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.




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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.




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RFC 4421           RTP Payload for Uncompressed Video      February 2006


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: [email protected]












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Acknowledgement

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