[Webkit-unassigned] [Bug 98396] [CSS Shaders] Make custom filters use a premultiplied buffer for performance optimization.

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=98396





--- Comment #2 from Max Vujovic <mvujovic at adobe.com>  2012-10-04 11:35:34 PST ---
(From update of attachment 167071)
Nice patch!

View in context: https://bugs.webkit.org/attachment.cgi?id=167071&action=review

> Source/WebCore/platform/graphics/filters/FECustomFilter.cpp:227
> +    Uint8ClampedArray* dstPixelArray = createPremultipliedImageResult();

When an author is not using the CSS mix function in his or her shader, the author is writing directly to gl_FragColor. For example:

CSS:
-webkit-filter: custom(url(shader.vs) url(shader.fs));

shader.fs:
void main()
{
    gl_FragColor = vec4(0.5);
}

In this case, we can't guarantee the author has premultiplied the gl_FragColor output. Thus, we have to add a condition:

Uint8ClampedArray* dstPixelArray = (m_validatedProgram->programInfo().programType == PROGRAM_TYPE_NO_ELEMENT_TEXTURE ? createUnmultipliedImageResult() : createPremultipliedImageResult());

To be clear, PROGRAM_TYPE_NO_ELEMENT_TEXTURE means the author is not using the CSS mix function and is writing directly to gl_FragColor. PROGRAM_TYPE_BLENDS_ELEMENT_TEXTURE means the the author is using the CSS mix function and writing to css_MixColor. 

> LayoutTests/css3/filters/custom/custom-filter-composite-source-over-expected.html:4
> +    <title>Tests the source-over composite operator between source and destination color with alpha.</title>

Nice test! I think this should say:
"Tests the source-atop composite operator when source alpha is a fractional value."

I would also rename the test to something like:
custom-filter-composite-fractional-source-alpha.html

> LayoutTests/css3/filters/custom/custom-filter-composite-source-over.html:42
> +        -webkit-filter: custom(none mix(url('../resources/mix-color.fs') normal source-over), mix_color 0.0 0.8 0.5 0.6);

If you land this before bug 97859, you'll want to use source-atop, since we don't have source-over yet.

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