Instead of clearing the whole output region after a repaint, clear only the regions that were actually painted. This way, the damage added when a surface moves from the primary plane to another one is kept while this region is obscured by the opaque region. This allows the contents below an overlaid surface to be culled, but to make this work properly, it is also necessary to change the way previous damage is drawn. Consider the following scenario: a surface is moved to an overlay plane leaving some damage in the primary plane. On the following frame, the surface on the overlay moves, revealing part of the damaged region on the primary plane. On the frame after that, the overlaid surface moves back to its previous position obscuring the region of the primary plane repainted before. At this point, the repainted region was added to the output's previous damage so that it is draw to both buffers. But since this region is now obscured, the redrawing is skipped. If the overlaid surface moves again revealing this region, one of the buffers actually contains the wrong content. To fix this problem, this patch ensures that any previous damage that would be lost is actually preserved by folding it back into the primary plane damage just before repainting.
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