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Michael Niedermayer
0293b8a027
Blending took the colour's own alpha as the value written into the alpha plane, weighted by that same alpha once more, so a translucent colour left less alpha behind than it covered: half transparent red over a transparent canvas ended with alpha 64 instead of 128, and drawn over opaque content it even punched a hole into the alpha plane. The colour's transparency is already part of the coverage, so the alpha plane takes full coverage, which is source-over compositing. The drawutils test program checks this and runs under FATE. Assisted-by: Claude