Don't unpremultiply alpha for Mask calculation
#987
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We were multiplying by alpha directly after again.
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Hmm but the specification says:
But I guess it makes sense that since you can just extract the /255 they cancel each other out, but then I'm wondering why they have that sentence in the spec. |
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The calculations are performed on non-premultiplied color values.
But I guess it makes sense that since you can just extract the /255 they cancel each other out, but then I'm wondering why they have that sentence in the spec.
For the calculations in that section generally it matters, and the luminance-to-alpha as calculated in that section only takes luminance into account, ignoring alpha, which does require the color to be non-premultiplied.
However, to use that value for calculating a mask, you multiply that value by the original alpha again (Masking Module Level 1 § 7.10.1). In this case that works out to be the same as doing the entire thing with the premultiplied color we already had.
We were multiplying by alpha directly after again.