Re: [csswg-drafts] [css-color-4] Premultiplication in cylindrical spaces and mixing (#11238)

> One is whether the `over` operator is defined over arbitrary color spaces or a select few, and if the former, how it should be defined for cylindrical color spaces.

CSS does not currently define composition except in https://www.w3.org/TR/compositing-1/ and only in the context of RGB spaces. It has not been defined or applied outside of that. So `over` is currently only defined in this context. This does not apply to general interpolation.

> I'm still not convinced that holding out hue from premultiplication is not needless complexity that makes things worse, though it's also possible I'm missing something. In particular, I don't follow the argument above.

Premultiplication is used essentially to weight how much each color influences the interpolation. This works well in rectangular coordinate systems. The actual hue of a color does not change in this process (speaking relative to the color space). That's why when you convert said cylindrical color to rectangular coordinates and apply premultiplication, then convert back, hue is unchanged. You should not premultiply hue because if you do, you are now interpolating something very different. 

If you cherry pick certain cases, it could look okay if we apply premultiplication to hues. For instance, you gave a specific case in OkLCh which does seem to look okay:

<img width="1208" alt="Screenshot 2024-11-19 at 10 16 12 AM" src="https://github.com/user-attachments/assets/5c99baf6-b70d-462d-a356-0a3acc64ac0a">

But if we use some other colors, it quickly falls apart:

<img width="1208" alt="Screenshot 2024-11-19 at 10 17 04 AM" src="https://github.com/user-attachments/assets/30997e3a-ee17-47c5-a33e-303e3c42db77">



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Received on Tuesday, 19 November 2024 17:20:32 UTC