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  1. README.md
docs/native-code/rtp-hdrext/color-space/README.md

Color Space

The color space extension is used to communicate color space information and optionally also metadata that is needed in order to properly render a high dynamic range (HDR) video stream. Contact kron@google.com for more info.

Name: “Color space” ; “RTP Header Extension for color space”

Formal name: http://www.webrtc.org/experiments/rtp-hdrext/color-space

Status: This extension is defined here to allow for experimentation. Once experience has shown that it is useful, we intend to make a proposal based on it for standardization in the IETF.

RTP header extension format

Data layout overview

Data layout without HDR metadata (one-byte RTP header extension) 1-byte header + 4 bytes of data:

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  ID   | L = 3 |   primaries   |   transfer    |    matrix     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |range+chr.sit. |
 +-+-+-+-+-+-+-+-+

Data layout of color space with HDR metadata (two-byte RTP header extension) 2-byte header + 28 bytes of data:

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      ID       |   length=28   |   primaries   |   transfer    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    matrix     |range+chr.sit. |         luminance_max         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         luminance_min         |            mastering_metadata.|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |primary_r.x and .y             |            mastering_metadata.|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |primary_g.x and .y             |            mastering_metadata.|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |primary_b.x and .y             |            mastering_metadata.|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |white.x and .y                 |    max_content_light_level    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | max_frame_average_light_level |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Data layout details

The data is written in the following order, Color space information (4 bytes):

  • Color primaries value according to ITU-T H.273 Table 2.
  • Transfer characteristic value according to ITU-T H.273 Table 3.
  • Matrix coefficients value according to ITU-T H.273 Table 4.
  • Range and chroma siting as specified at https://www.webmproject.org/docs/container/#colour. Range (range), horizontal (horz) and vertical (vert) siting are merged to one byte by the operation: (range << 4) + (horz << 2) + vert.

The extension may optionally include HDR metadata written in the following order, Mastering metadata (20 bytes):

  • Luminance max, specified in nits, where 1 nit = 1 cd/m2. (16-bit unsigned integer)
  • Luminance min, scaled by a factor of 10000 and specified in the unit 1/10000 nits. (16-bit unsigned integer)
  • CIE 1931 xy chromaticity coordinates of the primary red, scaled by a factor of 50000. (2x 16-bit unsigned integers)
  • CIE 1931 xy chromaticity coordinates of the primary green, scaled by a factor of 50000. (2x 16-bit unsigned integers)
  • CIE 1931 xy chromaticity coordinates of the primary blue, scaled by a factor of 50000. (2x 16-bit unsigned integers)
  • CIE 1931 xy chromaticity coordinates of the white point, scaled by a factor of 50000. (2x 16-bit unsigned integers)

Followed by max light levels (4 bytes):

  • Max content light level, specified in nits. (16-bit unsigned integer)
  • Max frame average light level, specified in nits. (16-bit unsigned integer)

Note, the byte order for all integers is big endian.

See the standard SMPTE ST 2086 for more information about these entities.

Notes: Extension should be present only in the last packet of video frames. If attached to other packets it should be ignored.