Re: [fxtf-drafts] [css-compositing-2] Remove plus-darker (#447)

I wrote a test in that tests the composite modes in canvas.  It turns out my test objected to the formula in because it wasn't producing properly premultiplied colors:  there were cases where the color result was greater than the alpha result, which isn't allowed.

However, my formula was sort of close; it appears (based on fiddling with my test) that what Safari implements for `plus-darker` is:

co = min(1, αs + αb) - min(1, αs x (1 - Cs) + αb x (1 - Cb));
αo = min(1, αs + αb);

(It's possible there's a simpler way to write this, but that's the expression I figured out that leads to Safari passing the test for `plus-darker`.)

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Received on Wednesday, 30 August 2023 14:27:56 UTC