Match other browsers by interpolating colors in floating point space rather than uint8_t space

[Sam Weinig]

Match other browsers by interpolating colors in floating point space rather than uint8_t space

[Sam Weinig]

Created attachment 423133 [details]
Patch

[Sam Weinig]

This does seem to fix WPT results, though I am curious if there is a way to do this and stay in uint8_t space using more clever rounding.

For each color channel, we are doing:

premultiplication:

  float:
    color.red = color.red * color.alpha

  uint8_t:
    color.red = static_cast<uint8_t>(fastDivideBy255(color.red * color.alpha + 254))


blending:

  float:
    from + (to - from) * progress

  uint8_t:
    static_cast<int>(lround(static_cast<double>(from) + static_cast<double>(to - from) * progress))


unpremultiplication:

  float:
    color.red = color.red / color.alpha

  uint8_t:
    color.red = static_cast<uint8_t>((fastMultiplyBy255(color.red) + color.alpha - 1) / color.alpha)

[EWS]

Committed r274408: <https://commits.webkit.org/r274408>

All reviewed patches have been landed. Closing bug and clearing flags on attachment 423133 [details].

[Radar WebKit Bug Importer]

<rdar://problem/75419227>