[Webkit-unassigned] [Bug 25956] Canvas' composition is incorrect when alpha=0

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=25956


hamaji at chromium.org changed:

           What    |Removed                     |Added
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  Attachment #30577|                            |review?
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------- Comment #2 from hamaji at chromium.org  2009-05-22 00:53 PDT -------
(From update of attachment 30577)
This is the reason why I introduced failing test cases in the previous patch:

https://bugs.webkit.org/show_bug.cgi?id=25417

This optimization was introduced 3 years ago. At this time, fillRect() always
calls fillRectSourceOver() so that this optimization was OK. So, I guess the
current behavior (when alpha is zero, all fillrect operations are skipped even
if the composite mode is 'copy') is not intentional.

http://trac.webkit.org/changeset/13992/trunk/WebCore/platform/mac/GraphicsContextMac.mm

Fortunately, Skia has the similar optimization path inside Skia and Skia checks
the composition mode if it is safe to do this optimization. I don't know if CG
has similar logic, but anyway I think this patch won't reduce performance
significantly as alpha=0 cases may be rare.

I also made error messages of the layout test case better, and added test case
for path and fill.


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