RTP Payload Format for Uncompressed Video: Additional Colour Sampling Modes
updates
- rfc-4175 — RTP Payload Format for Uncompressed Video
Extracted elements (9)
The new modes are motivated by the inability of RFC 4175's signalling to express formats where colour components have differing bit depths. The primary use case is efficient packing of pixels into small pgroups, most commonly 'depth=5' where an RGB565 pixel fits exactly into two octets without padding.
This memo updates RFC 4175; all features of the RFC 4175 payload format remain unchanged. The new colour sampling modes extend only the signalling parameters and are fully compatible with the existing packetisation scheme.
Six new colour sampling modes ('RGB+', 'RG+B', 'R+GB', 'BGR+', 'BG+R', 'B+GR') MAY be signalled for use with RFC 4175. These modes use the same sample packing order as RGB and BGR modes respectively, except that an additional bit per sample 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 RFC 4175.
SDP signalling for the new modes uses the 'sampling' parameter of the video/raw media type, e.g., 'a=fmtp:99 sampling=RG+B; width=1024; height=768; depth=5; colorimetry=SMPTE240M'.
Six new colour sampling mode identifiers are defined: 'RGB+', 'RG+B', 'R+GB', 'BGR+', 'BG+R', and 'B+GR'. The '+' symbol designates the component that carries one additional bit of colour depth relative to the depth parameter N.
The video/raw media type 'sampling' parameter registry (governed by RFC 4175 Section 6.2) is extended with six new values: 'RGB+', 'RG+B', 'R+GB', 'BGR+', 'BG+R', and 'B+GR'.
The security considerations of RFC 4175 apply in full. No additional security considerations are introduced by supporting the new colour sampling modes defined in this memo.
For 'depth=5' with an RG+B sampling mode, the Green component uses 6 bits and Red/Blue each use 5 bits, yielding 16 bits (2 octets) per pixel. Sample packing order follows RFC 4175 Section 4.3 RGB conventions.