Network Working Group                                         C. Perkins
Request for Comments: 4421                         University of Glasgow
Updates: 4175                                              February 2006
Category: Standards Track


               RTP Payload Format for Uncompressed Video:
                    Additional Colour Sampling Modes

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   The RFC Payload Format for Uncompressed Video, RFC 4175, defines a
   scheme to packetise uncompressed, studio-quality, video streams for
   transport using RTP.  This memo extends the format 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
   using RTP [2].  A range of standard and high-definition video formats
   is 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
   (Section 4.3 of [1]), 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 6 bits for the Green component, for a total of 16
   bits per pixel.  Using the sampling modes defined in this memo, this
   can be signalled in Session Description Protocol (SDP) according to
   the following example:

       v=0
       o=jdoe 2890844526 2890842807 IN IP4 192.0.2.5
       s=-
       c=IN IP4 192.0.2.6
       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]  Gharai, L. and C. Perkins, "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.

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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