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Re: [csswg-drafts] Don't force non-legacy colors to interpolate in a gamma-encoded space (#5883)

From: Lea Verou via GitHub <sysbot+gh@w3.org>
Date: Wed, 20 Jan 2021 19:56:44 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-763894996-1611172603-sysbot+gh@w3.org>
> By the way, as CSSWG already discussed, how to interpolate colors also depends on why you are interpolating for. As an example, compositing should be done in a linear-light space like XYZ. So Compositing-1 needs updating there; it currently only allows compositing in gamma-encoded sRGB. Filters-1 likewise needs updating (it currently only allows linear-light sRGB). And the definitions of gradients in CSS Images-4 need updating (again they are restricted to gamma-encoded sRGB, so you can't interpolate sRGB to P3 for example).
> I'm not prepared to hold up CSS Color 4 while all those other specs get their wider-than-sRGB upgrades.

We should **definitely** not hold up Color 4 until other specs update, but that's not what I was proposing. Color 4 needs to have an interpolation section, which defines interpolation for the common case (gradients, transitions etc), and clarifies that other specs may wish to override, leaving it up to them. We typically define how values interpolate in the same spec where the values are defined. Otherwise, if a UA wishes to implement Color 4 but not Color 5, how are they supposed to implement e.g. gradients between Lab colors? Surely they can't (and shouldn't) follow the Images spec and interpolate them in (s)RGBA.

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