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Re: [csswg-drafts] [css-color] Working Color Space (#300)

From: Chris Lilley via GitHub <sysbot+gh@w3.org>
Date: Wed, 09 Dec 2020 12:35:54 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-741743420-1607517353-sysbot+gh@w3.org>
To catch up on this long-standing issue:

There cannot be a per-document or even per-element `working colorspace` because different options require calculations to be done in different colorspaces:

- [color interpolation is now defined in CSS Color 5](https://drafts.csswg.org/css-color-5/#interpolation) and uses Lab by default, but individual properties or functions can override this
- [`color-mix()`](https://drafts.csswg.org/css-color-5/#color-mix) overrides the interpolation default to LCH, but has syntax to specify a different colorspace
- [compositing](https://drafts.fxtf.org/compositing-1/) currently uses gamma-encodes sRGB (which is terrible, but Web compatible) and will need to be upgraded to at minimum allow a linear-light option. CIE XYZ is the obvious choice here. Implementations can calculate in another linear-light space, like linear-light display-p3, and get the same result modulo gamut clipping. See https://github.com/w3c/csswg-drafts/issues/5669
- [gradients currently use alpha-premultiplied gamma-encoded sRGB](https://www.w3.org/TR/css-images-3/#coloring-gradient-line) for Web compat, and will need to be upgraded to allow a perceptually uniform (still alpha premultiplied) option. Lab and LCH are the obvious candidates there. CSS Color 5 defines [alpha-premultiplication for all color spaces](https://drafts.csswg.org/css-color-5/#interpolation-alpha)

Also the need for a working colorspace for serialization was removed [serialized value is in the colorspace it was declared](https://drafts.csswg.org/css-color-4/Overview.html#serializing-color-values).

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