Currently if the native drawing does not support the clipping region from the context we do a costly fallback to draw twice, read back, and compare to extract alpha information. A much more efficient approach would be to blit the background to a single matching temporary surface, draw once, and blit with the clip to the context.

I'd like to be able to find a visual to match the format of image surfaces. This adds a static method to gfxXlibSurface similar to FindRenderFormat.

This adds two new strategies that can be used to avoid alpha-extraction: 1. Copying the background from the target to the temp drawing surface. 2. Using a temp drawing surface with an alpha channel.

Requesting blocking2.0: 1. Bug 574583 significantly slowed down transparent flash performance in some situations and this recovers much of that. 2. Since retained layers, clips are more involved with drawing and so are causing fallback more often. 3. A number of reftests were modified in bug 573933 due to inaccuracies in the alpha-extraction path. This is needed to restore the tests.

Comment on attachment 458903 [details] [diff] [review] copy and draw over background to avoid alpha extraction when possible > if (!drawIsOpaque) { > tmpCtx = new gfxContext(tempXlibSurface); >- tmpCtx->SetOperator(gfxContext::OPERATOR_CLEAR); >+ if (method == eCopyBackground) { >+ tmpCtx->SetOperator(gfxContext::OPERATOR_SOURCE); >+ tmpCtx->SetSource(target, -(offset + matrix.GetTranslation())); >+ // The copy from the tempXlibSurface to the target context should >+ // use operator SOURCE, but that would need a mask to bound the >+ // operation. Here we only copy opaque backgrounds so operator >+ // OVER will behave like SOURCE masked by the surface. >+ NS_ASSERTION(tempXlibSurface->GetContentType() >+ == gfxASurface::CONTENT_COLOR, >+ "Don't copy background with a transparent surface"); >+ } else { >+ tmpCtx->SetOperator(gfxContext::OPERATOR_CLEAR); >+ } > tmpCtx->Paint(); > } > > if (!DrawOntoTempSurface(tempXlibSurface, -drawingRect.TopLeft())) { > return; > } > >- if (drawIsOpaque) { >+ if (method != eAlphaExtraction) { > ctx->SetSource(tempXlibSurface, offset); > ctx->Paint(); I guess that should say something like "would need a mask or fill path to bound the operation".

Comment on attachment 458903 [details] [diff] [review] copy and draw over background to avoid alpha extraction when possible + && visual->bits_per_rgb == 8 + && visual->red_mask == 0xff0000 + && visual->green_mask == 0xff00 + && visual->blue_mask == 0xff + && gfxXlibSurface::DepthOfVisual(screen, visual) == 32; + || visual->c_class != TrueColor + || format->type != PictTypeDirect + || gfxXlibSurface::DepthOfVisual(screen, visual) != format->depth) Style is to put operator at end of line. I don't like it, but that's what it is. Happens in various other places too.

http://hg.mozilla.org/mozilla-central/rev/92559a5573bd http://hg.mozilla.org/mozilla-central/rev/8fd528e132d7 Included the changes requested in comment 5. Tested by http://hg.mozilla.org/mozilla-central/rev/02d92df7381d