[Webkit-unassigned] [Bug 73952] canvas/philip/tests/2d.imageData.put.unchanged.html expects putImageData/getImageData to exactly preserve unpremul-alpha pixel values

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=73952





--- Comment #2 from Brian Salomon <bsalomon at google.com>  2011-12-07 06:50:17 PST ---
(In reply to comment #1)
> Actually, I think the test link is referring to this statement:
> 
> "The handling of pixel rounding when the specified coordinates do not exactly map to the device coordinate space is not defined by this specification, except that the following must result in no visible changes to the rendering # 2d.imageData.put.unchanged :
> 
> context.putImageData(context.getImageData(x, y, w, h), p, q);"
> 
> So although the mult/premult changes are lossy, in order to be spec-compliant, I think you have to be value-preserving over one roundtrip.

My take is that the rounding language refers to rounding coordinates to the pixel grid. The test seems to be exercising this by doing its get/put at coordinate (.1, .2) rather than (0, 0). Also it says "no visible change" not no change at all. The later language about premultiplying seems to indicate that a roundtrip may come back as "different values."

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