Round in the range [-0.5, 0.5) when computing offsetFromRenderer and subpixelAccumulation

Source/core/rendering/compositing/CompositedLayerMapping.cpp

 /*
  * Copyright (C) 2009, 2010, 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 538 matching lines @@
 
         squashedBounds.move(offsetFromSquashingLayer);
         totalSquashBounds.unite(squashedBounds);
     }
 
     *offsetFromTransformedAncestor = referenceOffsetFromTransformedAncestor;
     offsetFromTransformedAncestor->moveBy(totalSquashBounds.location());
 
     // The totalSquashBounds is positioned with respect to referenceLayer of this CompositedLayerMapping.
     // But the squashingLayer needs to be positioned with respect to the ancestor CompositedLayerMapping.
-    // The conversion between referenceLayer and the ancestor CLM is already computed in the caller as
-    // offsetFromReferenceLayerToCompositedAncestor.
+    // The conversion between referenceLayer and the ancestor CLM is already computed as
+    // offsetFromReferenceLayerToParentGraphicsLayer.
     totalSquashBounds.moveBy(offsetFromReferenceLayerToParentGraphicsLayer);
     IntRect squashLayerBounds = enclosingIntRect(totalSquashBounds);
     IntPoint squashLayerOrigin = squashLayerBounds.location();
     LayoutSize squashLayerOriginInOwningLayerSpace = squashLayerOrigin - offsetFromReferenceLayerToParentGraphicsLayer;
 
     squashingLayer->setPosition(squashLayerBounds.location());
     squashingLayer->setSize(squashLayerBounds.size());
 
     // Now that the squashing bounds are known, we can convert the RenderLayer painting offsets
     // from CLM owning layer space to the squashing layer space.
     //
     // The painting offset we want to compute for each squashed RenderLayer is essentially the position of
     // the squashed RenderLayer described w.r.t. referenceLayer's origin. For this purpose we already cached
     // offsetFromSquashingCLM before, which describes where the squashed RenderLayer is located w.r.t.
     // referenceLayer. So we just need to convert that point from referenceLayer space to referenceLayer
     // space. This is simply done by subtracing squashLayerOriginInOwningLayerSpace, but then the offset
     // overall needs to be negated because that's the direction that the painting code expects the
     // offset to be.
     for (size_t i = 0; i < layers.size(); ++i) {
         LayoutPoint offsetFromTransformedAncestorForSquashedLayer = layers[i].renderLayer->computeOffsetFromTransformedAncestor();
         LayoutSize offsetFromSquashLayerOrigin = (offsetFromTransformedAncestorForSquashedLayer - referenceOffsetFromTransformedAncestor) - squashLayerOriginInOwningLayerSpace;
 
         // It is ok to repaint here, because all of the geometry needed to correctly repaint is computed by this point.
-        IntSize newOffsetFromRenderer = -flooredIntSize(offsetFromSquashLayerOrigin);
+        IntSize newOffsetFromRenderer = -IntSize(offsetFromSquashLayerOrigin.width().round(), offsetFromSquashLayerOrigin.height().round());
+        LayoutSize subpixelAccumulation = offsetFromSquashLayerOrigin + newOffsetFromRenderer;
         if (layers[i].offsetFromRendererSet && layers[i].offsetFromRenderer != newOffsetFromRenderer)
             layers[i].renderLayer->repainter().repaintIncludingNonCompositingDescendants();
         layers[i].offsetFromRenderer = newOffsetFromRenderer;
         layers[i].offsetFromRendererSet = true;
 
-        layers[i].renderLayer->setSubpixelAccumulation(offsetFromSquashLayerOrigin.fraction());
-        ASSERT(layers[i].renderLayer->subpixelAccumulation() ==
-            toLayoutSize(computeOffsetFromCompositedAncestor(layers[i].renderLayer, layers[i].renderLayer->ancestorCompositingLayer())).fraction());

[leviw_travelin_and_unemployed, 2014/06/18 17:33:35]

I'm sad to see the assert leave, but I'll just cross my fingers on our test
coverage.

LGTM.

+        layers[i].renderLayer->setSubpixelAccumulation(subpixelAccumulation);
 
         // FIXME: find a better design to avoid this redundant value - most likely it will make
         // sense to move the paint task info into RenderLayer's m_compositingProperties.
         layers[i].renderLayer->setOffsetFromSquashingLayerOrigin(layers[i].offsetFromRenderer);
     }
 
     for (size_t i = 0; i < layers.size(); ++i)
         layers[i].localClipRectForSquashedLayer = localClipRectForSquashedLayer(referenceLayer, layers[i], layers);
 }
 
@@ skipping 1663 matching lines @@
     } else if (graphicsLayer == m_scrollingBlockSelectionLayer.get()) {
         name = "Scrolling Block Selection Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace WebCore