Add WCG color spaces.

[sandersdan]

This implements the color spaces portion of #384, but does not add pixel formats or HDR metadata.

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Preview | Diff

[sandersdan]

The alternative names here are "smpte2084" and "arib-std-b67", which would align with FFmpeg naming.

[chrisn]

Looks OK as is, but if you're considering changing, I'd suggest referencing BT2100, so something like bt2100-2-pq and bt2100-2-hlg.

[dalecurtis]

Is that more legible? Even Wikipedia just calls it PQ/HLG.

[chrisn]

To clarify, my preference is for pq and hlg

[dalecurtis]

Ah great :) I think Dan was just playing devils advocate above.

[chrisn]

I see! My comment was really: if you're going in that direction, prefer BT.2100 rather than the SMPTE and ARIB standards.

[chrisn]

Should we align these with Media Capabilities ColorGamut and TransferFunction? Are there cases where WebCodecs needs a different set of possible values than Media Capabilities?

[sandersdan]

Should we align these with Media Capabilities ColorGamut and TransferFunction?

The short answer is that we diverged with the first version of WebCodecs, which we decided was acceptable since WebCodecs does not integrate with MediaCapabilities. Some of the values I picked in this PR do match between the two, at least.

Perhaps more relevant is alignment with https://github.com/w3c/ColorWeb-CG/blob/master/hdr_html_canvas_element.md, but that is not yet finalized and also has a different technical interpretation.

[dalecurtis]

Names lgtm since we're using what's named in H.273.

[dalecurtis]

Any last comments on these names? It's been 3 weeks, so we'll go ahead and merge tomorrow otherwise.

[chrisn]

Thanks @dalecurtis, sorry for the delay. I left one minor comment to consider, but otherwise looks good.

[dalecurtis]

Thanks all. I think we have sufficient approval to merge now.