[Webkit-unassigned] [Bug 150940] New: Color interpolation for colors with alpha incorrect (gradients)

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=150940

            Bug ID: 150940
           Summary: Color interpolation for colors with alpha incorrect
                    (gradients)
    Classification: Unclassified
           Product: WebKit
           Version: Safari 9
          Hardware: All
                OS: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: CSS
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: christian.schnorr at me.com

Currently the following gradients produce different results despite they should render the same:
 - opaque black to transparent black
 - opaque black to transparent white
 - opaque black to transparent anything

All transparent anythings are in fact the same color, a clear color, no matter what the underlying RGB components say. It seems all four components are currently linearly interpolated, resulting in a very odd gradient -- the second one has a semitransparent gray in the middle. For semitransparent endpoint colors we have the same issue: their RBG components affect the interpolated colors too much, in an unnatural way.

No matter in which color space colors are interpolated, their components should be weighted according to their alpha component.

Firefox seems to use the following weighted approach, producing a much more visually appealing gradient:

float r3 = (r1 * a1 * (1.0 - fraction) + r2 * a2 * fraction) / ((1.0 - fraction) * a1 + fraction * a2);
float g3 = (g1 * a1 * (1.0 - fraction) + g2 * a2 * fraction) / ((1.0 - fraction) * a1 + fraction * a2);
float b3 = (b1 * a1 * (1.0 - fraction) + b2 * a2 * fraction) / ((1.0 - fraction) * a1 + fraction * a2);
float a3 = a1 * (1.0 - fraction) + a2 * fraction;

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