[Webkit-unassigned] [Bug 168425] REGRESSION(r205841): [GTK] Test fast/images/animated-png.html is failing since r205841

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=168425

Miguel Gomez <magomez at igalia.com> changed:

           What    |Removed                     |Added
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                 CC|                            |sabouhallawa at apple.com

--- Comment #3 from Miguel Gomez <magomez at igalia.com> ---
Once I found the fix for the animation not running, I've been debugging why one of the images of the test fails. The problem seems to be how alpha premultiplied components are calculated.

Before r205841, premultiplied alpha components were calculated in PNGImageDecoder::setPixelPremultipliedRGBA as

fastDivideBy255(colorComponent * alpha);

But after r205841 this was moved to Color::premultipliedChannel, where it's calculated as

fastDivideBy255(colorComponent * alpha + 254);

which is not producing the same result as before the changes. This seems to be causing problems with animations that blend frames into the previous ones.

I'm not sure why 254 is added for the calculation. I've always seen it as just colorComponent * alpha, so I may be missing something here. Said, what do you think?

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