Gecko:TransparentLayersAndSubpixelAA

Problem

Subpixel AA rendered to transparent surfaces needs component alpha to not look bad. Graphics backends do not support component alpha in a performant way.

Retained layers made us use transparent surfaces a lot more, e.g. when content without an opaque background is being scrolled.

Plan

• Do more analysis to make surfaces opaque in more cases. In particular, for a layer that has transparent content but is over a background that is completely one solid color, we can pull that solid color up into the layer and draw it into the layer's background. This will make most cases of scrolling over a solid color background not use transparent surfaces. This is a no-brainer since working with opaque surfaces is always a performance win.

• For each ThebesLayer, make FrameLayerBuilder track if we have text over transparent background, and mark that via a new ThebesLayer API, say ThebesLayer::SetHasSubpixelAntialiasing.

• In BasicLayerManager, when painting a ThebesLayer marked with SetHasSubpixelAntialiasing with aOpacity 1.0, treat it as non-retained. That is, we'll have no buffer and just repaint the entire layer (that intersects the paint dirty region) directly to the backbuffer or nearest enclosing temporary buffer. This will eliminate quality regressions due to retained layers. Retained layer scrolling speedups should mostly be retained since many common cases will still be a lot faster. Only pages where text moves relative to its background during scrolling should be slowed, and they shouldn't get slower than they were before retained layers.

• The same trick should work for LayerManagerD3D9 and D2D, and for LayerManagerOGL and X, since on those combinations we can render directly into the backbuffer or temporary surface (right?).

• For LayerManagerOGL and Quartz, and LayerManagerD3D9 and GDI, we still have a problem. We can't render directly onto the layer manager's backbuffer or temporary surfaces as far as I know. However, in some situations we can still win. In particular, if our transparent SetHasSubpixelAntialiasing ThebesLayer is directly over another ThebesLayer, we can draw the transparent ThebesLayer into the lower ThebesLayer's buffer directly (and then invalidate the drawn area of the lower ThebesLayer after compositing with it). Or we could copy the contents of the lower ThebesLayer's buffer into the transparent ThebesLayer before drawing. There are performance tradeoffs here.