Add gamut mapping details

[palemieux]

From w3ctag/design-reviews#917 (comment)

While we appreciated the discussion on volume mapping, we still had questions about the specifics. We see several cases where gamut/volume/tone mapping come into play, and we’d love some more detail on how these are handled (actually, the same questions apply to wider/smaller gamuts too):

• HDR image painted on a non-HDR canvas
• HDR canvas displayed non-HDR screen
• Author painting an HDR CSS color on a non-HDR canvas

[palemieux]

Annex A already contains many details. I have asked for clarification on the comment at w3ctag/design-reviews#917 (comment)

[ccameron-chromium]

From w3ctag/design-reviews#917 (comment)

While we appreciated the discussion on volume mapping, we still had questions about the specifics. We see several cases where gamut/volume/tone mapping come into play, and we’d love some more detail on how these are handled (actually, the same questions apply to wider/smaller gamuts too):

• HDR image painted on a non-HDR canvas

I filed whatwg/html#9112 about this (and the general issue of displaying HDR images).

Suppose we have HDR image that fits within the sRGB gamut (for simplicity).
I think the following should be equivalent:

• The image painted to a non-HDR (srgb) canvas
• The image displayed as an <img> element on an sRGB monitor

• HDR canvas displayed non-HDR screen

Over in the WebGPU review, we ended up replacing CanvasColorMetadata with CanvasToneMapping, which had a mode variable, which provides the answer. (In the context of WebGPU, there wasn't consensus about anything except "standard" and "extended", but there are notes in the explainer about how "hdr10" would be added).

• Author painting an HDR CSS color on a non-HDR canvas

I'd want to discuss this in the context of defining HDR CSS colors.

[palemieux]

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