[Webkit-unassigned] [Bug 51928] [chromium] Enable reuse of RenderSurface for drawing.

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=51928





--- Comment #2 from W. James MacLean <wjmaclean at chromium.org>  2011-01-05 07:55:09 PST ---
(In reply to comment #1)
> Created an attachment (id=78006)
 --> (https://bugs.webkit.org/attachment.cgi?id=78006&action=review) [details]

This patch suggest a modification to RenderSurfaceChromium that allows the object to be selected for drawing multiple times via LayerRendererChromium::useRenderSurface. This use case arises when compositing to an offscreen texture (atlhough not often it appears ...the test compositing/geometry/fixed-position.html is the only case I know of thus far).

This patch also contains two minor modifications to 
LayerRendererChromium::drawLayers(), which I would like to *discuss* (I'm not necessarily proposing they would be part of the final patch).

The mods are marked with comments that begin "// WJM". While testing with fixed-position.html and using offscreen compositing, I found that there was a problem with the alpha layers, and it seems to be solved if we turn on the alpha channel mask during drawLayers. The test places two transparent rectangles on the tab, and a piece of text below them. As the text is scrolled up into the region of the rectangles, it is visible 'through the colored layers', and the black text takes on the colours of the rectangles. Without the two commented lines, the text changes color, but the rectangles are not visible, and thus the test fails. Again, this only occurs during offscreen compositing. The change does not change test results for any other GPU test *during offscreen compositing*.

I'd be interested in thoughts/suggestions regarding this apparent difference between compositing directly to the display versus to an offscreen texture. It may be we will need to address this in a separate patch.

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