[Webkit-unassigned] [Bug 30333] [Color] (Un)premultiplication code is not optimal and broken in alpha==0 case

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=30333





--- Comment #3 from Sebastian Dröge <slomo at circular-chaos.org>  2009-10-13 21:47:49 PDT ---
(In reply to comment #2)
> (From update of attachment 41101 [details])
> Thanks for this contribution!
> 
> The change seems OK, but we need a test case that demonstrates the effects this
> has. We don't take bug fixes without test cases. So please add a test case
> demonstrating the effect this has. One way to create a test case may be to use
> the <canvas> element, because that supports reading back pixel values from the
> canvas buffer.
> 
> LayoutTests/fast/canvas/script-tests/set-colors.js is an example of a test case
> that might be adaptable to test this behavior.

I just noticed this while working on other code that needs alpha
premultiplication. I have absolutely *no* idea where this code here is used and
how.

I could provide a small test case that produces different results with/without
the patch by just calling those functions but no idea how to do it from a
canvas element.

> An alternative would be to explain why no test case is possible. That seems
> unlikely, though.
> 
> Also, you need to run all existing tests and generate new results for any tests
> that now will have different results.

How can I do that?

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