Network Working Group S. Midtskogen
Internet-Draft Cisco
Intended status: Standards Track August 8, 2016
Expires: February 9, 2017
Improved chroma prediction
draft-midtskogen-netvc-chromapred-01
Abstract
This document describes the technique used to improve the chroma
prediction in the Thor video codec.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 2
4. Computing the improved prediction . . . . . . . . . . . . . . 3
5. Performance . . . . . . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
Modern video coding standards such as Thor [I-D.fuldseth-netvc-thor]
form predictions for the luma channel (Y) and chroma channels (U and
V) which are encoded separately (in that order). The prediction for
each channel has spatial or temporal dependencies only in its own
channel. Most of the perceived information of a video is to be found
in the luma channel, but there still remain correlations between the
luma and chroma channels. For instance, the same shape of an object
can often be seen in all three channels, and if this correlation is
not exploited, some structural information will be transmitted three
times. Thor will attempt to improve the chroma prediction by finding
linear relationships between the each of the initial chroma
predictions and the luma prediction, and if certain criteria are
satisfied, use that relationship to form a new prediction based on
the reconstructed luma samples.
2. Definitions
2.1. Requirements Language
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].
3. Background
The improved predictions are derived from the reconstructed luma
samples using a mapping. The underlying assumption is that the
colours can be identified by their luminosities. Informally we can
say that a new chroma prediction is formed from the reconstructed
luma block painted with the colours of the initial chroma prediction.
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There is often a linear correlation between the luma and chroma
channel, so that a chroma sample c can be expressed by the linear
function
c = a*y + b
Figure 1: Linear relationship
where y is the corresponding luma sample. This observation has been
previously been used in techniques to convert YUV 4:2:0 and YUV 4:2:2
images to YUV 4:4:4, and in a (rejected) proposal for HEVC as a
special intra mode. Thor, however, generalises the prediction, so it
does not depend on the coding mode (i.e. whether inter or intra, or
the kind of inter/intra mode).
Since it would be too costly to transmit the values a and b in the
linear mapping, and since both the encoder and decoder must be able
to compute identical predictions, a and b are derived from data
available to both using linear regression.
4. Computing the improved prediction
Since the assumption that the correlation is the same in the
predicted block and in the reconstructed block is not always true,
the new prediction from luma might not be better even when there is a
very good correlation in the predicted block. Therefore, we can only
expected an improvement if the initial prediction is bad, and the
luma residual is used as an estimate for this. The initial chroma
prediction is kept unless the average squared difference between the
reconstructed luma samples yr and the predicted y samples for an N*N
prediction block is above 64:
_N_ _N_
\ \
/__ /__ (yr(i, j) - y(i, j)) ^ 2
i=1 j=1
-------------------------------- > 64
N*N
Figure 2: Requirement for improvement 1
The encoder and decoder must compute a and b using the same least
square fit for an N*N prediction block, where y and c denote the luma
and chroma samples in the initial prediction:
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_N_ _N_ _N_ _N_
\ \ \ \
Ysum = /__ /__ y(i, j) Csum = /__ /__ c(i, j)
i=1 j=1 i=1 j=1
_N_ _N_ _N_ _N_
\ \ \ \
YYsum = /__ /__ y(i, j) ^ 2 CCsum = /__ /__ c(i, j) ^ 2
i=1 j=1 i=1 j=1
_N_ _N_
\ \
YCsum = /__ /__ y(i, j) * c(i, j)
i=1 j=1
Figure 3: Equations for linear regression 1
These sums will all be contained within a 32 bit signed integer.
Then the following must be computed using 64 bit arithmetic:
SSyy = YYsum - ((Ysum * Ysum) >> 2*log2(N))
SScc = CCsum - ((Csum * Csum) >> 2*log2(N))
SSyc = YCsum - ((Ysum * Csum) >> 2*log2(N))
Figure 4: Equations for linear regression 2
Still using 64 bit arithmetic, if
SSyy > 0 /\ 2 * SSyy * SSyy > SSyy * SScc
Figure 5: Requirement for improvement 2
then it is assumed that the correlation is reasonably good and a new
prediction will be computed and used. Otherwise, the initial
prediction will be kept. First, a and b must be computed. 2^15 is
added to b to ensure correct rounding later on.
a = (SSyc << 16) / SSyy
b = (((Csum << 16) - a * Ysum) >> 2*log2(N)) + (1 << 15)
Figure 6: Equation for linear regression 3
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The final operations are performed with 32 bit arithmetic, so a must
be clipped to [-2^23, 2^23] and b must be clipped to [-2^31, 2^31-1].
The a new chroma prediction c' is computed using the reconstructed
luma samples yr, a and b, and a clipping function saturating the
results to an 8 bit value:
c'(i, j) = clip((a * yr(i, j) + b) >> 16)
Figure 7: Improved chroma prediction
The above assumes 4:4:4 format. For the 4:2:0 format the predicted
luma block must be subsampled first:
y'(i,j) = (y(2*i, 2*j) + y(2*i+i, 2j) +
y(2*i, 2*j+1) + y(2*i+1, 2*j+1) + 2) >> 2
Figure 8: Subsampling of predicted luma block
The resulting new chroma prediction must also be subsampled. The
clipping is performed before the subsampling.
c'(i, j) = (clip((a*yr(2*i, 2*j) + b) >> 16) +
clip((a*yr(2*i+1, 2*j) + b) >> 16) +
clip((a*yr(2*i, 2*j+1) + b) >> 16) +
clip((a*yr(2*i+1, 2*j+1) + b) >> 16) + 2) >> 2
Figure 9: Subsampling of improved chroma prediction
In intra mode the chroma prediction improvement must be performed
right after each transform, since the new chroma reconstruction will
be used to predict the next block.
5. Performance
The improved chroma prediction may significantly improve the
compression efficiency for images or video containing high
correlations between the channels. It is particularly useful for
encoding screen content, 4:4:4 content, high frequency content and
"difficult" content where traditional prediction techniques perform
poorly. Little quality change is seen for content not in these
categories, but there is a general small increase in chroma PSNR.
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An encoded configured for low delay and high complexity was used for
the following results. The numbers have been computed using the
Bjontegaard Delta Rate (BDR [BDR]). The rates for Y, U and V have
been shown separately.
+--------------+--------------------+--------------------+
| | 4:4:4 | 4:2:0 |
+--------------+------+------+------+------+------+------+
|Sequence | Y | U | V | Y | U | V |
+--------------+------+------+------+------+------+------+
|cad_waveform |-21.3%|-27.0%|-24.0%| 0.5%| -1.3%| -1.1%|
|pcb_layout | -9.2%|-13.3%|-10.6%| -1.6%| -3.1%| -3.5%|
|ppt_doc_xls | -6.3%|-14.1%|-12.7%| -0.1%| -0.8%| -0.8%|
|vc_doc_sharing| -2.9%| -6.4%| -6.9%| 0.3%| -1.2%| -0.6%|
|web_browsing | -0.5%| -1.1%| -1.5%| 0.3%| -0.5%| -1.0%|
|wordEditing | -1.8%| -5.9%| -4.8%| 1.5%| 1.2%| 1.1%|
|park_joy | -0.5%| -2.6%| -0.9%| -0.0%| -0.8%| 0.4%|
|old_town_cross| -0.1%| -2.2%| -1.2%| 0.0%| -0.6%| -0.2%|
+--------------+------+------+------+------+------+------+
|Average | -5.3%| -9.1%| -7.8%| 0.1%| -0.9%| -0.7%|
+--------------+------+------+------+------+------+------+
Figure 10: Compression Performance, improved prediction for intra
blocks only
+--------------+--------------------+--------------------+
| | 4:4:4 | 4:2:0 |
+--------------+------+------+------+------+------+------+
|Sequence | Y | U | V | Y | U | V |
+--------------+------+------+------+------+------+------+
|cad_waveform |-23.1%|-28.9%|-26.1%| -2.6%| -3.6%| -3.5%|
|pcb_layout |-21.0%|-29.0%|-21.0%| -5.4%| -7.9%| -5.4%|
|ppt_doc_xls | -9.0%|-19.0%|-17.5%| -0.2%| -0.2%| -1.2%|
|vc_doc_sharing| -4.7%| -9.6%| -9.6%| -0.1%| -1.0%| -0.4%|
|web_browsing | -0.6%| -1.5%| -1.5%| -0.5%| -1.2%| -1.2%|
|wordEditing |-11.3%|-13.7%|-11.7%| -3.0%| -4.2%| -3.2%|
|park_joy | -5.5%| -7.4%| -7.1%| -0.9%| -1.9%| -1.6%|
|old_town_cross| -1.7%| -3.6%| -2.2%| -0.3%| -4.1%| -1.6%|
+--------------+------+------+------+------+------+------+
|Average | -9.6%|-14.1%|-12.1%| -1.6%| -3.0%| -2.3%|
+--------------+------+------+------+------+------+------+
Figure 11: Compression Performance, improved prediction using intra
only coding
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+--------------+--------------------+--------------------+
| | 4:4:4 | 4:2:0 |
+--------------+------+------+------+------+------+------+
|Sequence | Y | U | V | Y | U | V |
+--------------+------+------+------+------+------+------+
|cad_waveform |-11.5%|-14.4%|-12.7%| 0.0%| -1.8%| -1.7%|
|pcb_layout | -3.2%| -5.5%| -4.8%| -0.9%| -2.4%| -3.4%|
|ppt_doc_xls | -0.1%| -0.7%| -0.3%| 0.0%| -0.2%| -0.6%|
|vc_doc_sharing| -0.4%| -0.6%| -1.6%| -0.0%| -0.4%| -0.6%|
|web_browsing | 0.1%| 0.2%| 0.1%| 0.5%| -0.0%| -0.9%|
|wordEditing | -3.7%| -5.8%| -6.2%| 0.4%| -0.9%| -1.4%|
|park_joy | -1.6%| -8.6%| -1.5%| 0.0%| -3.5%| -0.2%|
|old_town_cross| -0.0%| -0.4%| -0.1%| 0.0%| 0.1%| -0.2%|
+--------------+------+------+------+------+------+------+
|Average | -2.5%| -4.5%| -3.4%| 0.0%| -1.1%| -1.1%|
+--------------+------+------+------+------+------+------+
Figure 12: Compression Performance, improved prediction for inter
blocks only
+--------------+--------------------+--------------------+
| | 4:4:4 | 4:2:0 |
+--------------+------+------+------+------+------+------+
|Sequence | Y | U | V | Y | U | V |
+--------------+------+------+------+------+------+------+
|cad_waveform |-25.8%|-31.7%|-28.2%| -2.4%| -5.5%| -5.4%|
|pcb_layout |-11.5%|-16.1%|-13.5%| -2.4%| -4.1%| -5.6%|
|ppt_doc_xls | -6.3%|-14.3%|-13.2%| -0.2%| -0.8%| -0.8%|
|vc_doc_sharing| -3.0%| -6.7%| -8.2%| 0.1%| -0.9%| -1.1%|
|web_browsing | -0.5%| -1.2%| -1.5%| 0.2%| -0.3%| -2.0%|
|wordEditing | -3.4%| -6.8%| -6.6%| 0.6%| -0.5%| -1.4%|
|park_joy | -1.7%| -9.2%| -1.7%| -0.0%| -4.0%| 0.0%|
|old_town_cross| -0.1%| -2.2%| -1.0%| 0.1%| -0.5%| -0.1%|
+--------------+------+------+------+------+------+------+
|Average | -6.5%|-11.0%| -9.2%| -0.5%| -2.1%| -2.0%|
+--------------+------+------+------+------+------+------+
Figure 13: Compression Performance, improved prediction for intra and
inter blocks
6. IANA Considerations
This document has no IANA considerations yet. TBD
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7. Security Considerations
This document has no security considerations yet. TBD
8. Acknowledgments
The author would like to thank Arild Fuldseth and Mo Zanaty for
reviewing this document, and Timothy Terriberry for pointing a couple
of errors in the first draft.
9. References
9.1. Normative References
[I-D.fuldseth-netvc-thor]
Fuldseth, A., Bjontegaard, G., Midtskogen, S., Davies, T.,
and M. Zanaty, "Thor Video Codec", draft-fuldseth-netvc-
thor-02 (work in progress), March 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
9.2. Informative References
[BDR] Bjontegaard, G., "Calculation of average PSNR differences
between RD-curves", ITU-T SG16 Q6 VCEG-M33 , April 2001.
Author's Address
Steinar Midtskogen
Cisco
Lysaker
Norway
Email: stemidts@cisco.com
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