Re: [w3ctag/design-reviews] WG New Spec: HDR on the web (CSS, Canvas, WebGL, WebGPU) (Issue #1194)

jyasskin left a comment (w3ctag/design-reviews#1194)

We greatly appreciate your effort to come up with a coherent design for adding HDR across the whole web platform! Overall, we think you've assembled a good set of experts in the Color on the Web CG, and that we shouldn't try to second-guess your technical details. We have two remaining comments:

We think https://github.com/w3c/ColorWeb-CG/issues/129#issuecomment-3883959470 is correct that every headroom-related property should include whether it's linear or log2 in its name, to prevent incorrect assumptions. For example, we noticed that https://github.com/w3c/ColorWeb-CG/blob/main/canvas-smpte-st-2094-50.md uses `baselineHdrHeadroom` without specifying which it is, and that's probably a mistake. Ideally, you'll make the whole platform [consistent](https://www.w3.org/TR/design-principles/#consistency) in the choice of linear vs log2. However, it's possible that the correct design is, for example, to use `linear` in most places for consistency with color values, but to use log2 in places that are closely tied to image formats that already use log2. As long as you're trying for as much consistency as possible with surrounding platform features, we think you'll make reasonable choices.

We noticed that https://github.com/w3c/ColorWeb-CG/blob/main/hdr-big-picture.md#css-hdr-colors says "The [hdr-color](https://drafts.csswg.org/css-color-hdr/#funcdef-hdr-color) functional ... allows an CSS color to color match a pixel in an HDR image at all HDR headrooms." However, [color-hdr()](https://drafts.csswg.org/css-color-hdr/#funcdef-hdr-color) appears to only allow geometric-mean interpolation (or whatever the right term is) between specified headrooms, while the SMPTE ST 2086 tone mapping seems more flexible. Have we misread the specs, is there a feature missing from the CSS function, or does the CSS function do a "good enough" job that websites are unlikely to care about the difference?

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