[Impeller] Color::Blend appears to have bugs and only be used in tests #128606
Description
@jonahwilliams @bdero fyi
We should remove this if it's really test-only.
If it's not test-only, we should fix it. Comparing, for example, the kOverlay implementation to skia:
Impeller:
return apply_rgb_srcover_alpha([&](auto s, auto d) {
if (d * 2 < dst.alpha) {
return 2 * s * d;
}
return src.alpha * dst.alpha - 2 * (dst.alpha - s) * (src.alpha - d);
});Skia:
return apply_rgb_srcover_a([&](auto s, auto d) {
return mma(s, 1-dst.a, d, 1-src.a) +
select(two(d) <= dst.a,
two(s * d),
src.a * dst.a - two((dst.a - d) * (src.a - s)));
});Skia has dst.a - d where we have dst.alpha - s (and a similar swap in the next one). Skia also has if 2 * d <= dst.a where we have if 2 * d < dst.alpha (so we're missing the equals part of less than or equals).