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# 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/m<sup>2</sup>.
(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.
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