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.
Perkins Standards Track [Page 1]
RFC 4421 RTP Payload for Uncompressed Video February 2006
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: csp@csperkins.org
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RFC 4421 RTP Payload for Uncompressed Video February 2006
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Perkins Standards Track [Page 4]
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