RTP Payload Format for Uncompressed Video: Additional Colour Sampling Modes
draft-ietf-avt-uncomp-video-ext-01

Network Working Group                                         C. Perkins
Internet-Draft                                     University of Glasgow
Updates: 4175 (if approved)                           September 25, 2005
Expires: March 29, 2006

 RTP Payload Format for Uncompressed Video: Additional Colour Sampling
                                 Modes
                 draft-ietf-avt-uncomp-video-ext-01.txt

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Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This memo extends the RTP Payload Format for Uncompressed Video to
   support additional colour sampling modes.

1.  Introduction

   The RTP Payload Format for Uncompressed Video [1] defines a scheme to
   packetise uncompressed, studio-quality, video streams for transport

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   using RTP [2].  A range of standard and high definition video formats
   are supported, and parameters are defined so sender and receiver can
   negotiate the image size, colour space, pixel depth, and colour
   sampling mode.

   A limitation of the signalling is that the number of bits per sample
   is assumed to be the same for each colour component.  For example, it
   is possible to signal video using RGB colour sampling with 8-bits for
   each of the Red, Green and Blue components (24 bits per pixel), but
   not video using RGB colour sampling with 5-bits each for the Red and
   Blue components, but 6-bits for the Green component (16 bits per
   pixel).  Such video formats can easily be transported by the payload
   format, but cannot be signalled using the parameters defined.  This
   memo extends [1] with additional colour sampling modes, to signal
   such video formats.

2.  Conventions Used in this Document

   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 [3].

3.  Payload Format Parameters

   This memo defines six new colour sampling modes that MAY be signalled
   for use with [1].  The new modes are "RGB+", "RG+B", "R+GB", "BGR+",
   "BG+R" and "B+GR".  These sampling modes use the same packing order
   of samples as do the RGB and BGR colour sampling modes respectively
   ([1] Section 4.3), except that an additional bit per sample of colour
   depth MUST be used for the component marked by the + symbol.  The
   mandatory parameter "depth=N" indicates that N bits per sample are
   used by the unmarked components, but N+1 bits are used by the marked
   component.  All other features of the payload format are as defined
   in [1].

   The primary use of these colour sampling modes is to enable efficient
   packing of data into small pixel groups ("pgroups").  The most common
   use case is expected to be video with "depth=5", where the additional
   bit of colour depth for the marked component enables a single pixel
   to fit into two octets without padding.  The new colour sampling
   modes MAY be used for other depths, however, should that prove
   useful.

4.  Example

   A common uncompressed video format is RGB with 5 bits for the Red and
   Blue components and six bits for the Green component, for a total of
   16 bits per pixel.  Using the sampling modes defined in this memo,

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   this can be signalled in SDP according to the following example:

       v=0
       o=jdoe 2890844526 2890842807 IN IP4 10.47.16.5
       s=-
       c=IN IP4 10.47.16.6
       t=2873397496 2873404696
       m=video 51372 RTP/AVP 99
       a=rtpmap:99 raw/90000
       a=fmtp:99 sampling=RG+B; width=1024; height=768; depth=5;
         colorimetry=SMPTE240M

   The last line has been wrapped due to formatting constraints of this
   memo, and forms one complete line in the SDP file.

5.  Security Considerations

   The security considerations of [1] apply.  No additional security
   considerations are introduced by support for new colour sampling
   modes.

6.  IANA Considerations

   The video/raw media type is extended with six new values for the
   "sampling" parameter according to the rules defined in section 6.2 of
   [1].  The new values are "RGB+", "RG+B", "R+GB", "BGR+", "BG+R" and
   "B+GR" as described in this memo.

7.  Acknowledgements

   Thanks to Jeremy Searle and Andrew Lee at Westland Helicopters.

8.  Normative References

   [1]  Perkins, C. and L. Gharai, "RTP Payload Format for Uncompressed
        Video", RFC 4175, September 2005.

   [2]  Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
        "RTP: A Transport Protocol for Real-Time Applications", STD 64,
        RFC 3550, July 2003.

   [3]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

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Author's Address

   Colin Perkins
   University of Glasgow
   Department of Computing Science
   17 Lilybank Gardens
   Glasgow  G12 8QQ
   UK

   Email: csp@csperkins.org

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