP:
       t = left[left_context_index[i]] + above[above_context_index[i]];
       c = !type; /* all blocks start at 0 except type 0, which starts
                   * at 1. */

       prob = type_probs;
       prob += t * ENTROPY_NODES;

   DO_WHILE:
       prob += bands_x[c];
       DECODE_AND_BRANCH_IF_ZERO(prob[EOB_CONTEXT_NODE],
         BLOCK_FINISHED);

   CHECK_0_:
       DECODE_AND_LOOP_IF_ZERO(prob[ZERO_CONTEXT_NODE], CHECK_0_);
       DECODE_AND_BRANCH_IF_ZERO(prob[ONE_CONTEXT_NODE],
                                 ONE_CONTEXT_NODE_0_);
       DECODE_AND_BRANCH_IF_ZERO(prob[LOW_VAL_CONTEXT_NODE],
                                 LOW_VAL_CONTEXT_NODE_0_);
       DECODE_AND_BRANCH_IF_ZERO(prob[HIGH_LOW_CONTEXT_NODE],
                                 HIGH_LOW_CONTEXT_NODE_0_);
       DECODE_AND_BRANCH_IF_ZERO(prob[CAT_THREEFOUR_CONTEXT_NODE],
                                 CAT_THREEFOUR_CONTEXT_NODE_0_);
       DECODE_AND_BRANCH_IF_ZERO(prob[CAT_FIVE_CONTEXT_NODE],
                                 CAT_FIVE_CONTEXT_NODE_0_);
       val = extrabits[DCT_VAL_CATEGORY6].min_val;
       bits_count = extrabits[DCT_VAL_CATEGORY6].length;

       do
       {
           DECODE_EXTRABIT_AND_ADJUST_VAL(DCT_VAL_CATEGORY6,
             bits_count);
           bits_count --;
       }
       while (bits_count >= 0);

       DECODE_SIGN_WRITE_COEFF_AND_CHECK_EXIT(val);

   CAT_FIVE_CONTEXT_NODE_0_:
       val = extrabits[DCT_VAL_CATEGORY5].min_val;
       DECODE_EXTRABIT_AND_ADJUST_VAL(DCT_VAL_CATEGORY5, 4);
       DECODE_EXTRABIT_AND_ADJUST_VAL(DCT_VAL_CATEGORY5, 3);
       DECODE_EXTRABIT_AND_ADJUST_VAL(DCT_VAL_CATEGORY5, 2);
       DECODE_EXTRABIT_AND_ADJUST_VAL(DCT_VAL_CATEGORY5, 1);
       DECODE_EXTRABIT_AND_ADJUST_VAL(DCT_VAL_CATEGORY5, 0);
       DECODE_SIGN_WRITE_COEFF_AND_CHECK_EXIT(val);

Bankoski, et al.              Informational                   [Page 236]
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   CAT_THREEFOUR_CONTEXT_NODE_0_:
       DECODE_AND_BRANCH_IF_ZERO(prob[CAT_THREE_CONTEXT_NODE],
                                 CAT_THREE_CONTEXT_NODE_0_);
       val = extrabits[DCT_VAL_CATEGORY4].min_val;
       DECODE_EXTRABIT_AND_ADJUST_VAL(DCT_VAL_CATEGORY4, 3);
       DECODE_EXTRABIT_AND_ADJUST_VAL(DCT_VAL_CATEGORY4, 2);
       DECODE_EXTRABIT_AND_ADJUST_VAL(DCT_VAL_CATEGORY4, 1);
       DECODE_EXTRABIT_AND_ADJUST_VAL(DCT_VAL_CATEGORY4, 0);
       DECODE_SIGN_WRITE_COEFF_AND_CHECK_EXIT(val);

   CAT_THREE_CONTEXT_NODE_0_:
       val = extrabits[DCT_VAL_CATEGORY3].min_val;
       DECODE_EXTRABIT_AND_ADJUST_VAL(DCT_VAL_CATEGORY3, 2);
       DECODE_EXTRABIT_AND_ADJUST_VAL(DCT_VAL_CATEGORY3, 1);
       DECODE_EXTRABIT_AND_ADJUST_VAL(DCT_VAL_CATEGORY3, 0);
       DECODE_SIGN_WRITE_COEFF_AND_CHECK_EXIT(val);

   HIGH_LOW_CONTEXT_NODE_0_:
       DECODE_AND_BRANCH_IF_ZERO(prob[CAT_ONE_CONTEXT_NODE],
                                 CAT_ONE_CONTEXT_NODE_0_);

       val = extrabits[DCT_VAL_CATEGORY2].min_val;
       DECODE_EXTRABIT_AND_ADJUST_VAL(DCT_VAL_CATEGORY2, 1);
       DECODE_EXTRABIT_AND_ADJUST_VAL(DCT_VAL_CATEGORY2, 0);
       DECODE_SIGN_WRITE_COEFF_AND_CHECK_EXIT(val);

   CAT_ONE_CONTEXT_NODE_0_:
       val = extrabits[DCT_VAL_CATEGORY1].min_val;
       DECODE_EXTRABIT_AND_ADJUST_VAL(DCT_VAL_CATEGORY1, 0);
       DECODE_SIGN_WRITE_COEFF_AND_CHECK_EXIT(val);

   LOW_VAL_CONTEXT_NODE_0_:
       DECODE_AND_BRANCH_IF_ZERO(prob[TWO_CONTEXT_NODE],
                                 TWO_CONTEXT_NODE_0_);
       DECODE_AND_BRANCH_IF_ZERO(prob[THREE_CONTEXT_NODE],
                                 THREE_CONTEXT_NODE_0_);
       DECODE_SIGN_WRITE_COEFF_AND_CHECK_EXIT(4);

   THREE_CONTEXT_NODE_0_:
       DECODE_SIGN_WRITE_COEFF_AND_CHECK_EXIT(3);

   TWO_CONTEXT_NODE_0_:
       DECODE_SIGN_WRITE_COEFF_AND_CHECK_EXIT(2);

   ONE_CONTEXT_NODE_0_:
       DECODE_AND_APPLYSIGN(1);
       prob = type_probs + ENTROPY_NODES;

Bankoski, et al.              Informational                   [Page 237]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

       if (c < 15)
       {
           b_tokens[zigzag[c]] = v;
           ++c;
           goto DO_WHILE;
       }

       b_tokens[zigzag[15]] = v;
   BLOCK_FINISHED:
       eob_mask |= (c > 1) << i;
       t = (c != !type);   // any non-zero data?
       eob_mask |= t << 31;

       left[left_context_index[i]] = above[above_context_index[i]] = t;
       b_tokens += 16;

       i++;

       if (i < stop)
           goto BLOCK_LOOP;

       if (i == 25)
       {
           type = 0;
           i = 0;
           stop = 16;
           type_probs = probs[type][0][0];
           b_tokens = tokens;
           dqf = factor[TOKEN_BLOCK_Y1];
           goto BLOCK_LOOP;
       }

       if (i == 16)
       {
           type = 2;
           type_probs = probs[type][0][0];
           stop = 24;
           dqf = factor[TOKEN_BLOCK_UV];
           goto BLOCK_LOOP;
       }

       return eob_mask;
   }

Bankoski, et al.              Informational                   [Page 238]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   static void
   reset_row_context(token_entropy_ctx_t *left)
   {
       memset(left, 0, sizeof(*left));
   }

   static void
   reset_above_context(token_entropy_ctx_t *above, unsigned int cols)
   {
       memset(above, 0, cols * sizeof(*above));
   }

   static void
   reset_mb_context(token_entropy_ctx_t  *left,
                    token_entropy_ctx_t  *above,
                    enum prediction_mode  mode)
   {
       /* Reset the macroblock context on the left and right.  We have
        * to preserve the context of the second order block if this mode
        * would not have updated it.
        */
       memset(left, 0, sizeof((*left)[0]) * 8);
       memset(above, 0, sizeof((*above)[0]) * 8);

       if (mode != B_PRED && mode != SPLITMV)
       {
           (*left)[8] = 0;
           (*above)[8] = 0;
       }
   }

   void
   vp8_dixie_tokens_process_row(struct vp8_decoder_ctx *ctx,
                                unsigned int            partition,
                                unsigned int            row,
                                unsigned int            start_col,
                                unsigned int            num_cols)
   {
       struct token_decoder *tokens = &ctx->tokens[partition];
       short              coeffs = tokens->coeffs + 25 * 16 * start_col;
       unsigned int       col;
       token_entropy_ctx_t  *above = ctx->above_token_entropy_ctx
                                     + start_col;
       token_entropy_ctx_t  *left = &tokens->left_token_entropy_ctx;
       struct mb_info       *mbi = ctx->mb_info_rows[row] + start_col;

Bankoski, et al.              Informational                   [Page 239]
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       if (row == 0)
           reset_above_context(above, num_cols);

       if (start_col == 0)
           reset_row_context(left);

       for (col = start_col; col < start_col + num_cols; col++)
       {
           memset(coeffs, 0, 25 * 16 * sizeof(short));

           if (mbi->base.skip_coeff)
           {
               reset_mb_context(left, above, mbi->base.y_mode);
               mbi->base.eob_mask = 0;
           }
           else
           {
               struct dequant_factors *dqf;

               dqf = ctx->dequant_factors  + mbi->base.segment_id;
               mbi->base.eob_mask =
                   decode_mb_tokens(&tokens->bool,
                                    *left, *above,
                                    coeffs,
                                    mbi->base.y_mode,
                                    ctx->entropy_hdr.coeff_probs,
                                    dqf->factor);
           }

           above++;
           mbi++;
           coeffs += 25 * 16;
       }
   }

   void
   vp8_dixie_tokens_init(struct vp8_decoder_ctx *ctx)
   {
       unsigned int  partitions = ctx->token_hdr.partitions;

       if (ctx->frame_hdr.frame_size_updated)
       {
           unsigned int i;
           unsigned int coeff_row_sz =
               ctx->mb_cols * 25 * 16 * sizeof(short);

Bankoski, et al.              Informational                   [Page 240]
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           for (i = 0; i < partitions; i++)
           {
               free(ctx->tokens[i].coeffs);
               ctx->tokens[i].coeffs = memalign(16, coeff_row_sz);

               if (!ctx->tokens[i].coeffs)
                   vpx_internal_error(&ctx->error, VPX_CODEC_MEM_ERROR,
                                      NULL);
           }

           free(ctx->above_token_entropy_ctx);
           ctx->above_token_entropy_ctx =
               calloc(ctx->mb_cols,
               sizeof(*ctx->above_token_entropy_ctx));

           if (!ctx->above_token_entropy_ctx)
               vpx_internal_error(&ctx->error,
               VPX_CODEC_MEM_ERROR, NULL);
       }
   }

   void
   vp8_dixie_tokens_destroy(struct vp8_decoder_ctx *ctx)
   {
       int i;

       for (i = 0; i < MAX_PARTITIONS; i++)
           free(ctx->tokens[i].coeffs);

       free(ctx->above_token_entropy_ctx);
   }

   ---- End code block ----------------------------------------

Bankoski, et al.              Informational                   [Page 241]
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20.17.  tokens.h

   ---- Begin code block --------------------------------------

   /*
    *  Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    *  Use of this source code is governed by a BSD-style license
    *  that can be found in the LICENSE file in the root of the source
    *  tree.  An additional intellectual property rights grant can be
    *  found in the file PATENTS.  All contributing project authors may
    *  be found in the AUTHORS file in the root of the source tree.
    */
   #ifndef TOKENS_H
   #define TOKENS_H

   void
   vp8_dixie_tokens_init(struct vp8_decoder_ctx *ctx);

   void
   vp8_dixie_tokens_destroy(struct vp8_decoder_ctx *ctx);

   void
   vp8_dixie_tokens_process_row(struct vp8_decoder_ctx *ctx,
                                unsigned int            partition,
                                unsigned int            row,
                                unsigned int            start_col,
                                unsigned int            num_cols);

   #endif

   ---- End code block ----------------------------------------

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20.18.  vp8_prob_data.h

   ---- Begin code block --------------------------------------

   /*
    *  Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    *  Use of this source code is governed by a BSD-style license
    *  that can be found in the LICENSE file in the root of the source
    *  tree.  An additional intellectual property rights grant can be
    *  found in the file PATENTS.  All contributing project authors may
    *  be found in the AUTHORS file in the root of the source tree.
    */

   static const
   unsigned char k_coeff_entropy_update_probs[BLOCK_TYPES][COEFF_BANDS]
   [PREV_COEFF_CONTEXTS]
   [ENTROPY_NODES] =
   {
       {
           {
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
           },
           {
               {176, 246, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {223, 241, 252, 255, 255, 255, 255, 255, 255, 255, 255},
               {249, 253, 253, 255, 255, 255, 255, 255, 255, 255, 255},
           },
           {
               {255, 244, 252, 255, 255, 255, 255, 255, 255, 255, 255},
               {234, 254, 254, 255, 255, 255, 255, 255, 255, 255, 255},
               {253, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
           },
           {
               {255, 246, 254, 255, 255, 255, 255, 255, 255, 255, 255},
               {239, 253, 254, 255, 255, 255, 255, 255, 255, 255, 255},
               {254, 255, 254, 255, 255, 255, 255, 255, 255, 255, 255},
           },
           {
               {255, 248, 254, 255, 255, 255, 255, 255, 255, 255, 255},
               {251, 255, 254, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
           },

Bankoski, et al.              Informational                   [Page 243]
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           {
               {255, 253, 254, 255, 255, 255, 255, 255, 255, 255, 255},
               {251, 254, 254, 255, 255, 255, 255, 255, 255, 255, 255},
               {254, 255, 254, 255, 255, 255, 255, 255, 255, 255, 255},
           },
           {
               {255, 254, 253, 255, 254, 255, 255, 255, 255, 255, 255},
               {250, 255, 254, 255, 254, 255, 255, 255, 255, 255, 255},
               {254, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
           },
           {
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
           },
       },
       {
           {
               {217, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {225, 252, 241, 253, 255, 255, 254, 255, 255, 255, 255},
               {234, 250, 241, 250, 253, 255, 253, 254, 255, 255, 255},
           },
           {
               {255, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {223, 254, 254, 255, 255, 255, 255, 255, 255, 255, 255},
               {238, 253, 254, 254, 255, 255, 255, 255, 255, 255, 255},
           },
           {
               {255, 248, 254, 255, 255, 255, 255, 255, 255, 255, 255},
               {249, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
           },
           {
               {255, 253, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {247, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
           },
           {
               {255, 253, 254, 255, 255, 255, 255, 255, 255, 255, 255},
               {252, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
           },
           {
               {255, 254, 254, 255, 255, 255, 255, 255, 255, 255, 255},
               {253, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
           },

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           {
               {255, 254, 253, 255, 255, 255, 255, 255, 255, 255, 255},
               {250, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {254, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
           },
           {
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
           },
       },
       {
           {
               {186, 251, 250, 255, 255, 255, 255, 255, 255, 255, 255},
               {234, 251, 244, 254, 255, 255, 255, 255, 255, 255, 255},
               {251, 251, 243, 253, 254, 255, 254, 255, 255, 255, 255},
           },
           {
               {255, 253, 254, 255, 255, 255, 255, 255, 255, 255, 255},
               {236, 253, 254, 255, 255, 255, 255, 255, 255, 255, 255},
               {251, 253, 253, 254, 254, 255, 255, 255, 255, 255, 255},
           },
           {
               {255, 254, 254, 255, 255, 255, 255, 255, 255, 255, 255},
               {254, 254, 254, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
           },
           {
               {255, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {254, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {254, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
           },
           {
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {254, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
           },
           {
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
           },
           {
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
           },

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           {
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
           },
       },
       {
           {
               {248, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {250, 254, 252, 254, 255, 255, 255, 255, 255, 255, 255},
               {248, 254, 249, 253, 255, 255, 255, 255, 255, 255, 255},
           },
           {
               {255, 253, 253, 255, 255, 255, 255, 255, 255, 255, 255},
               {246, 253, 253, 255, 255, 255, 255, 255, 255, 255, 255},
               {252, 254, 251, 254, 254, 255, 255, 255, 255, 255, 255},
           },
           {
               {255, 254, 252, 255, 255, 255, 255, 255, 255, 255, 255},
               {248, 254, 253, 255, 255, 255, 255, 255, 255, 255, 255},
               {253, 255, 254, 254, 255, 255, 255, 255, 255, 255, 255},
           },
           {
               {255, 251, 254, 255, 255, 255, 255, 255, 255, 255, 255},
               {245, 251, 254, 255, 255, 255, 255, 255, 255, 255, 255},
               {253, 253, 254, 255, 255, 255, 255, 255, 255, 255, 255},
           },
           {
               {255, 251, 253, 255, 255, 255, 255, 255, 255, 255, 255},
               {252, 253, 254, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 254, 255, 255, 255, 255, 255, 255, 255, 255, 255},
           },
           {
               {255, 252, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {249, 255, 254, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 254, 255, 255, 255, 255, 255, 255, 255, 255},
           },
           {
               {255, 255, 253, 255, 255, 255, 255, 255, 255, 255, 255},
               {250, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
           },

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           {
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {254, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
               {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255},
           },
       },
   };

   static const
   unsigned char k_default_y_mode_probs        [] =
   { 112,  86, 140,  37};

   static const
   unsigned char k_default_uv_mode_probs       [] =
   { 162, 101, 204};

   static const
   unsigned char k_default_coeff_probs [BLOCK_TYPES][COEFF_BANDS]
   [PREV_COEFF_CONTEXTS][ENTROPY_NODES] =
   {
       { /* block type 0 */
           { /* coeff band 0 */
               { 128, 128, 128, 128, 128, 128, 128, 128, 128, 128, 128},
               { 128, 128, 128, 128, 128, 128, 128, 128, 128, 128, 128},
               { 128, 128, 128, 128, 128, 128, 128, 128, 128, 128, 128}
           },
           { /* coeff band 1 */
               { 253, 136, 254, 255, 228, 219, 128, 128, 128, 128, 128},
               { 189, 129, 242, 255, 227, 213, 255, 219, 128, 128, 128},
               { 106, 126, 227, 252, 214, 209, 255, 255, 128, 128, 128}
           },
           { /* coeff band 2 */
               {   1,  98, 248, 255, 236, 226, 255, 255, 128, 128, 128},
               { 181, 133, 238, 254, 221, 234, 255, 154, 128, 128, 128},
               {  78, 134, 202, 247, 198, 180, 255, 219, 128, 128, 128}
           },
           { /* coeff band 3 */
               {   1, 185, 249, 255, 243, 255, 128, 128, 128, 128, 128},
               { 184, 150, 247, 255, 236, 224, 128, 128, 128, 128, 128},
               {  77, 110, 216, 255, 236, 230, 128, 128, 128, 128, 128}
           },

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           { /* coeff band 4 */
               {   1, 101, 251, 255, 241, 255, 128, 128, 128, 128, 128},
               { 170, 139, 241, 252, 236, 209, 255, 255, 128, 128, 128},
               {  37, 116, 196, 243, 228, 255, 255, 255, 128, 128, 128}
           },
           { /* coeff band 5 */
               {   1, 204, 254, 255, 245, 255, 128, 128, 128, 128, 128},
               { 207, 160, 250, 255, 238, 128, 128, 128, 128, 128, 128},
               { 102, 103, 231, 255, 211, 171, 128, 128, 128, 128, 128}
           },
           { /* coeff band 6 */
               {   1, 152, 252, 255, 240, 255, 128, 128, 128, 128, 128},
               { 177, 135, 243, 255, 234, 225, 128, 128, 128, 128, 128},
               {  80, 129, 211, 255, 194, 224, 128, 128, 128, 128, 128}
           },
           { /* coeff band 7 */
               {   1,   1, 255, 128, 128, 128, 128, 128, 128, 128, 128},
               { 246,   1, 255, 128, 128, 128, 128, 128, 128, 128, 128},
               { 255, 128, 128, 128, 128, 128, 128, 128, 128, 128, 128}
           }
       },
       { /* block type 1 */
           { /* coeff band 0 */
               { 198,  35, 237, 223, 193, 187, 162, 160, 145, 155,  62},
               { 131,  45, 198, 221, 172, 176, 220, 157, 252, 221,   1},
               {  68,  47, 146, 208, 149, 167, 221, 162, 255, 223, 128}
           },
           { /* coeff band 1 */
               {   1, 149, 241, 255, 221, 224, 255, 255, 128, 128, 128},
               { 184, 141, 234, 253, 222, 220, 255, 199, 128, 128, 128},
               {  81,  99, 181, 242, 176, 190, 249, 202, 255, 255, 128}
           },
           { /* coeff band 2 */
               {   1, 129, 232, 253, 214, 197, 242, 196, 255, 255, 128},
               {  99, 121, 210, 250, 201, 198, 255, 202, 128, 128, 128},
               {  23,  91, 163, 242, 170, 187, 247, 210, 255, 255, 128}
           },
           { /* coeff band 3 */
               {   1, 200, 246, 255, 234, 255, 128, 128, 128, 128, 128},
               { 109, 178, 241, 255, 231, 245, 255, 255, 128, 128, 128},
               {  44, 130, 201, 253, 205, 192, 255, 255, 128, 128, 128}
           },
           { /* coeff band 4 */
               {   1, 132, 239, 251, 219, 209, 255, 165, 128, 128, 128},
               {  94, 136, 225, 251, 218, 190, 255, 255, 128, 128, 128},
               {  22, 100, 174, 245, 186, 161, 255, 199, 128, 128, 128}
           },

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           { /* coeff band 5 */
               {   1, 182, 249, 255, 232, 235, 128, 128, 128, 128, 128},
               { 124, 143, 241, 255, 227, 234, 128, 128, 128, 128, 128},
               {  35,  77, 181, 251, 193, 211, 255, 205, 128, 128, 128}
           },
           { /* coeff band 6 */
               {   1, 157, 247, 255, 236, 231, 255, 255, 128, 128, 128},
               { 121, 141, 235, 255, 225, 227, 255, 255, 128, 128, 128},
               {  45,  99, 188, 251, 195, 217, 255, 224, 128, 128, 128}
           },
           { /* coeff band 7 */
               {   1,   1, 251, 255, 213, 255, 128, 128, 128, 128, 128},
               { 203,   1, 248, 255, 255, 128, 128, 128, 128, 128, 128},
               { 137,   1, 177, 255, 224, 255, 128, 128, 128, 128, 128}
           }
       },
       { /* block type 2 */
           { /* coeff band 0 */
               { 253,   9, 248, 251, 207, 208, 255, 192, 128, 128, 128},
               { 175,  13, 224, 243, 193, 185, 249, 198, 255, 255, 128},
               {  73,  17, 171, 221, 161, 179, 236, 167, 255, 234, 128}
           },
           { /* coeff band 1 */
               {   1,  95, 247, 253, 212, 183, 255, 255, 128, 128, 128},
               { 239,  90, 244, 250, 211, 209, 255, 255, 128, 128, 128},
               { 155,  77, 195, 248, 188, 195, 255, 255, 128, 128, 128}
           },
           { /* coeff band 2 */
               {   1,  24, 239, 251, 218, 219, 255, 205, 128, 128, 128},
               { 201,  51, 219, 255, 196, 186, 128, 128, 128, 128, 128},
               {  69,  46, 190, 239, 201, 218, 255, 228, 128, 128, 128}
           },
           { /* coeff band 3 */
               {   1, 191, 251, 255, 255, 128, 128, 128, 128, 128, 128},
               { 223, 165, 249, 255, 213, 255, 128, 128, 128, 128, 128},
               { 141, 124, 248, 255, 255, 128, 128, 128, 128, 128, 128}
           },
           { /* coeff band 4 */
               {   1,  16, 248, 255, 255, 128, 128, 128, 128, 128, 128},
               { 190,  36, 230, 255, 236, 255, 128, 128, 128, 128, 128},
               { 149,   1, 255, 128, 128, 128, 128, 128, 128, 128, 128}
           },
           { /* coeff band 5 */
               {   1, 226, 255, 128, 128, 128, 128, 128, 128, 128, 128},
               { 247, 192, 255, 128, 128, 128, 128, 128, 128, 128, 128},
               { 240, 128, 255, 128, 128, 128, 128, 128, 128, 128, 128}
           },

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           { /* coeff band 6 */
               {   1, 134, 252, 255, 255, 128, 128, 128, 128, 128, 128},
               { 213,  62, 250, 255, 255, 128, 128, 128, 128, 128, 128},
               {  55,  93, 255, 128, 128, 128, 128, 128, 128, 128, 128}
           },
           { /* coeff band 7 */
               { 128, 128, 128, 128, 128, 128, 128, 128, 128, 128, 128},
               { 128, 128, 128, 128, 128, 128, 128, 128, 128, 128, 128},
               { 128, 128, 128, 128, 128, 128, 128, 128, 128, 128, 128}
           }
       },
       { /* block type 3 */
           { /* coeff band 0 */
               { 202,  24, 213, 235, 186, 191, 220, 160, 240, 175, 255},
               { 126,  38, 182, 232, 169, 184, 228, 174, 255, 187, 128},
               {  61,  46, 138, 219, 151, 178, 240, 170, 255, 216, 128}
           },
           { /* coeff band 1 */
               {   1, 112, 230, 250, 199, 191, 247, 159, 255, 255, 128},
               { 166, 109, 228, 252, 211, 215, 255, 174, 128, 128, 128},
               {  39,  77, 162, 232, 172, 180, 245, 178, 255, 255, 128}
           },
           { /* coeff band 2 */
               {   1,  52, 220, 246, 198, 199, 249, 220, 255, 255, 128},
               { 124,  74, 191, 243, 183, 193, 250, 221, 255, 255, 128},
               {  24,  71, 130, 219, 154, 170, 243, 182, 255, 255, 128}
           },
           { /* coeff band 3 */
               {   1, 182, 225, 249, 219, 240, 255, 224, 128, 128, 128},
               { 149, 150, 226, 252, 216, 205, 255, 171, 128, 128, 128},
               {  28, 108, 170, 242, 183, 194, 254, 223, 255, 255, 128}
           },
           { /* coeff band 4 */
               {   1,  81, 230, 252, 204, 203, 255, 192, 128, 128, 128},
               { 123, 102, 209, 247, 188, 196, 255, 233, 128, 128, 128},
               {  20,  95, 153, 243, 164, 173, 255, 203, 128, 128, 128}
           },
           { /* coeff band 5 */
               {   1, 222, 248, 255, 216, 213, 128, 128, 128, 128, 128},
               { 168, 175, 246, 252, 235, 205, 255, 255, 128, 128, 128},
               {  47, 116, 215, 255, 211, 212, 255, 255, 128, 128, 128}
           },
           { /* coeff band 6 */
               {   1, 121, 236, 253, 212, 214, 255, 255, 128, 128, 128},
               { 141,  84, 213, 252, 201, 202, 255, 219, 128, 128, 128},
               {  42,  80, 160, 240, 162, 185, 255, 205, 128, 128, 128}
           },

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           { /* coeff band 7 */
               {   1,   1, 255, 128, 128, 128, 128, 128, 128, 128, 128},
               { 244,   1, 255, 128, 128, 128, 128, 128, 128, 128, 128},
               { 238,   1, 255, 128, 128, 128, 128, 128, 128, 128, 128}
           }
       }
   };

   static const
   unsigned char k_mv_entropy_update_probs[2][MV_PROB_CNT] =
   {
       {
           237,
           246,
           253, 253, 254, 254, 254, 254, 254,
           254, 254, 254, 254, 254, 250, 250, 252, 254, 254
       },
       {
           231,
           243,
           245, 253, 254, 254, 254, 254, 254,
           254, 254, 254, 254, 254, 251, 251, 254, 254, 254
       }
   };

   static const
   unsigned char k_default_mv_probs[2][MV_PROB_CNT] =
   {
       {                                                  // row
           162,                                           // is short
           128,                                           // sign
           225, 146, 172, 147, 214,  39, 156,             // short tree
           128, 129, 132,  75, 145, 178, 206, 239, 254, 254 // long bits
       },
       {
           164,
           128,
           204, 170, 119, 235, 140, 230, 228,
           128, 130, 130,  74, 148, 180, 203, 236, 254, 254

       }
   };

   ---- End code block ----------------------------------------

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20.19.  vpx_codec_internal.h

   ---- Begin code block --------------------------------------

   /*
    *  Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    *  Use of this source code is governed by a BSD-style license
    *  that can be found in the LICENSE file in the root of the source
    *  tree.  An additional intellectual property rights grant can be
    *  found in the file PATENTS.  All contributing project authors may
    *  be found in the AUTHORS file in the root of the source tree.
    */

   /*!\file vpx_codec_internal.h
    * \brief Describes the decoder algorithm interface for algorithm
    *        implementations.
    *
    * This file defines the private structures and data types that are
    * only relevant to implementing an algorithm, as opposed to using
    * it.
    *
    * To create a decoder algorithm class, an interface structure is put
    * into the global namespace:
    *     <pre>
    *     my_codec.c:
    *       vpx_codec_iface_t my_codec = {
    *           "My Codec v1.0",
    *           VPX_CODEC_ALG_ABI_VERSION,
    *           ...
    *       };
    *     </pre>
    *
    * An application instantiates a specific decoder instance by using
    * vpx_codec_init() and a pointer to the algorithm's interface
    * structure:
    *     <pre>
    *     my_app.c:
    *       extern vpx_codec_iface_t my_codec;
    *       {
    *           vpx_codec_ctx_t algo;
    *           res = vpx_codec_init(&algo, &my_codec);
    *       }
    *     </pre>
    *

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    * Once initialized, the instance is managed using other functions
    * from the vpx_codec_* family.
    */
   #ifndef VPX_CODEC_INTERNAL_H
   #define VPX_CODEC_INTERNAL_H
   #include "vpx_decoder.h"
   #include <stdarg.h>

   /*!\brief Current ABI version number
    *
    * \internal
    * If this file is altered in any way that changes the Application
    * Binary Interface (ABI), this value must be bumped.  Examples
    * include, but are not limited to, changing types, removing or
    * reassigning enums, adding/removing/rearranging fields to
    * structures.
    */
   #define VPX_CODEC_INTERNAL_ABI_VERSION (3)

   typedef struct vpx_codec_alg_priv  vpx_codec_alg_priv_t;

   /*!\brief init function pointer prototype
    *
    * Performs algorithm-specific initialization of the decoder context.
    * This function is called by the generic vpx_codec_init() wrapper
    * function, so plugins implementing this interface may trust the
    * input parameters to be properly initialized.
    *
    * \param[in] ctx   Pointer to this instance's context
    * \retval #VPX_CODEC_OK
    *     The input stream was recognized and decoder initialized.
    * \retval #VPX_CODEC_MEM_ERROR
    *     Memory operation failed.
    */
   typedef vpx_codec_err_t (*vpx_codec_init_fn_t)(vpx_codec_ctx_t *ctx);

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   /*!\brief destroy function pointer prototype
    *
    * Performs algorithm-specific destruction of the decoder context.
    * This function is called by the generic vpx_codec_destroy() wrapper
    * function, so plugins implementing this interface may trust the
    * input parameters to be properly initialized.
    *
    * \param[in] ctx   Pointer to this instance's context
    * \retval #VPX_CODEC_OK
    *     The input stream was recognized and decoder initialized.
    * \retval #VPX_CODEC_MEM_ERROR
    *     Memory operation failed.
    */
   typedef vpx_codec_err_t (*vpx_codec_destroy_fn_t)(
       vpx_codec_alg_priv_t *ctx);

   /*!\brief parse stream info function pointer prototype
    *
    * Performs high level parsing of the bitstream.  This function is
    * called by the generic vpx_codec_parse_stream() wrapper function,
    * so plugins implementing this interface may trust the input
    * parameters to be properly initialized.
    *
    * \param[in]      data    Pointer to a block of data to parse
    * \param[in]      data_sz Size of the data buffer
    * \param[in,out]  si      Pointer to stream info to update.  The
    *                         size member \ref MUST be properly
    *                         initialized, but \ref MAY be clobbered by
    *                         the algorithm.  This parameter \ref MAY
    *                         be NULL.
    *
    * \retval #VPX_CODEC_OK
    *     Bitstream is parsable and stream information updated
    */
   typedef vpx_codec_err_t (*vpx_codec_peek_si_fn_t)(
       const uint8_t         *data,
       unsigned int           data_sz,
       vpx_codec_stream_info_t *si);

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   /*!\brief Return information about the current stream.
    *
    * Returns information about the stream that has been parsed during
    * decoding.
    *
    * \param[in]      ctx     Pointer to this instance's context
    * \param[in,out]  si      Pointer to stream info to update.  The
    *                         size member \ref MUST be properly
    *                         initialized, but \ref MAY be clobbered by
    *                         the algorithm.  This parameter \ref MAY
    *                         be NULL.
    *
    * \retval #VPX_CODEC_OK
    *     Bitstream is parsable and stream information updated
    */
   typedef vpx_codec_err_t (*vpx_codec_get_si_fn_t)(
       vpx_codec_alg_priv_t    *ctx,
       vpx_codec_stream_info_t *si);

   /*!\brief control function pointer prototype
    *
    * This function is used to exchange algorithm-specific data with the
    * decoder instance.  This can be used to implement features specific
    * to a particular algorithm.
    *
    * This function is called by the generic vpx_codec_control() wrapper
    * function, so plugins implementing this interface may trust the
    * input parameters to be properly initialized.  However, this
    * interface does not provide type safety for the exchanged data or
    * assign meanings to the control codes.  Those details should be
    * specified in the algorithm's header file.  In particular, the
    * ctrl_id parameter is guaranteed to exist in the algorithm's
    * control mapping table, and the data parameter may be NULL.
    *
    *
    * \param[in]     ctx       Pointer to this instance's context
    * \param[in]     ctrl_id   Algorithm-specific control identifier
    * \param[in,out] data      Data to exchange with algorithm instance.
    *
    * \retval #VPX_CODEC_OK
    *     The internal state data was deserialized.
    */
   typedef vpx_codec_err_t (*vpx_codec_control_fn_t)(
       vpx_codec_alg_priv_t  *ctx,
       int                   ctrl_id,
       va_list               ap);

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   /*!\brief control function pointer mapping
    *
    * This structure stores the mapping between control identifiers and
    * implementing functions.  Each algorithm provides a list of these
    * mappings.  This list is searched by the vpx_codec_control()
    * wrapper function to determine which function to invoke.  The
    * special value {0, NULL} is used to indicate end-of-list, and must
    * be present.  The special value {0, <non-null>} can be used as a
    * catch-all mapping.  This implies that ctrl_id values chosen by the
    * algorithm \ref MUST be non-zero.
    */
   typedef const struct
   {
       int                    ctrl_id;
       vpx_codec_control_fn_t   fn;
   } vpx_codec_ctrl_fn_map_t;

   /*!\brief decode data function pointer prototype
    *
    * Processes a buffer of coded data.  If the processing results in a
    * new decoded frame becoming available, #VPX_CODEC_CB_PUT_SLICE and
    * #VPX_CODEC_CB_PUT_FRAME events are generated as appropriate.
    * This function is called by the generic vpx_codec_decode() wrapper
    * function, so plugins implementing this interface may trust the
    * input parameters to be properly initialized.
    *
    * \param[in] ctx         Pointer to this instance's context
    * \param[in] data        Pointer to this block of new coded data.
    *                        If NULL, a #VPX_CODEC_CB_PUT_FRAME event is
    *                        posted for the previously decoded frame.
    * \param[in] data_sz     Size of the coded data, in bytes.
    *
    * \return Returns #VPX_CODEC_OK if the coded data was processed
    *         completely and future pictures can be decoded without
    *         error.  Otherwise, see the descriptions of the other error
    *         codes in ::vpx_codec_err_t for recoverability
    *         capabilities.
    */
   typedef vpx_codec_err_t (*vpx_codec_decode_fn_t)(
       vpx_codec_alg_priv_t  *ctx,
       const uint8_t         *data,
       unsigned int     data_sz,
       void        *user_priv,
       long         deadline);

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   /*!\brief Decoded frames iterator
    *
    * Iterates over a list of the frames available for display.  The
    * iterator storage should be initialized to NULL to start the
    * iteration.  Iteration is complete when this function returns NULL.
    *
    * The list of available frames becomes valid upon completion of the
    * vpx_codec_decode call, and remains valid until the next call to
    * vpx_codec_decode.
    *
    * \param[in]     ctx      Pointer to this instance's context
    * \param[in out] iter     Iterator storage, initialized to NULL
    *
    * \return Returns a pointer to an image, if one is ready for
    *         display.  Frames produced will always be in PTS
    *         (presentation time stamp) order.
    */
   typedef vpx_image_t*(*vpx_codec_get_frame_fn_t)(
       vpx_codec_alg_priv_t *ctx,
       vpx_codec_iter_t     *iter);

   /*\brief External Memory Allocation memory map get iterator
    *
    * Iterates over a list of the memory maps requested by the decoder.
    * The iterator storage should be initialized to NULL to start the
    * iteration.  Iteration is complete when this function returns NULL.
    *
    * \param[in out] iter     Iterator storage, initialized to NULL
    *
    * \return Returns a pointer to a memory segment descriptor, or NULL
    *         to indicate end-of-list.
    */
   typedef vpx_codec_err_t (*vpx_codec_get_mmap_fn_t)(
       const vpx_codec_ctx_t      *ctx,
       vpx_codec_mmap_t           *mmap,
       vpx_codec_iter_t           *iter);

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   /*\brief External Memory Allocation memory map set iterator
    *
    * Sets a memory descriptor inside the decoder instance.
    *
    * \param[in] ctx      Pointer to this instance's context
    * \param[in] mmap     Memory map to store.
    *
    * \retval #VPX_CODEC_OK
    *     The memory map was accepted and stored.
    * \retval #VPX_CODEC_MEM_ERROR
    *     The memory map was rejected.
    */
   typedef vpx_codec_err_t (*vpx_codec_set_mmap_fn_t)(
       vpx_codec_ctx_t         *ctx,
       const vpx_codec_mmap_t  *mmap);

   typedef vpx_codec_err_t (*vpx_codec_encode_fn_t)(
       vpx_codec_alg_priv_t  *ctx,
       const vpx_image_t     *img,
       vpx_codec_pts_t        pts,
       unsigned long          duration,
       vpx_enc_frame_flags_t  flags,
       unsigned long          deadline);
   typedef const vpx_codec_cx_pkt_t*(*vpx_codec_get_cx_data_fn_t)(
       vpx_codec_alg_priv_t *ctx,
       vpx_codec_iter_t     *iter);

   typedef vpx_codec_err_t
   (*vpx_codec_enc_config_set_fn_t)(
       vpx_codec_alg_priv_t       *ctx,
       const vpx_codec_enc_cfg_t  *cfg);
   typedef vpx_fixed_buf_t *
   (*vpx_codec_get_global_headers_fn_t)(vpx_codec_alg_priv_t   *ctx);

   typedef vpx_image_t *
   (*vpx_codec_get_preview_frame_fn_t)(vpx_codec_alg_priv_t   *ctx);

   /*!\brief usage configuration mapping
    *
    * This structure stores the mapping between usage identifiers and
    * configuration structures.  Each algorithm provides a list of these
    * mappings.  This list is searched by the
    * vpx_codec_enc_config_default() wrapper function to determine which

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    * config to return.  The special value {-1, {0}} is used to indicate
    * end-of-list, and must be present.  At least one mapping must be
    * present, in addition to the end-of-list.
    *
    */
   typedef const struct
   {
       int                 usage;
       vpx_codec_enc_cfg_t cfg;
   } vpx_codec_enc_cfg_map_t;

   #define NOT_IMPLEMENTED 0

   /*!\brief Decoder algorithm interface
    *
    * All decoders \ref MUST expose a variable of this type.
    */
   struct vpx_codec_iface
   {
       const char               *name;
       int                       abi_version;
       vpx_codec_caps_t          caps;
       vpx_codec_init_fn_t       init;
       vpx_codec_destroy_fn_t    destroy;
       vpx_codec_ctrl_fn_map_t  *ctrl_maps;
       vpx_codec_get_mmap_fn_t   get_mmap;
       vpx_codec_set_mmap_fn_t   set_mmap;
       struct
       {
           vpx_codec_peek_si_fn_t    peek_si;
           vpx_codec_get_si_fn_t     get_si;
           vpx_codec_decode_fn_t     decode;
           vpx_codec_get_frame_fn_t  get_frame;
       } dec;
       struct
       {
           vpx_codec_enc_cfg_map_t           *cfg_maps;
           vpx_codec_encode_fn_t              encode;
           vpx_codec_get_cx_data_fn_t         get_cx_data;
           vpx_codec_enc_config_set_fn_t      cfg_set;
           vpx_codec_get_global_headers_fn_t  get_glob_hdrs;
           vpx_codec_get_preview_frame_fn_t   get_preview;
       } enc;
   };

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   /*!\brief Callback function pointer / user data pair storage */
   typedef struct vpx_codec_priv_cb_pair
   {
       union
       {
           vpx_codec_put_frame_cb_fn_t    put_frame;
           vpx_codec_put_slice_cb_fn_t    put_slice;
       };
       void                            *user_priv;
   } vpx_codec_priv_cb_pair_t;

   /*!\brief Instance private storage
    *
    * This structure is allocated by the algorithm's init function.  It
    * can be extended in one of two ways.  First, a second, algorithm
    * specific structure can be allocated and the priv member pointed to
    * it.  Alternatively, this structure can be made the first member of
    * the algorithm-specific structure, and the pointer casted to the
    * proper type.
    */
   struct vpx_codec_priv
   {
       unsigned int                    sz;
       vpx_codec_iface_t              *iface;
       struct vpx_codec_alg_priv      *alg_priv;
       const char                     *err_detail;
       vpx_codec_flags_t               init_flags;
       struct
       {
           vpx_codec_priv_cb_pair_t    put_frame_cb;
           vpx_codec_priv_cb_pair_t    put_slice_cb;
       } dec;
       struct
       {
           struct vpx_fixed_buf        cx_data_dst_buf;
           unsigned int                cx_data_pad_before;
           unsigned int                cx_data_pad_after;
           vpx_codec_cx_pkt_t          cx_data_pkt;
       } enc;
   };

   #undef VPX_CTRL_USE_TYPE
   #define VPX_CTRL_USE_TYPE(id, typ) \
       static typ id##__value(va_list args) \
       {return va_arg(args, typ);} \
       static typ id##__convert(void *x)\
       {\

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           union\
           {\
               void *x;\
               typ   d;\
           } u;\
           u.x = x;\
           return u.d;\
       }

   #undef VPX_CTRL_USE_TYPE_DEPRECATED
   #define VPX_CTRL_USE_TYPE_DEPRECATED(id, typ) \
       static typ id##__value(va_list args) \
       {return va_arg(args, typ);} \
       static typ id##__convert(void *x)\
       {\
           union\
           {\
               void *x;\
               typ   d;\
           } u;\
           u.x = x;\
           return u.d;\
       }

   #define CAST(id, arg) id##__value(arg)
   #define RECAST(id, x) id##__convert(x)

   /* Internal Utility Functions
    *
    * The following functions are intended to be used inside algorithms
    * as utilities for manipulating vpx_codec_* data structures.
    */
   struct vpx_codec_pkt_list
   {
       unsigned int            cnt;
       unsigned int            max;
       struct vpx_codec_cx_pkt pkts[1];
   };

   #define vpx_codec_pkt_list_decl(n)\
       union {struct vpx_codec_pkt_list head;\
           struct {struct vpx_codec_pkt_list head;\
               struct vpx_codec_cx_pkt    pkts[n];} alloc;}

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   #define vpx_codec_pkt_list_init(m)\
       (m)->alloc.head.cnt = 0,\
       (m)->alloc.head.max = \
       sizeof((m)->alloc.pkts) / sizeof((m)->alloc.pkts[0])

   int
   vpx_codec_pkt_list_add(struct vpx_codec_pkt_list *,
                          const struct vpx_codec_cx_pkt *);

   const vpx_codec_cx_pkt_t*
   vpx_codec_pkt_list_get(struct vpx_codec_pkt_list *list,
                          vpx_codec_iter_t           *iter);

   #include <stdio.h>
   #include <setjmp.h>
   struct vpx_internal_error_info
   {
       vpx_codec_err_t  error_code;
       int              has_detail;
       char             detail[80];
       int              setjmp;
       jmp_buf          jmp;
   };

   static void vpx_internal_error(struct vpx_internal_error_info *info,
                                  vpx_codec_err_t                 error,
                                  const char                     *fmt,
                                  ...)
   {
       va_list ap;

       info->error_code = error;
       info->has_detail = 0;

       if (fmt)
       {
           size_t  sz = sizeof(info->detail);

           info->has_detail = 1;
           va_start(ap, fmt);
           vsnprintf(info->detail, sz - 1, fmt, ap);
           va_end(ap);
           info->detail[sz-1] = '\0';
       }

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       if (info->setjmp)
           longjmp(info->jmp, info->error_code);
   }
   #endif

   ---- End code block ----------------------------------------

20.20.  vpx_decoder.h

   ---- Begin code block --------------------------------------

   /*
    *  Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    *  Use of this source code is governed by a BSD-style license
    *  that can be found in the LICENSE file in the root of the source
    *  tree.  An additional intellectual property rights grant can be
    *  found in the file PATENTS.  All contributing project authors may
    *  be found in the AUTHORS file in the root of the source tree.
    */

   /*!\defgroup decoder Decoder Algorithm Interface
    * \ingroup codec
    * This abstraction allows applications using this decoder to easily
    * support multiple video formats with minimal code duplication.
    * This section describes the interface common to all decoders.
    * @{
    */

   /*!\file vpx_decoder.h
    * \brief Describes the decoder algorithm interface to applications.
    *
    * This file describes the interface between an application and a
    * video decoder algorithm.
    *
    */
   #ifdef __cplusplus
   extern "C" {
   #endif
   #ifndef VPX_DECODER_H
   #define VPX_DECODER_H
   #include "vpx_codec.h"

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       /*!\brief Current ABI version number
        *
        * \internal
        * If this file is altered in any way that changes the ABI, this
        * value must be bumped.  Examples include, but are not limited
        * to, changing types, removing or reassigning enums,
        * adding/removing/rearranging fields to structures
        */
   #define VPX_DECODER_ABI_VERSION (2 + VPX_CODEC_ABI_VERSION)

       /*! \brief Decoder capabilities bitfield
        *
        *  Each decoder advertises the capabilities it supports as part
        *  of its ::vpx_codec_iface_t interface structure.  Capabilities
        *  are extra interfaces or functionality, and are not required
        *  to be supported by a decoder.
        *
        *  The available flags are specified by VPX_CODEC_CAP_* defines.
        */
   #define VPX_CODEC_CAP_PUT_SLICE  0x10000 /**< Will issue put_slice
       callbacks */
   #define VPX_CODEC_CAP_PUT_FRAME  0x20000 /**< Will issue put_frame
       callbacks */
   #define VPX_CODEC_CAP_POSTPROC   0x40000 /**< Can postprocess decoded
       frame */

       /*! \brief Initialization-time Feature Enabling
        *
        *  Certain codec features must be known at initialization time,
        *  to allow for proper memory allocation.
        *
        *  The available flags are specified by VPX_CODEC_USE_* defines.
        */
   #define VPX_CODEC_USE_POSTPROC   0x10000 /**< Postprocess decoded
       frame */

       /*!\brief Stream properties
        *
        * This structure is used to query or set properties of the
        * decoded stream.  Algorithms may extend this structure with
        * data specific to their bitstream by setting the sz member
        * appropriately.
        */

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       typedef struct vpx_codec_stream_info
       {
           unsigned int sz;    /**< Size of this structure */
           unsigned int w;     /**< Width (or 0 for unknown/default) */
           unsigned int h;     /**< Height (or 0 for unknown/default) */
           unsigned int is_kf; /**< Current frame is a keyframe */
       } vpx_codec_stream_info_t;

       /* REQUIRED FUNCTIONS
        *
        * The following functions are required to be implemented for all
        * decoders.  They represent the base case functionality expected
        * of all decoders.
        */

       /*!\brief Initialization Configurations
        *
        * This structure is used to pass init time configuration options
        * to the decoder.
        */
       typedef struct vpx_codec_dec_cfg
       {
           unsigned int threads; /**< Maximum number of threads to use,
               default 1 */
           unsigned int w;      /**< Width */
           unsigned int h;      /**< Height */
       } vpx_codec_dec_cfg_t; /**< alias for struct vpx_codec_dec_cfg */

       /*!\brief Initialize a decoder instance
        *
        * Initializes a decoder context using the given interface.
        * Applications should call the vpx_codec_dec_init convenience
        * macro instead of this function directly, to ensure that the
        * ABI version number parameter is properly initialized.
        *
        * In XMA mode (activated by setting VPX_CODEC_USE_XMA in the
        * flags parameter), the storage pointed to by the cfg parameter
        * must be kept readable and stable until all memory maps have
        * been set.
        *
        * \param[in]    ctx     Pointer to this instance's context.
        * \param[in]    iface   Pointer to the algorithm interface to
        *                       use.
        * \param[in]    cfg     Configuration to use, if known.  May be
        *                       NULL.

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        * \param[in]    flags   Bitfield of VPX_CODEC_USE_* flags
        * \param[in]    ver     ABI version number.  Must be set to
        *                       VPX_DECODER_ABI_VERSION
        * \retval #VPX_CODEC_OK
        *     The decoder algorithm initialized.
        * \retval #VPX_CODEC_MEM_ERROR
        *     Memory allocation failed.
        */
       vpx_codec_err_t vpx_codec_dec_init_ver(
           vpx_codec_ctx_t      *ctx,
           vpx_codec_iface_t    *iface,
           vpx_codec_dec_cfg_t  *cfg,
           vpx_codec_flags_t     flags,
           int                   ver);

       /*!\brief Convenience macro for vpx_codec_dec_init_ver()
        *
        * Ensures the ABI version parameter is properly set.
        */
   #define vpx_codec_dec_init(ctx, iface, cfg, flags) \
       vpx_codec_dec_init_ver(ctx, iface, cfg, flags, \
       VPX_DECODER_ABI_VERSION)

       /*!\brief Parse stream info from a buffer
        *
        * Performs high level parsing of the bitstream.  Construction of
        * a decoder context is not necessary.  Can be used to determine
        * if the bitstream is of the proper format, and to extract
        * information from the stream.
        *
        * \param[in]      iface   Pointer to the algorithm interface
        * \param[in]      data    Pointer to a block of data to parse
        * \param[in]      data_sz Size of the data buffer
        * \param[in,out]  si      Pointer to stream info to update.  The
        *                         size member
        *                         \ref MUST be properly initialized, but
        *                         \ref MAY be clobbered by the
        *                         algorithm.  This parameter \ref MAY be
        *                         NULL.
        *
        * \retval #VPX_CODEC_OK
        *     Bitstream is parsable and stream information updated
        */

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       vpx_codec_err_t vpx_codec_peek_stream_info(
           vpx_codec_iface_t       *iface,
           const uint8_t           *data,
           unsigned int             data_sz,
           vpx_codec_stream_info_t *si);

       /*!\brief Return information about the current stream.
        *
        * Returns information about the stream that has been parsed
        * during decoding.
        *
        * \param[in]      ctx     Pointer to this instance's context
        * \param[in,out]  si      Pointer to stream info to update.  The
        *                         size member \ref MUST be properly
        *                         initialized, but \ref MAY be clobbered
        *                         by the algorithm.  This parameter \ref
        *                         MAY be NULL.
        *
        * \retval #VPX_CODEC_OK
        *     Bitstream is parsable and stream information updated
        */
       vpx_codec_err_t vpx_codec_get_stream_info(
           vpx_codec_ctx_t         *ctx,
           vpx_codec_stream_info_t *si);

       /*!\brief Decode data
        *
        * Processes a buffer of coded data.  If the processing results
        * in a new decoded frame becoming available, PUT_SLICE and
        * PUT_FRAME events may be generated, as appropriate.  Encoded
        * data \ref MUST be passed in DTS (decode time stamp) order.
        * Frames produced will always be in PTS (presentation time
        * stamp) order.
        *
        * \param[in] ctx          Pointer to this instance's context
        * \param[in] data         Pointer to this block of new coded
        *                         data.  If NULL, a
        *                         VPX_CODEC_CB_PUT_FRAME event is posted
        *                         for the previously decoded frame.
        * \param[in] data_sz      Size of the coded data, in bytes.
        * \param[in] user_priv    Application-specific data to associate
        *                         with this frame.
        * \param[in] deadline     Soft deadline the decoder should
        *                         attempt to meet, in us.  Set to zero
        *                         for unlimited.
        *

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        * \return Returns #VPX_CODEC_OK if the coded data was processed
        *         completely and future pictures can be decoded without
        *         error.  Otherwise, see the descriptions of the other
        *         error codes in ::vpx_codec_err_t for recoverability
        *         capabilities.
        */
       vpx_codec_err_t vpx_codec_decode(vpx_codec_ctx_t    *ctx,
                                        const uint8_t        *data,
                                        unsigned int            data_sz,
                                        void               *user_priv,
                                        long                deadline);

       /*!\brief Decoded frames iterator
        *
        * Iterates over a list of the frames available for display.  The
        * iterator storage should be initialized to NULL to start the
        * iteration.  Iteration is complete when this function returns
        * NULL.
        *
        * The list of available frames becomes valid upon completion of
        * the vpx_codec_decode call, and remains valid until the next
        * call to vpx_codec_decode.
        *
        * \param[in]     ctx      Pointer to this instance's context
        * \param[in,out] iter     Iterator storage, initialized to NULL
        *
        * \return Returns a pointer to an image, if one is ready for
        *         display.  Frames produced will always be in PTS
        *         (presentation time stamp) order.
        */
       vpx_image_t *vpx_codec_get_frame(vpx_codec_ctx_t  *ctx,
                                        vpx_codec_iter_t *iter);

       /*!\defgroup cap_put_frame Frame-Based Decoding Functions
        *
        * The following functions are required to be implemented for all
        * decoders that advertise the VPX_CODEC_CAP_PUT_FRAME
        * capability.  Calling these functions for codecs that don't
        * advertise this capability will result in an error code being
        * returned, usually VPX_CODEC_ERROR
        * @{
        */

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       /*!\brief put frame callback prototype
        *
        * This callback is invoked by the decoder to notify the
        * application of the availability of decoded image data.
        */
       typedef void (*vpx_codec_put_frame_cb_fn_t)(
           void        *user_priv,
           const vpx_image_t *img);

       /*!\brief Register for notification of frame completion.
        *
        * Registers a given function to be called when a decoded frame
        * is available.
        *
        * \param[in] ctx          Pointer to this instance's context
        * \param[in] cb           Pointer to the callback function
        * \param[in] user_priv    User's private data
        *
        * \retval #VPX_CODEC_OK
        *     Callback successfully registered.
        * \retval #VPX_CODEC_ERROR
        *     Decoder context not initialized, or algorithm not capable
        *     of posting slice completion.
        */
       vpx_codec_err_t vpx_codec_register_put_frame_cb(
           vpx_codec_ctx_t             *ctx,
           vpx_codec_put_frame_cb_fn_t  cb,
           void                        *user_priv);

       /*!@} - end defgroup cap_put_frame */

       /*!\defgroup cap_put_slice Slice-Based Decoding Functions
        *
        * The following functions are required to be implemented for all
        * decoders that advertise the VPX_CODEC_CAP_PUT_SLICE
        * capability.  Calling these functions for codecs that don't
        * advertise this capability will result in an error code being
        * returned, usually VPX_CODEC_ERROR
        * @{
        */

       /*!\brief put slice callback prototype
        *
        * This callback is invoked by the decoder to notify the
        * application of the availability of partially decoded image
        * data.
        */

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       typedef void (*vpx_codec_put_slice_cb_fn_t)(
           void         *user_priv,
           const vpx_image_t      *img,
           const vpx_image_rect_t *valid,
           const vpx_image_rect_t *update);

       /*!\brief Register for notification of slice completion.
        *
        * Registers a given function to be called when a decoded slice
        * is available.
        *
        * \param[in] ctx          Pointer to this instance's context
        * \param[in] cb           Pointer to the callback function
        * \param[in] user_priv    User's private data
        *
        * \retval #VPX_CODEC_OK
        *     Callback successfully registered.
        * \retval #VPX_CODEC_ERROR
        *     Decoder context not initialized, or algorithm not capable
        *     of posting slice completion.
        */
       vpx_codec_err_t vpx_codec_register_put_slice_cb(
           vpx_codec_ctx_t             *ctx,
           vpx_codec_put_slice_cb_fn_t  cb,
           void                        *user_priv);

       /*!@} - end defgroup cap_put_slice*/

       /*!@} - end defgroup decoder*/

   #endif

   #ifdef __cplusplus
   }
   #endif

   #if !defined(VPX_CODEC_DISABLE_COMPAT) || !VPX_CODEC_DISABLE_COMPAT
   #include "vpx_decoder_compat.h"
   #endif

   ---- End code block ----------------------------------------

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20.21.  vpx_decoder_compat.h

   ---- Begin code block --------------------------------------

   /*
    * Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    * Use of this source code is governed by a BSD-style license
    * that can be found in the LICENSE file in the root of the source
    * tree.  An additional intellectual property rights grant can be
    * found in the file PATENTS.  All contributing project authors may
    * be found in the AUTHORS file in the root of the source tree.
    */

   /*!\defgroup decoder Common Decoder Algorithm Interface
    * This abstraction allows applications using this decoder to easily
    * support multiple video formats with minimal code duplication.
    * This section describes the interface common to all codecs.
    * @{
    */

   /*!\file
    * \brief Provides a compatibility layer between version 1 and 2 of
    * this API.
    *
    * This interface has been deprecated.  Only existing code should
    * make use of this interface, and therefore, it is only thinly
    * documented.  Existing code should be ported to the vpx_codec_*
    * API.
    */
   #ifdef __cplusplus
   extern "C" {
   #endif

   #ifndef VPX_DECODER_COMPAT_H
   #define VPX_DECODER_COMPAT_H

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       /*!\brief Decoder algorithm return codes */
       typedef enum {
           /*!\brief Operation completed without error */
           VPX_DEC_OK = VPX_CODEC_OK,

           /*!\brief Unspecified error */
           VPX_DEC_ERROR = VPX_CODEC_ERROR,

           /*!\brief Memory operation failed */
           VPX_DEC_MEM_ERROR = VPX_CODEC_MEM_ERROR,

           /*!\brief ABI version mismatch */
           VPX_DEC_ABI_MISMATCH = VPX_CODEC_ABI_MISMATCH,

           /*!\brief The given bitstream is not supported.
            *
            * The bitstream was unable to be parsed at the highest
            * level.  The decoder is unable to proceed.  This error \ref
            * SHOULD be treated as fatal to the stream.
            */
           VPX_DEC_UNSUP_BITSTREAM = VPX_CODEC_UNSUP_BITSTREAM,

           /*!\brief Encoded bitstream uses an unsupported feature
            *
            * The decoder does not implement a feature required by the
            * encoder.  This return code should only be used for
            * features that prevent future pictures from being properly
            * decoded.  This error \ref MAY be treated as fatal to the
            * stream or \ref MAY be treated as fatal to the current
            * Group of Pictures (GOP).
            */
           VPX_DEC_UNSUP_FEATURE = VPX_CODEC_UNSUP_FEATURE,

           /*!\brief The coded data for this stream is corrupt or
            * incomplete
            *
            * There was a problem decoding the current frame.  This
            * return code should only be used for failures that prevent
            * future pictures from being properly decoded.  This error
            * \ref MAY be treated as fatal to the stream or \ref MAY be
            * treated as fatal to the current GOP.  If decoding is
            * continued for the current GOP, artifacts may be present.
            */
           VPX_DEC_CORRUPT_FRAME = VPX_CODEC_CORRUPT_FRAME,

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           /*!\brief An application-supplied parameter is not valid.
            *
            */
           VPX_DEC_INVALID_PARAM = VPX_CODEC_INVALID_PARAM,

           /*!\brief An iterator reached the end of list.
            *
            */
           VPX_DEC_LIST_END = VPX_CODEC_LIST_END

       }
       vpx_dec_err_t;

       /*! \brief Decoder capabilities bitfield
        *
        *  Each decoder advertises the capabilities it supports as part
        *  of its ::vpx_dec_iface_t interface structure.  Capabilities
        *  are extra interfaces or functionality, and are not required
        *  to be supported by a decoder.
        *
        *  The available flags are specified by VPX_DEC_CAP_* defines.
        */
       typedef int vpx_dec_caps_t;
   #define VPX_DEC_CAP_PUT_SLICE  0x0001 /**< Will issue put_slice
                                            callbacks */
   #define VPX_DEC_CAP_PUT_FRAME  0x0002 /**< Will issue put_frame
                                            callbacks */

   #define VPX_DEC_CAP_XMA        0x0004 /**< Supports External Memory
                                            Allocation */

       /*!\brief Stream properties
        *
        * This structure is used to query or set properties of the
        * decoded stream.  Algorithms may extend this structure with
        * data specific to their bitstream by setting the sz member
        * appropriately.
        */
   #if 1
       typedef vpx_codec_stream_info_t vpx_dec_stream_info_t;
   #else
       typedef struct
       {
           unsigned int sz;    /**< Size of this structure */
           unsigned int w;     /**< Width (or 0 for unknown/default) */
           unsigned int h;     /**< Height (or 0 for unknown/default) */
           unsigned int is_kf; /**< Current frame is a keyframe */
       } vpx_dec_stream_info_t;

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   #endif

       /*!\brief Decoder interface structure.
        *
        * Contains function pointers and other data private to the
        * decoder implementation.  This structure is opaque to the
        * application.
        */
       typedef const struct vpx_codec_iface vpx_dec_iface_t;
       typedef       struct vpx_codec_priv  vpx_dec_priv_t;

       /*!\brief Iterator
        *
        * Opaque storage used for iterating over lists.
        */
       typedef vpx_codec_iter_t vpx_dec_iter_t;

       /*!\brief Decoder context structure
        *
        * All decoders \ref MUST support this context structure fully.
        * In general, this data should be considered private to the
        * decoder algorithm, and not be manipulated or examined by the
        * calling application.  Applications may reference the 'name'
        * member to get a printable description of the algorithm.
        */
   #if 1
       typedef vpx_codec_ctx_t vpx_dec_ctx_t;
   #else
       typedef struct
       {
           const char          *name;  /**< Printable interface name */
           vpx_dec_iface_t     *iface; /**< Interface pointers */
           vpx_dec_err_t        err;   /**< Last returned error */
           vpx_dec_priv_t      *priv;  /**< Algorithm private storage */
       } vpx_dec_ctx_t;
   #endif

       /*!\brief Return the build configuration
        *
        * Returns a printable string containing an encoded version of
        * the build configuration.  This may be useful to vpx support.
        *
        */
       const char *vpx_dec_build_config(void) DEPRECATED;

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       /*!\brief Return the name for a given interface
        *
        * Returns a human readable string for name of the given decoder
        * interface.
        *
        * \param[in]    iface     Interface pointer
        *
        */
       const char *vpx_dec_iface_name(
           vpx_dec_iface_t *iface) DEPRECATED;

       /*!\brief Convert error number to printable string
        *
        * Returns a human readable string for the last error returned
        * by the algorithm.  The returned error will be one line and
        * will not contain any newline characters.
        *
        *
        * \param[in]    err     Error number.
        *
        */
       const char *vpx_dec_err_to_string(vpx_dec_err_t  err) DEPRECATED;

       /*!\brief Retrieve error synopsis for decoder context
        *
        * Returns a human readable string for the last error returned by
        * the algorithm.  The returned error will be one line and will
        * not contain any newline characters.
        *
        *
        * \param[in]    ctx     Pointer to this instance's context.
        *
        */
       const char *vpx_dec_error(vpx_dec_ctx_t  *ctx) DEPRECATED;

       /*!\brief Retrieve detailed error information for decoder context
        *
        * Returns a human readable string providing detailed information
        * about the last error.
        *
        * \param[in]    ctx     Pointer to this instance's context.
        *

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        * \retval NULL
        *     No detailed information is available.
        */
       const char *vpx_dec_error_detail(vpx_dec_ctx_t  *ctx) DEPRECATED;

       /* REQUIRED FUNCTIONS
        *
        * The following functions are required to be implemented for all
        * decoders.  They represent the base case functionality expected
        * of all decoders.
        */

       /*!\brief Initialize a decoder instance
        *
        * Initializes a decoder context using the given interface.
        * Applications should call the vpx_dec_init convenience macro
        * instead of this function directly, to ensure that the ABI
        * version number parameter is properly initialized.
        *
        * \param[in]   ctx    Pointer to this instance's context.
        * \param[in]   iface  Pointer to the algorithm interface to use.
        * \param[in]   ver    ABI version number.  Must be set to
        *                       VPX_DECODER_ABI_VERSION
        * \retval #VPX_DEC_OK
        *     The decoder algorithm initialized.
        * \retval #VPX_DEC_MEM_ERROR
        *     Memory allocation failed.
        */
       vpx_dec_err_t vpx_dec_init_ver(
           vpx_dec_ctx_t    *ctx,
           vpx_dec_iface_t  *iface,
           int               ver) DEPRECATED;

   #define vpx_dec_init(ctx, iface) \
       vpx_dec_init_ver(ctx, iface, VPX_DECODER_ABI_VERSION)

       /*!\brief Destroy a decoder instance
        *
        * Destroys a decoder context, freeing any associated memory
        * buffers.
        *
        * \param[in] ctx   Pointer to this instance's context
        *
        * \retval #VPX_DEC_OK
        *     The decoder algorithm initialized.

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        * \retval #VPX_DEC_MEM_ERROR
        *     Memory allocation failed.
        */
       vpx_dec_err_t vpx_dec_destroy(vpx_dec_ctx_t *ctx) DEPRECATED;

       /*!\brief Get the capabilities of an algorithm.
        *
        * Retrieves the capabilities bitfield from the algorithm's
        * interface.
        *
        * \param[in] iface   Pointer to the algorithm interface
        *
        */
       vpx_dec_caps_t vpx_dec_get_caps(
           vpx_dec_iface_t *iface) DEPRECATED;

       /*!\brief Parse stream info from a buffer
        *
        * Performs high level parsing of the bitstream.  Construction of
        * a decoder context is not necessary.  Can be used to determine
        * if the bitstream is of the proper format, and to extract
        * information from the stream.
        *
        * \param[in]      iface   Pointer to the algorithm interface
        * \param[in]      data    Pointer to a block of data to parse
        * \param[in]      data_sz Size of the data buffer
        * \param[in,out]  si      Pointer to stream info to update.  The
        *                         size member \ref MUST be properly
        *                         initialized, but \ref MAY be
        *                         clobbered by the algorithm.  This
        *                         parameter \ref MAY be NULL.
        *
        * \retval #VPX_DEC_OK
        *     Bitstream is parsable and stream information updated
        */
       vpx_dec_err_t vpx_dec_peek_stream_info(
                                vpx_dec_iface_t       *iface,
                                const uint8_t         *data,
                                unsigned int           data_sz,
                                vpx_dec_stream_info_t *si) DEPRECATED;

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       /*!\brief Return information about the current stream.
        *
        * Returns information about the stream that has been parsed
        * during decoding.
        *
        * \param[in]      ctx     Pointer to this instance's context
        * \param[in,out]  si      Pointer to stream info to update.
        *                         The size member \ref MUST be properly
        *                         initialized, but \ref MAY be clobbered
        *                         by the algorithm.  This parameter \ref
        *                         MAY be NULL.
        *
        * \retval #VPX_DEC_OK
        *     Bitstream is parsable and stream information updated
        */
       vpx_dec_err_t vpx_dec_get_stream_info(
           vpx_dec_ctx_t         *ctx,
           vpx_dec_stream_info_t *si) DEPRECATED;

       /*!\brief Control algorithm
        *
        * This function is used to exchange algorithm-specific data with
        * the decoder instance.  This can be used to implement features
        * specific to a particular algorithm.
        *
        * This wrapper function dispatches the request to the helper
        * function associated with the given ctrl_id.  It tries to call
        * this function transparently, but will return #VPX_DEC_ERROR if
        * the request could not be dispatched.
        *
        * \param[in]     ctx          Pointer to this instance's context
        * \param[in]     ctrl_id      Algorithm-specific control
        *                             identifier
        * \param[in,out] data         Data to exchange with algorithm
        *                             instance.
        *
        * \retval #VPX_DEC_OK
        *     The control request was processed.
        * \retval #VPX_DEC_ERROR
        *     The control request was not processed.
        * \retval #VPX_DEC_INVALID_PARAM
        *     The data was not valid.
        */
       vpx_dec_err_t vpx_dec_control(vpx_dec_ctx_t  *ctx,
                                     int             ctrl_id,
                                     void           *data) DEPRECATED;

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       /*!\brief Decode data
        *
        * Processes a buffer of coded data.  If the processing results
        * in a new decoded frame becoming available,
        * #VPX_DEC_CB_PUT_SLICE and #VPX_DEC_CB_PUT_FRAME events may be
        * generated, as appropriate.  Encoded data \ref MUST be passed
        * in DTS (decode time stamp) order.  Frames produced will always
        * be in PTS (presentation time stamp) order.
        *
        * \param[in] ctx          Pointer to this instance's context
        * \param[in] data         Pointer to this block of new coded
        *                         data.  If NULL, a VPX_DEC_CB_PUT_FRAME
        *                         event is posted for the previously
        *                         decoded frame.
        * \param[in] data_sz      Size of the coded data, in bytes.
        * \param[in] user_priv    Application-specific data to associate
        *                         with this frame.
        * \param[in] rel_pts      PTS relative to the previous frame, in
        *                         us.  If unknown or unavailable, set to
        *                         zero.
        *
        * \return Returns #VPX_DEC_OK if the coded data was processed
        *         completely and future pictures can be decoded without
        *         error.  Otherwise, see the descriptions of the other
        *         error codes in ::vpx_dec_err_t for recoverability
        *         capabilities.
        */
       vpx_dec_err_t vpx_dec_decode(
           vpx_dec_ctx_t  *ctx,
           uint8_t        *data,
           unsigned int    data_sz,
           void           *user_priv,
           int             rel_pts) DEPRECATED;

       /*!\brief Decoded frames iterator
        *
        * Iterates over a list of the frames available for display.  The
        * iterator storage should be initialized to NULL to start the
        * iteration.  Iteration is complete when this function returns
        * NULL.
        *
        * The list of available frames becomes valid upon completion of
        * the vpx_dec_decode call, and remains valid until the next call
        * to vpx_dec_decode.
        *
        * \param[in]     ctx      Pointer to this instance's context
        * \param[in out] iter     Iterator storage, initialized to NULL

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        *
        * \return Returns a pointer to an image, if one is ready for
        *         display.  Frames produced will always be in PTS
        *         (presentation time stamp) order.
        */
       vpx_image_t *vpx_dec_get_frame(vpx_dec_ctx_t  *ctx,
                                      vpx_dec_iter_t *iter) DEPRECATED;

       /*!\defgroup cap_put_frame Frame-Based Decoding Functions
        *
        * The following functions are required to be implemented for all
        * decoders that advertise the VPX_DEC_CAP_PUT_FRAME capability.
        * Calling these functions for codecs that don't advertise this
        * capability will result in an error code being returned,
        * usually VPX_DEC_ERROR @{
        */

       /*!\brief put frame callback prototype
        *
        * This callback is invoked by the decoder to notify the
        * application of the availability of decoded image data.
        */
       typedef void (*vpx_dec_put_frame_cb_fn_t)(
               void          *user_priv,
               const vpx_image_t *img);

       /*!\brief Register for notification of frame completion.
        *
        * Registers a given function to be called when a decoded frame
        * is available.
        *
        * \param[in] ctx          Pointer to this instance's context
        * \param[in] cb           Pointer to the callback function
        * \param[in] user_priv    User's private data
        *
        * \retval #VPX_DEC_OK
        *     Callback successfully registered.
        * \retval #VPX_DEC_ERROR
        *     Decoder context not initialized, or algorithm not capable
        *     of posting slice completion.
        */
       vpx_dec_err_t vpx_dec_register_put_frame_cb(
               vpx_dec_ctx_t             *ctx,
               vpx_dec_put_frame_cb_fn_t  cb,
               void                      *user_priv) DEPRECATED;

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       /*!@} - end defgroup cap_put_frame */

       /*!\defgroup cap_put_slice Slice-Based Decoding Functions
        *
        * The following functions are required to be implemented for all
        * decoders that advertise the VPX_DEC_CAP_PUT_SLICE capability.
        * Calling these functions for codecs that don't advertise this
        * capability will result in an error code being returned,
        * usually VPX_DEC_ERROR
        * @{
        */

       /*!\brief put slice callback prototype
        *
        * This callback is invoked by the decoder to notify the
        * application of the availability of partially decoded image
        * data.
        */
       typedef void (*vpx_dec_put_slice_cb_fn_t)(void        *user_priv,
               const vpx_image_t      *img,
               const vpx_image_rect_t *valid,
               const vpx_image_rect_t *update);

       /*!\brief Register for notification of slice completion.
        *
        * Registers a given function to be called when a decoded slice
        * is available.
        *
        * \param[in] ctx          Pointer to this instance's context
        * \param[in] cb           Pointer to the callback function
        * \param[in] user_priv    User's private data
        *
        * \retval #VPX_DEC_OK
        *     Callback successfully registered.
        * \retval #VPX_DEC_ERROR
        *     Decoder context not initialized, or algorithm not capable
        *     of posting slice completion.
        */
       vpx_dec_err_t vpx_dec_register_put_slice_cb(vpx_dec_ctx_t   *ctx,
               vpx_dec_put_slice_cb_fn_t  cb,
               void                      *user_priv) DEPRECATED;

       /*!@} - end defgroup cap_put_slice*/

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       /*!\defgroup cap_xma External Memory Allocation Functions
        *
        * The following functions are required to be implemented for all
        * decoders that advertise the VPX_DEC_CAP_XMA capability.
        * Calling these functions for codecs that don't advertise this
        * capability will result in an error code being returned,
        * usually VPX_DEC_ERROR
        * @{
        */

       /*!\brief Memory Map Entry
        *
        * This structure is used to contain the properties of a memory
        * segment.  It is populated by the decoder in the request phase,
        * and by the calling application once the requested allocation
        * has been performed.
        */
   #if 1
   #define VPX_DEC_MEM_ZERO     0x1  /**< Segment must be zeroed by
                                          allocation */
   #define VPX_DEC_MEM_WRONLY   0x2  /**< Segment need not be
                                          readable */
   #define VPX_DEC_MEM_FAST     0x4  /**< Place in fast memory, if
                                          available */
       typedef struct vpx_codec_mmap vpx_dec_mmap_t;
   #else
       typedef struct vpx_dec_mmap
       {
           /*
            * The following members are set by the codec when requesting
            * a segment
            */
           unsigned int   id;     /**< identifier for the segment's
                                       contents */
           unsigned long  sz;     /**< size of the segment, in bytes */
           unsigned int   align;  /**< required alignment of the
                                       segment, in bytes */
           unsigned int   flags;  /**< bitfield containing segment
                                       properties */
   #define VPX_DEC_MEM_ZERO     0x1  /**< Segment must be zeroed by
                                          allocation */
   #define VPX_DEC_MEM_WRONLY   0x2  /**< Segment need not be
                                          readable */
   #define VPX_DEC_MEM_FAST     0x4  /**< Place in fast memory, if
                                          available */

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           /* The following members are to be filled in by the
            * allocation function */
           void          *base;   /**< pointer to the allocated
                                       segment */
           void (*dtor)(struct vpx_dec_mmap *map);  /**< destructor to
                                                         call */
           void          *priv;   /**< allocator private storage */
       } vpx_dec_mmap_t;
   #endif

       /*!\brief Initialize a decoder instance in external allocation
        * mode
        *
        * Initializes a decoder context using the given interface.
        * Applications should call the vpx_dec_xma_init convenience
        * macro instead of this function directly, to ensure that the
        * ABI version number parameter is properly initialized.
        *
        * \param[in]    ctx     Pointer to this instance's context.
        * \param[in]    iface   Pointer to the algorithm interface to
        *                       use.
        * \param[in]    ver     ABI version number.  Must be set to
        *                       VPX_DECODER_ABI_VERSION
        * \retval #VPX_DEC_OK
        *     The decoder algorithm initialized.
        * \retval #VPX_DEC_ERROR
        *     Decoder does not support XMA mode.
        */
       vpx_dec_err_t vpx_dec_xma_init_ver(vpx_dec_ctx_t    *ctx,
                                          vpx_dec_iface_t  *iface,
                                          int           ver) DEPRECATED;
   #define vpx_dec_xma_init(ctx, iface) \
       vpx_dec_xma_init_ver(ctx, iface, VPX_DECODER_ABI_VERSION)

       /*!\brief Iterate over the list of segments to allocate.
        *
        * Iterates over a list of the segments to allocate.  The
        * iterator storage should be initialized to NULL to start the
        * iteration.  Iteration is complete when this function returns
        * VPX_DEC_LIST_END.  The amount of memory needed to allocate is
        * dependent upon the size of the encoded stream.  This means
        * that the stream info structure must be known at allocation
        * time.  It can be populated with the vpx_dec_peek_stream_info()
        * function.  In cases where the stream to be decoded is not
        * available at allocation time, a fixed size must be requested.
        * The decoder will not be able to decode streams larger than the
        * size used at allocation time.

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        *
        * \param[in]      ctx     Pointer to this instance's context.
        * \param[out]     mmap    Pointer to the memory map entry to
        *                         populate.
        * \param[in]      si      Pointer to the stream info.
        * \param[in out]  iter    Iterator storage, initialized to NULL
        *
        * \retval #VPX_DEC_OK
        *     The memory map entry was populated.
        * \retval #VPX_DEC_ERROR
        *     Decoder does not support XMA mode.
        * \retval #VPX_DEC_MEM_ERROR
        *     Unable to determine segment size from stream info.
        */
       vpx_dec_err_t vpx_dec_get_mem_map(
           vpx_dec_ctx_t                *ctx,
           vpx_dec_mmap_t               *mmap,
           const vpx_dec_stream_info_t  *si,
           vpx_dec_iter_t               *iter) DEPRECATED;

       /*!\brief Identify allocated segments to decoder instance
        *
        * Stores a list of allocated segments in the decoder.  Segments
        * \ref MUST be passed in the order they are read from
        * vpx_dec_get_mem_map(), but may be passed in groups of any
        * size.  Segments \ref MUST be set only once.  The allocation
        * function \ref MUST ensure that the vpx_dec_mmap_t::base member
        * is non-NULL.  If the segment requires cleanup handling (e.g.,
        * calling free() or close()) then the vpx_dec_mmap_t::dtor
        * member \ref MUST be populated.
        *
        * \param[in]      ctx       Pointer to this instance's context.
        * \param[in]      mmaps     Pointer to the first memory map
        *                           entry in the list.
        * \param[in]      num_maps  Number of entries being set at this
        *                           time
        *
        * \retval #VPX_DEC_OK
        *     The segment was stored in the decoder context.
        * \retval #VPX_DEC_ERROR
        *     Decoder does not support XMA mode.
        * \retval #VPX_DEC_MEM_ERROR
        *     Segment base address was not set, or segment was already
        * stored.

        */

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       vpx_dec_err_t  vpx_dec_set_mem_map(
           vpx_dec_ctx_t   *ctx,
           vpx_dec_mmap_t  *mmaps,
           unsigned int     num_maps) DEPRECATED;

       /*!@} - end defgroup cap_xma*/
       /*!@} - end defgroup decoder*/

   #endif
   #ifdef __cplusplus
   }
   #endif

   ---- End code block ----------------------------------------

20.22.  vpx_image.c

   ---- Begin code block --------------------------------------

   /*
    * Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    * Use of this source code is governed by a BSD-style license
    * that can be found in the LICENSE file in the root of the source
    * tree.  An additional intellectual property rights grant can be
    * found in the file PATENTS.  All contributing project authors may
    * be found in the AUTHORS file in the root of the source tree.
    */

   #include <stdlib.h>
   #include <string.h>
   #include "vpx/vpx_image.h"

   static vpx_image_t *img_alloc_helper(vpx_image_t  *img,
                                        vpx_img_fmt_t fmt,
                                        unsigned int  d_w,
                                        unsigned int  d_h,
                                        unsigned int  stride_align,
                                        unsigned char      *img_data)
   {

Bankoski, et al.              Informational                   [Page 285]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

       unsigned int  h, w, s, xcs, ycs, bps;
       int           align;

       /* Treat align==0 like align==1 */
       if (!stride_align)
           stride_align = 1;

       /* Validate alignment (must be power of 2) */
       if (stride_align & (stride_align - 1))
           goto fail;

       /* Get sample size for this format */
       switch (fmt)
       {
       case VPX_IMG_FMT_RGB32:
       case VPX_IMG_FMT_RGB32_LE:
       case VPX_IMG_FMT_ARGB:
       case VPX_IMG_FMT_ARGB_LE:
           bps = 32;
           break;
       case VPX_IMG_FMT_RGB24:
       case VPX_IMG_FMT_BGR24:
           bps = 24;
           break;
       case VPX_IMG_FMT_RGB565:
       case VPX_IMG_FMT_RGB565_LE:
       case VPX_IMG_FMT_RGB555:
       case VPX_IMG_FMT_RGB555_LE:
       case VPX_IMG_FMT_UYVY:
       case VPX_IMG_FMT_YUY2:
       case VPX_IMG_FMT_YVYU:
           bps = 16;
           break;
       case VPX_IMG_FMT_I420:
       case VPX_IMG_FMT_YV12:
       case VPX_IMG_FMT_VPXI420:
       case VPX_IMG_FMT_VPXYV12:
           bps = 12;
           break;
       default:
           bps = 16;
           break;
       }

Bankoski, et al.              Informational                   [Page 286]
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       /* Get chroma shift values for this format */
       switch (fmt)
       {
       case VPX_IMG_FMT_I420:
       case VPX_IMG_FMT_YV12:
       case VPX_IMG_FMT_VPXI420:
       case VPX_IMG_FMT_VPXYV12:
           xcs = 1;
           break;
       default:
           xcs = 0;
           break;
       }

       switch (fmt)
       {
       case VPX_IMG_FMT_I420:
       case VPX_IMG_FMT_YV12:
       case VPX_IMG_FMT_VPXI420:
       case VPX_IMG_FMT_VPXYV12:
           ycs = 1;
           break;
       default:
           ycs = 0;
           break;
       }

       /* Calculate storage sizes given the chroma subsampling */
       align = (1 << xcs) - 1;
       w = (d_w + align) & ~align;
       align = (1 << ycs) - 1;
       h = (d_h + align) & ~align;
       s = (fmt & VPX_IMG_FMT_PLANAR) ? w : bps * w / 8;
       s = (s + stride_align - 1) & ~(stride_align - 1);

       /* Allocate the new image */
       if (!img)
       {
           img = (vpx_image_t *)calloc(1, sizeof(vpx_image_t));

           if (!img)
               goto fail;

           img->self_allocd = 1;
       }

Bankoski, et al.              Informational                   [Page 287]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

       else
       {
           memset(img, 0, sizeof(vpx_image_t));
       }

       img->img_data = img_data;

       if (!img_data)
       {
           img->img_data = malloc((fmt & VPX_IMG_FMT_PLANAR) ?
             h * w * bps / 8 : h * s);
           img->img_data_owner = 1;
       }

       if (!img->img_data)
           goto fail;

       img->fmt = fmt;
       img->w = w;
       img->h = h;
       img->x_chroma_shift = xcs;
       img->y_chroma_shift = ycs;
       img->bps = bps;

       /* Calculate strides */
       img->stride[VPX_PLANE_Y] = img->stride[VPX_PLANE_ALPHA] = s;
       img->stride[VPX_PLANE_U] = img->stride[VPX_PLANE_V] = s >> xcs;

       /* Default viewport to entire image */
       if (!vpx_img_set_rect(img, 0, 0, d_w, d_h))
           return img;

   fail:
       vpx_img_free(img);
       return NULL;
   }

   vpx_image_t *vpx_img_alloc(vpx_image_t  *img,
                              vpx_img_fmt_t fmt,
                              unsigned int  d_w,
                              unsigned int  d_h,
                              unsigned int  stride_align)
   {
       return img_alloc_helper(img, fmt, d_w, d_h, stride_align, NULL);
   }

Bankoski, et al.              Informational                   [Page 288]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   vpx_image_t *vpx_img_wrap(vpx_image_t  *img,
                             vpx_img_fmt_t fmt,
                             unsigned int  d_w,
                             unsigned int  d_h,
                             unsigned int  stride_align,
                             unsigned char       *img_data)
   {
       return img_alloc_helper(img, fmt, d_w, d_h, stride_align,
         img_data);
   }

   int vpx_img_set_rect(vpx_image_t  *img,
                        unsigned int  x,
                        unsigned int  y,
                        unsigned int  w,
                        unsigned int  h)
   {
       unsigned char      *data;

       if (x + w <= img->w && y + h <= img->h)
       {
           img->d_w = w;
           img->d_h = h;

           /* Calculate plane pointers */
           if (!(img->fmt & VPX_IMG_FMT_PLANAR))
           {
               img->planes[VPX_PLANE_PACKED] =
                   img->img_data + x * img->bps / 8 + y *
                     img->stride[VPX_PLANE_PACKED];
           }
           else
           {
               data = img->img_data;

               if (img->fmt & VPX_IMG_FMT_HAS_ALPHA)
               {
                   img->planes[VPX_PLANE_ALPHA] =
                       data + x + y * img->stride[VPX_PLANE_ALPHA];
                   data += img->h * img->stride[VPX_PLANE_ALPHA];
               }

               img->planes[VPX_PLANE_Y] =
                 data + x + y * img->stride[VPX_PLANE_Y];
               data += img->h * img->stride[VPX_PLANE_Y];

Bankoski, et al.              Informational                   [Page 289]
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               if (!(img->fmt & VPX_IMG_FMT_UV_FLIP))
               {
                   img->planes[VPX_PLANE_U] = data
                                          + (x >> img->x_chroma_shift)
                                          + (y >> img->y_chroma_shift) *
                                            img->stride[VPX_PLANE_U];
                   data += (img->h >> img->y_chroma_shift) *
                                            img->stride[VPX_PLANE_U];
                   img->planes[VPX_PLANE_V] = data
                                          + (x >> img->x_chroma_shift)
                                          + (y >> img->y_chroma_shift) *
                                            img->stride[VPX_PLANE_V];
               }
               else
               {
                   img->planes[VPX_PLANE_V] = data
                                          + (x >> img->x_chroma_shift)
                                          + (y >> img->y_chroma_shift) *
                                            img->stride[VPX_PLANE_V];
                   data += (img->h >> img->y_chroma_shift) *
                                            img->stride[VPX_PLANE_V];
                   img->planes[VPX_PLANE_U] = data
                                          + (x >> img->x_chroma_shift)
                                          + (y >> img->y_chroma_shift) *
                                            img->stride[VPX_PLANE_U];
               }
           }

           return 0;
       }

       return -1;
   }

   void vpx_img_flip(vpx_image_t *img)
   {
       /* Note: In the calculation pointer adjustment calculation, we
        * want the rhs to be promoted to a signed type.  Section 6.3.1.8
        * of the ISO C99 standard [ISO-C99] indicates that if the
        * adjustment parameter is unsigned, the stride parameter will be
        * promoted to unsigned, causing errors when the lhs is a larger
        * type than the rhs.
        */
       img->planes[VPX_PLANE_Y] += (signed)
         (img->d_h - 1) * img->stride[VPX_PLANE_Y];
       img->stride[VPX_PLANE_Y] = -img->stride[VPX_PLANE_Y];

Bankoski, et al.              Informational                   [Page 290]
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       img->planes[VPX_PLANE_U] += (signed)
         ((img->d_h >> img->y_chroma_shift) - 1)
                               * img->stride[VPX_PLANE_U];
       img->stride[VPX_PLANE_U] = -img->stride[VPX_PLANE_U];

       img->planes[VPX_PLANE_V] += (signed)
                               ((img->d_h >> img->y_chroma_shift) - 1) *
                               img->stride[VPX_PLANE_V];
       img->stride[VPX_PLANE_V] = -img->stride[VPX_PLANE_V];

       img->planes[VPX_PLANE_ALPHA] += (signed)
         (img->d_h - 1) * img->stride[VPX_PLANE_ALPHA];
       img->stride[VPX_PLANE_ALPHA] = -img->stride[VPX_PLANE_ALPHA];
   }

   void vpx_img_free(vpx_image_t *img)
   {
       if (img)
       {
           if (img->img_data && img->img_data_owner)
               free(img->img_data);

           if (img->self_allocd)
               free(img);
       }
   }

   ---- End code block ----------------------------------------

20.23.  vpx_image.h

   ---- Begin code block --------------------------------------

   /*
    * Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    * Use of this source code is governed by a BSD-style license
    * that can be found in the LICENSE file in the root of the source
    * tree.  An additional intellectual property rights grant can be
    * found in the file PATENTS.  All contributing project authors may
    * be found in the AUTHORS file in the root of the source tree.
    */

   /*!\file
    * \brief Describes the vpx image descriptor and associated
    * operations
    *
    */

Bankoski, et al.              Informational                   [Page 291]
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   #ifdef __cplusplus
   extern "C" {
   #endif

   #ifndef VPX_IMAGE_H
   #define VPX_IMAGE_H

       /*!\brief Current ABI version number
        *
        * \internal
        * If this file is altered in any way that changes the ABI, this
        * value must be bumped.  Examples include, but are not limited
        * to, changing types, removing or reassigning enums,
        * adding/removing/rearranging fields to structures
        */
   #define VPX_IMAGE_ABI_VERSION (1) /**<\hideinitializer*/

   #define VPX_IMG_FMT_PLANAR     0x100  /**< Image is a planar
                                              format */
   #define VPX_IMG_FMT_UV_FLIP    0x200  /**< V plane precedes U plane
                                              in memory */
   #define VPX_IMG_FMT_HAS_ALPHA  0x400  /**< Image has an alpha channel
                                              component */

       /*!\brief List of supported image formats */
       typedef enum vpx_img_fmt {
           VPX_IMG_FMT_NONE,
           VPX_IMG_FMT_RGB24,      /**< 24 bit per pixel packed RGB */
           VPX_IMG_FMT_RGB32,      /**< 32 bit per pixel packed 0RGB */
           VPX_IMG_FMT_RGB565,     /**< 16 bit per pixel, 565 */
           VPX_IMGFMT_RGB555,      /**< 16 bit per pixel, 555 */
           VPX_IMG_FMT_UYVY,       /**< UYVY packed YUV */
           VPX_IMG_FMT_YUY2,       /**< YUYV packed YUV */
           VPX_IMG_FMT_YVYU,       /**< YVYU packed YUV */
           VPX_IMG_FMT_BGR24,      /**< 24 bit per pixel packed BGR */
           VPX_IMG_FMT_RGB32_LE,   /**< 32 bit packed BGR0 */
           VPX_IMG_FMT_ARGB,       /**< 32 bit packed ARGB, alpha=255 */
           VPX_IMG_FMT_ARGB_LE,    /**< 32 bit packed BGRA, alpha=255 */
           VPX_IMG_FMT_RGB565_LE,  /**< 16 bit per pixel,
                                        gggbbbbb rrrrrggg */
           VPX_IMG_FMT_RGB555_LE,  /**< 16 bit per pixel,
                                        gggbbbbb 0rrrrrgg */
           VPX_IMG_FMT_YV12    = VPX_IMG_FMT_PLANAR |
             VPX_IMG_FMT_UV_FLIP | 1, /**< planar YVU */
           VPX_IMG_FMT_I420    = VPX_IMG_FMT_PLANAR | 2,

Bankoski, et al.              Informational                   [Page 292]
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           VPX_IMG_FMT_VPXYV12 = VPX_IMG_FMT_PLANAR |
             VPX_IMG_FMT_UV_FLIP | 3, /** < planar 4:2:0 format with
                                            vpx color space */
           VPX_IMG_FMT_VPXI420 = VPX_IMG_FMT_PLANAR | 4   /** < planar
             4:2:0 format with vpx color space */
       }
       vpx_img_fmt_t; /**< alias for enum vpx_img_fmt */

   #if !defined(VPX_CODEC_DISABLE_COMPAT) || !VPX_CODEC_DISABLE_COMPAT
   /** \deprecated Use #VPX_IMG_FMT_PLANAR */
   #define IMG_FMT_PLANAR         VPX_IMG_FMT_PLANAR
   /** \deprecated Use #VPX_IMG_FMT_UV_FLIP */
   #define IMG_FMT_UV_FLIP        VPX_IMG_FMT_UV_FLIP
   /** \deprecated Use #VPX_IMG_FMT_HAS_ALPHA */
   #define IMG_FMT_HAS_ALPHA      VPX_IMG_FMT_HAS_ALPHA

       /*!\brief Deprecated list of supported image formats
        * \deprecated New code should use #vpx_img_fmt
        */
   #define img_fmt   vpx_img_fmt
       /*!\brief alias for enum img_fmt.
        * \deprecated New code should use #vpx_img_fmt_t
        */
   #define img_fmt_t vpx_img_fmt_t

   /** \deprecated Use #VPX_IMG_FMT_NONE */
   #define IMG_FMT_NONE       VPX_IMG_FMT_NONE
   /** \deprecated Use #VPX_IMG_FMT_RGB24 */
   #define IMG_FMT_RGB24      VPX_IMG_FMT_RGB24
   /** \deprecated Use #VPX_IMG_FMT_RGB32 */
   #define IMG_FMT_RGB32      VPX_IMG_FMT_RGB32
   /** \deprecated Use #VPX_IMG_FMT_RGB565 */
   #define IMG_FMT_RGB565     VPX_IMG_FMT_RGB565
   /** \deprecated Use #VPX_IMG_FMT_RGB555 */
   #define IMG_FMT_RGB555     VPX_IMG_FMT_RGB555
   /** \deprecated Use #VPX_IMG_FMT_UYVY */
   #define IMG_FMT_UYVY       VPX_IMG_FMT_UYVY
   /** \deprecated Use #VPX_IMG_FMT_YUY2 */
   #define IMG_FMT_YUY2       VPX_IMG_FMT_YUY2
   /** \deprecated Use #VPX_IMG_FMT_YVYU */
   #define IMG_FMT_YVYU       VPX_IMG_FMT_YVYU
   /** \deprecated Use #VPX_IMG_FMT_BGR24 */
   #define IMG_FMT_BGR24      VPX_IMG_FMT_BGR24
   /**< \deprecated Use #VPX_IMG_FMT_RGB32_LE */
   #define IMG_FMT_RGB32_LE   VPX_IMG_FMT_RGB32_LE
   /** \deprecated Use #VPX_IMG_FMT_ARGB */
   #define IMG_FMT_ARGB       VPX_IMG_FMT_ARGB

Bankoski, et al.              Informational                   [Page 293]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   /** \deprecated Use #VPX_IMG_FMT_ARGB_LE */
   #define IMG_FMT_ARGB_LE    VPX_IMG_FMT_ARGB_LE
   /** \deprecated Use #VPX_IMG_FMT_RGB565_LE */
   #define IMG_FMT_RGB565_LE  VPX_IMG_FMT_RGB565_LE
   /** \deprecated Use #VPX_IMG_FMT_RGB555_LE */
   #define IMG_FMT_RGB555_LE  VPX_IMG_FMT_RGB555_LE
   /** \deprecated Use #VPX_IMG_FMT_YV12 */
   #define IMG_FMT_YV12       VPX_IMG_FMT_YV12
   /** \deprecated Use #VPX_IMG_FMT_I420 */
   #define IMG_FMT_I420       VPX_IMG_FMT_I420
   /** \deprecated Use #VPX_IMG_FMT_VPXYV12 */
   #define IMG_FMT_VPXYV12    VPX_IMG_FMT_VPXYV12
   /** \deprecated Use #VPX_IMG_FMT_VPXI420 */
   #define IMG_FMT_VPXI420    VPX_IMG_FMT_VPXI420
   #endif /* VPX_CODEC_DISABLE_COMPAT */

       /**\brief Image Descriptor */
       typedef struct vpx_image
       {
           vpx_img_fmt_t fmt; /**< Image Format */

           /* Image storage dimensions */
           unsigned int  w;   /**< Stored image width */
           unsigned int  h;   /**< Stored image height */

           /* Image display dimensions */
           unsigned int  d_w;   /**< Displayed image width */
           unsigned int  d_h;   /**< Displayed image height */

           /* Chroma subsampling info */
           unsigned int  x_chroma_shift;   /**< subsampling order, X */
           unsigned int  y_chroma_shift;   /**< subsampling order, Y */

           /* Image data pointers. */
   #define VPX_PLANE_PACKED 0  /**< To be used for all packed formats */
   #define VPX_PLANE_Y      0  /**< Y (Luminance) plane */
   #define VPX_PLANE_U      1  /**< U (Chroma) plane */
   #define VPX_PLANE_V      2  /**< V (Chroma) plane */
   #define VPX_PLANE_ALPHA  3  /**< A (Transparency) plane */
   #if !defined(VPX_CODEC_DISABLE_COMPAT) || !VPX_CODEC_DISABLE_COMPAT
   #define PLANE_PACKED     VPX_PLANE_PACKED
   #define PLANE_Y          VPX_PLANE_Y
   #define PLANE_U          VPX_PLANE_U
   #define PLANE_V          VPX_PLANE_V
   #define PLANE_ALPHA      VPX_PLANE_ALPHA
   #endif

Bankoski, et al.              Informational                   [Page 294]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

           unsigned char *planes[4];  /**< pointer to the top-left pixel
           q                               for each plane */
           int    stride[4];  /**< stride between rows for each plane */

           int    bps; /**< bits per sample (for packed formats) */

           /* The following member may be set by the application to
            * associate data with this image.
            */
           void   *user_priv; /**< may be set by the application to
                                    associate data with this image. */

           /* The following members should be treated as private. */
           unsigned char *img_data;       /**< private */
           int      img_data_owner; /**< private */
           int      self_allocd;    /**< private */
       } vpx_image_t; /**< alias for struct vpx_image */

       /**\brief Representation of a rectangle on a surface */
       typedef struct vpx_image_rect
       {
           unsigned int x; /**< leftmost column */
           unsigned int y; /**< topmost row */
           unsigned int w; /**< width */
           unsigned int h; /**< height */
       } vpx_image_rect_t; /**< alias for struct vpx_image_rect */

       /*!\brief Open a descriptor, allocating storage for the
        * underlying image
        *
        * Returns a descriptor for storing an image of the given format.
        * The storage for the descriptor is allocated on the heap.
        *
        * \param[in]    img       Pointer to storage for descriptor.
        *                         If this parameter is NULL, the storage
        *                         for the descriptor will be allocated
        *                         on the heap.
        * \param[in]    fmt       Format for the image
        * \param[in]    d_w       Width of the image
        * \param[in]    d_h       Height of the image
        * \param[in]    align     Alignment, in bytes, of each row in
        *                         the image.
        *
        * \return Returns a pointer to the initialized image descriptor.
        *         If the img parameter is non-null, the value of the img
        *         parameter will be returned.
        */

Bankoski, et al.              Informational                   [Page 295]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

       vpx_image_t *vpx_img_alloc(vpx_image_t  *img,
                                  vpx_img_fmt_t fmt,
                                  unsigned int d_w,
                                  unsigned int d_h,
                                  unsigned int align);

       /*!\brief Open a descriptor, using existing storage for the
        * underlying image
        *
        * Returns a descriptor for storing an image of the given format.
        * The storage for descriptor has been allocated elsewhere, and a
        * descriptor is desired to "wrap" that storage.
        *
        * \param[in]    img       Pointer to storage for descriptor.
        *                         If this parameter is NULL, the storage
        *                         for the descriptor will be
        *                         allocated on the heap.
        * \param[in]    fmt       Format for the image
        * \param[in]    d_w       Width of the image
        * \param[in]    d_h       Height of the image
        * \param[in]    align     Alignment, in bytes, of each row in
        *                         the image.
        * \param[in]    img_data  Storage to use for the image
        *
        * \return Returns a pointer to the initialized image descriptor.
        *         If the img parameter is non-null, the value of the img
        *         parameter will be returned.
        */
       vpx_image_t *vpx_img_wrap(vpx_image_t  *img,
                                 vpx_img_fmt_t fmt,
                                 unsigned int d_w,
                                 unsigned int d_h,
                                 unsigned int align,
                                 unsigned char      *img_data);

       /*!\brief Set the rectangle identifying the displayed portion of
        * the image
        *
        * Updates the displayed rectangle (aka viewport) on the image
        * surface to match the specified coordinates and size.
        *
        * \param[in]    img       Image descriptor
        * \param[in]    x         leftmost column
        * \param[in]    y         topmost row
        * \param[in]    w         width
        * \param[in]    h         height
        *

Bankoski, et al.              Informational                   [Page 296]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

        * \return 0 if the requested rectangle is valid, non-zero
        * otherwise.
        */
       int vpx_img_set_rect(vpx_image_t  *img,
                            unsigned int  x,
                            unsigned int  y,
                            unsigned int  w,
                            unsigned int  h);

       /*!\brief Flip the image vertically (top for bottom)
        *
        * Adjusts the image descriptor's pointers and strides to make
        * the image be referenced upside-down.
        *
        * \param[in]    img       Image descriptor
        */
       void vpx_img_flip(vpx_image_t *img);

       /*!\brief Close an image descriptor
        *
        * Frees all allocated storage associated with an image
        * descriptor.
        *
        * \param[in]    img       Image descriptor
        */
       void vpx_img_free(vpx_image_t *img);

   #endif
   #ifdef __cplusplus
   }
   #endif

   ---- End code block ----------------------------------------

Bankoski, et al.              Informational                   [Page 297]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

20.24.  vpx_integer.h

   ---- Begin code block --------------------------------------

   /*
    *  Copyright (c) 2010, 2011, Google Inc.  All rights reserved.
    *
    *  Use of this source code is governed by a BSD-style license
    *  that can be found in the LICENSE file in the root of the source
    *  tree.  An additional intellectual property rights grant can be
    *  found in the file PATENTS.  All contributing project authors may
    *  be found in the AUTHORS file in the root of the source tree.
    */

   #ifndef VPX_INTEGER_H
   #define VPX_INTEGER_H

   /* get ptrdiff_t, size_t, wchar_t, NULL */
   #include <stddef.h>

   #if defined(_MSC_VER) || defined(VPX_EMULATE_INTTYPES)
   typedef signed char  int8_t;
   typedef signed short int16_t;
   typedef signed int   int32_t;
   typedef unsigned char  uint8_t;
   typedef unsigned short uint16_t;
   typedef unsigned int   uint32_t;

   #if defined(_MSC_VER)
   typedef signed __int64   int64_t;
   typedef unsigned __int64 uint64_t;
   #define PRId64 "I64d"
   #endif

   #ifdef HAVE_ARMV6
   typedef unsigned int int_fast16_t;
   #else
   typedef signed short int_fast16_t;
   #endif
   typedef signed char int_fast8_t;
   typedef unsigned char uint_fast8_t;

   #ifndef _UINTPTR_T_DEFINED
   typedef unsigned int   uintptr_t;
   #endif

Bankoski, et al.              Informational                   [Page 298]
RFC 6386           VP8 Data Format and Decoding Guide      November 2011

   #else

   /* Most platforms have the C99 standard integer types. */

   #if defined(__cplusplus) && !defined(__STDC_FORMAT_MACROS)
   #define __STDC_FORMAT_MACROS
   #endif
   #include <stdint.h>
   #include <inttypes.h>

   #endif

   #endif

   ---- End code block ----------------------------------------

20.25.  AUTHORS File

   Aaron Watry <awatry@gmail.com>

   Adrian Grange <agrange@google.com>

   Alex Converse <alex.converse@gmail.com>

   Andoni Morales Alastruey <ylatuya@gmail.com>

   Andres Mejia <mcitadel@gmail.com>

   Attila Nagy <attilanagy@google.com>

   Fabio Pedretti <fabio.ped@libero.it>

   Frank Galligan <fgalligan@google.com>

   Fredrik Soederquist <fs@opera.com>

   Fritz Koenig <frkoenig@google.com>

   Gaute Strokkenes <gaute.strokkenes@broadcom.com>

   Giuseppe Scrivano <gscrivano@gnu.org>

   Guillermo Ballester Valor <gbvalor@gmail.com>

   Henrik Lundin <hlundin@google.com>

   James Berry <jamesberry@google.com>

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   James Zern <jzern@google.com>

   Jan Kratochvil <jan.kratochvil@redhat.com>

   Jeff Muizelaar <jmuizelaar@mozilla.com>

   Jim Bankoski <jimbankoski@google.com>

   Johann Koenig <johannkoenig@google.com>

   John Koleszar <jkoleszar@google.com>

   Justin Clift <justin@salasaga.org>

   Justin Lebar <justin.lebar@gmail.com>

   Luca Barbato <lu_zero@gentoo.org>

   Makoto Kato <makoto.kt@gmail.com>

   Martin Ettl <ettl.martin78@googlemail.com>

   Michael Kohler <michaelkohler@live.com>

   Mikhal Shemer <mikhal@google.com>

   Pascal Massimino <pascal.massimino@gmail.com>

   Patrik Westin <patrik.westin@gmail.com>

   Paul Wilkins <paulwilkins@google.com>

   Pavol Rusnak <stick@gk2.sk>

   Philip Jaegenstedt <philipj@opera.com>

   Scott LaVarnway <slavarnway@google.com>

   Tero Rintaluoma <teror@google.com>

   Timothy B. Terriberry <tterribe@xiph.org>

   Tom Finegan <tomfinegan@google.com>

   Yaowu Xu <yaowu@google.com>

   Yunqing Wang <yunqingwang@google.com>

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   Google Inc.

   The Mozilla Foundation

   The Xiph.Org Foundation

20.26.  LICENSE

   Copyright (c) 2010, 2011, Google Inc.  All rights reserved.

   Redistribution and use in source and binary forms, with or without
   modification, are permitted provided that the following conditions
   are met:

   o  Redistributions of source code must retain the above copyright
      notice, this list of conditions and the following disclaimer.

   o  Redistributions in binary form must reproduce the above copyright
      notice, this list of conditions and the following disclaimer in
      the documentation and/or other materials provided with the
      distribution.

   o  Neither the name of Google nor the names of its contributors may
      be used to endorse or promote products derived from this software
      without specific prior written permission.

   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
   A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT
   HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
   INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
   BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
   OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
   AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY
   WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   POSSIBILITY OF SUCH DAMAGE.

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20.27.  PATENTS

   Additional IP Rights Grant (Patents)

   "This implementation" means the copyrightable works distributed by
   Google as part of the WebM Project.

   Google hereby grants to you a perpetual, worldwide, non-exclusive,
   no-charge, royalty-free, irrevocable (except as stated in this
   section) patent license to make, have made, use, offer to sell, sell,
   import, transfer, and otherwise run, modify and propagate the
   contents of this implementation of VP8, where such license applies
   only to those patent claims, both currently owned by Google and
   acquired in the future, licensable by Google that are necessarily
   infringed by this implementation of VP8.  This grant does not include
   claims that would be infringed only as a consequence of further
   modification of this implementation.  If you or your agent or
   exclusive licensee institute or order or agree to the institution of
   patent litigation against any entity (including a cross-claim or
   counterclaim in a lawsuit) alleging that this implementation of VP8
   or any code incorporated within this implementation of VP8
   constitutes direct or contributory patent infringement, or inducement
   of patent infringement, then any patent rights granted to you under
   this License for this implementation of VP8 shall terminate as of the
   date such litigation is filed.

21.  Security Considerations

   A VP8 decoder should take appropriate security considerations into
   account, as outlined in [RFC4732] and [RFC3552].  It is extremely
   important that a decoder be robust against malicious payloads.
   Malicious payloads must not cause the decoder to overrun its
   allocated memory or to consume inordinate resources.  Although
   encoder issues are typically rarer, the same applies to an encoder.
   Malicious stream data must not cause the encoder to misbehave, as
   this might allow an attacker access to transcoding gateways.

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22.  References

22.1.  Normative Reference

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

22.2.  Informative References

   [Bell]      Bell, T., Cleary, J., and I. Witten, "Text Compression",
               1990.

   [ISO-C99]   International Organization for Standardization,
               "Information technology --  Programming languages -- C",
               ISO/IEC 9899:1999, 1999.

   [ITU-R_BT.601]
               International Telecommunication Union, "ITU BT.601-7:
               Studio encoding parameters of digital television for
               standard 4:3 and wide screen 16:9 aspect ratios",
               March 2011.

   [Kernighan] Kernighan, B. and D. Ritchie, "The C Programming Language
               (2nd edition)", April 1988.

   [Loeffler]  Loeffler, C., Ligtenberg , A., and G. Moschytz,
               "Practical Fast 1-D DCT Algorithms with 11
               Multiplications", May 1989.

   [RFC3552]   Rescorla, E. and B. Korver, "Guidelines for Writing RFC
               Text on Security Considerations", BCP 72, RFC 3552,
               July 2003.

   [RFC4732]   Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet
               Denial-of-Service Considerations", RFC 4732,
               December 2006.

   [Shannon]   Shannon, C., "A Mathematical Theory of Communication",
               Bell System Technical Journal Vol. 27, pp. 379-423 and
               623-656, July and October 1948.

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Authors' Addresses

   James Bankoski
   Google Inc.

   EMail: jimbankoski@google.com

   John Koleszar
   Google Inc.

   EMail: jkoleszar@google.com

   Lou Quillio
   Google Inc.

   EMail: louquillio@google.com

   Janne Salonen
   Google Inc.

   EMail: jsalonen@google.com

   Paul Wilkins
   Google Inc.

   EMail: paulwilkins@google.com

   Yaowu Xu
   Google Inc.

   EMail: yaowu@google.com

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