Fix sub-pixel handling of transparency layers

Source/core/rendering/RenderLayer.cpp

Index: Source/core/rendering/RenderLayer.cpp
diff --git a/Source/core/rendering/RenderLayer.cpp b/Source/core/rendering/RenderLayer.cpp
index 512b4828f3961fc5612afcd8233161d04854eaad..d6840131c9283d1446108880432c09c108d989f7 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -1216,10 +1216,10 @@ enum TransparencyClipBoxMode {
     RootOfTransparencyClipBox
 };
 
-static LayoutRect transparencyClipBox(const RenderLayer*, const RenderLayer* rootLayer, TransparencyClipBoxBehavior, TransparencyClipBoxMode, PaintBehavior = 0);
+static LayoutRect transparencyClipBox(const RenderLayer*, const RenderLayer* rootLayer, TransparencyClipBoxBehavior, TransparencyClipBoxMode, const LayoutSize& subPixelAccumulation, PaintBehavior = 0);
 
 static void expandClipRectForDescendantsAndReflection(LayoutRect& clipRect, const RenderLayer* layer, const RenderLayer* rootLayer,
-    TransparencyClipBoxBehavior transparencyBehavior, PaintBehavior paintBehavior)
+    TransparencyClipBoxBehavior transparencyBehavior, const LayoutSize& subPixelAccumulation, PaintBehavior paintBehavior)
 {
     // If we have a mask, then the clip is limited to the border box area (and there is
     // no need to examine child layers).
@@ -1228,7 +1228,7 @@ static void expandClipRectForDescendantsAndReflection(LayoutRect& clipRect, cons
         // a stacking container. This means we can just walk the layer tree directly.
         for (RenderLayer* curr = layer->firstChild(); curr; curr = curr->nextSibling()) {
             if (!layer->reflectionInfo() || layer->reflectionInfo()->reflectionLayer() != curr)
-                clipRect.unite(transparencyClipBox(curr, rootLayer, transparencyBehavior, DescendantsOfTransparencyClipBox, paintBehavior));
+                clipRect.unite(transparencyClipBox(curr, rootLayer, transparencyBehavior, DescendantsOfTransparencyClipBox, subPixelAccumulation, paintBehavior));
         }
     }
 
@@ -1246,7 +1246,7 @@ static void expandClipRectForDescendantsAndReflection(LayoutRect& clipRect, cons
 }
 
 static LayoutRect transparencyClipBox(const RenderLayer* layer, const RenderLayer* rootLayer, TransparencyClipBoxBehavior transparencyBehavior,
-    TransparencyClipBoxMode transparencyMode, PaintBehavior paintBehavior)
+    TransparencyClipBoxMode transparencyMode, const LayoutSize& subPixelAccumulation, PaintBehavior paintBehavior)
 {
     // FIXME: Although this function completely ignores CSS-imposed clipping, we did already intersect with the
     // paintDirtyRect, and that should cut down on the amount we have to paint.  Still it
@@ -1261,14 +1261,16 @@ static LayoutRect transparencyClipBox(const RenderLayer* layer, const RenderLaye
         LayoutPoint delta;
         layer->convertToLayerCoords(rootLayerForTransform, delta);
 
+        delta.move(subPixelAccumulation);
+        IntPoint pixelSnappedDelta = roundedIntPoint(delta);
         TransformationMatrix transform;
-        transform.translate(delta.x(), delta.y());
+        transform.translate(pixelSnappedDelta.x(), pixelSnappedDelta.y());
         transform = transform * *layer->transform();
 
         // We don't use fragment boxes when collecting a transformed layer's bounding box, since it always
         // paints unfragmented.
         LayoutRect clipRect = layer->boundingBox(layer);
-        expandClipRectForDescendantsAndReflection(clipRect, layer, layer, transparencyBehavior, paintBehavior);
+        expandClipRectForDescendantsAndReflection(clipRect, layer, layer, transparencyBehavior, subPixelAccumulation, paintBehavior);
         layer->renderer()->style()->filterOutsets().expandRect(clipRect);
         LayoutRect result = transform.mapRect(clipRect);
         if (!paginationLayer)
@@ -1287,17 +1289,18 @@ static LayoutRect transparencyClipBox(const RenderLayer* layer, const RenderLaye
     }
 
     LayoutRect clipRect = layer->boundingBox(rootLayer, RenderLayer::UseFragmentBoxes);
-    expandClipRectForDescendantsAndReflection(clipRect, layer, rootLayer, transparencyBehavior, paintBehavior);
+    expandClipRectForDescendantsAndReflection(clipRect, layer, rootLayer, transparencyBehavior, subPixelAccumulation, paintBehavior);
     layer->renderer()->style()->filterOutsets().expandRect(clipRect);
+    clipRect.move(subPixelAccumulation);
     return clipRect;
 }
 
-LayoutRect RenderLayer::paintingExtent(const RenderLayer* rootLayer, const LayoutRect& paintDirtyRect, PaintBehavior paintBehavior)
+LayoutRect RenderLayer::paintingExtent(const RenderLayer* rootLayer, const LayoutRect& paintDirtyRect, const LayoutSize& subPixelAccumulation, PaintBehavior paintBehavior)
 {
-    return intersection(transparencyClipBox(this, rootLayer, PaintingTransparencyClipBox, RootOfTransparencyClipBox, paintBehavior), paintDirtyRect);
+    return intersection(transparencyClipBox(this, rootLayer, PaintingTransparencyClipBox, RootOfTransparencyClipBox, subPixelAccumulation, paintBehavior), paintDirtyRect);
 }
 
-void RenderLayer::beginTransparencyLayers(GraphicsContext* context, const RenderLayer* rootLayer, const LayoutRect& paintDirtyRect, PaintBehavior paintBehavior)
+void RenderLayer::beginTransparencyLayers(GraphicsContext* context, const RenderLayer* rootLayer, const LayoutRect& paintDirtyRect, const LayoutSize& subPixelAccumulation, PaintBehavior paintBehavior)
 {
     bool createTransparencyLayerForBlendMode = m_stackingNode->isStackingContext() && m_blendInfo.childLayerHasBlendMode();
     if (context->paintingDisabled() || ((paintsWithTransparency(paintBehavior) || paintsWithBlendMode() || createTransparencyLayerForBlendMode) && m_usedTransparency))
@@ -1305,12 +1308,12 @@ void RenderLayer::beginTransparencyLayers(GraphicsContext* context, const Render
 
     RenderLayer* ancestor = transparentPaintingAncestor();
     if (ancestor)
-        ancestor->beginTransparencyLayers(context, rootLayer, paintDirtyRect, paintBehavior);
+        ancestor->beginTransparencyLayers(context, rootLayer, paintDirtyRect, subPixelAccumulation, paintBehavior);
 
     if (paintsWithTransparency(paintBehavior) || paintsWithBlendMode() || createTransparencyLayerForBlendMode) {
         m_usedTransparency = true;
         context->save();
-        LayoutRect clipRect = paintingExtent(rootLayer, paintDirtyRect, paintBehavior);
+        LayoutRect clipRect = paintingExtent(rootLayer, paintDirtyRect, subPixelAccumulation, paintBehavior);
         context->clip(clipRect);
 
         if (paintsWithBlendMode())
@@ -1897,9 +1900,9 @@ void RenderLayer::paintLayer(GraphicsContext* context, const LayerPaintingInfo&
         // layer from the parent now, assuming there is a parent
         if (paintFlags & PaintLayerHaveTransparency) {
             if (parent())
-                parent()->beginTransparencyLayers(context, paintingInfo.rootLayer, paintingInfo.paintDirtyRect, paintingInfo.paintBehavior);
+                parent()->beginTransparencyLayers(context, paintingInfo.rootLayer, paintingInfo.paintDirtyRect, paintingInfo.subPixelAccumulation, paintingInfo.paintBehavior);
             else
-                beginTransparencyLayers(context, paintingInfo.rootLayer, paintingInfo.paintDirtyRect, paintingInfo.paintBehavior);
+                beginTransparencyLayers(context, paintingInfo.rootLayer, paintingInfo.paintDirtyRect, paintingInfo.subPixelAccumulation, paintingInfo.paintBehavior);
         }
 
         if (enclosingPaginationLayer()) {
@@ -2032,7 +2035,7 @@ void RenderLayer::paintLayerContents(GraphicsContext* context, const LayerPainti
     bool createTransparencyLayerForBlendMode = !renderer()->isRoot() && m_stackingNode->isStackingContext() && m_blendInfo.childLayerHasBlendMode();
 
     if (createTransparencyLayerForBlendMode)
-        beginTransparencyLayers(context, paintingInfo.rootLayer, paintingInfo.paintDirtyRect, paintingInfo.paintBehavior);
+        beginTransparencyLayers(context, paintingInfo.rootLayer, paintingInfo.paintDirtyRect, paintingInfo.subPixelAccumulation, paintingInfo.paintBehavior);
 
     LayerPaintingInfo localPaintingInfo(paintingInfo);
     FilterEffectRendererHelper filterPainter(filterRenderer() && paintsWithFilters());
@@ -2068,7 +2071,7 @@ void RenderLayer::paintLayerContents(GraphicsContext* context, const LayerPainti
 
     if (filterPainter.hasStartedFilterEffect() && haveTransparency) {
         // If we have a filter and transparency, we have to eagerly start a transparency layer here, rather than risk a child layer lazily starts one with the wrong context.
-        beginTransparencyLayers(transparencyLayerContext, localPaintingInfo.rootLayer, paintingInfo.paintDirtyRect, localPaintingInfo.paintBehavior);
+        beginTransparencyLayers(transparencyLayerContext, localPaintingInfo.rootLayer, paintingInfo.paintDirtyRect, paintingInfo.subPixelAccumulation, localPaintingInfo.paintBehavior);
     }
 
     // If this layer's renderer is a child of the paintingRoot, we render unconditionally, which
@@ -2302,7 +2305,7 @@ void RenderLayer::paintTransformedLayerIntoFragments(GraphicsContext* context, c
 {
     LayerFragments enclosingPaginationFragments;
     LayoutPoint offsetOfPaginationLayerFromRoot;
-    LayoutRect transformedExtent = transparencyClipBox(this, enclosingPaginationLayer(), PaintingTransparencyClipBox, RootOfTransparencyClipBox, paintingInfo.paintBehavior);
+    LayoutRect transformedExtent = transparencyClipBox(this, enclosingPaginationLayer(), PaintingTransparencyClipBox, RootOfTransparencyClipBox, paintingInfo.subPixelAccumulation, paintingInfo.paintBehavior);
     enclosingPaginationLayer()->collectFragments(enclosingPaginationFragments, paintingInfo.rootLayer, paintingInfo.paintDirtyRect,
         (paintFlags & PaintLayerTemporaryClipRects) ? TemporaryClipRects : PaintingClipRects, IgnoreOverlayScrollbarSize,
         (paintFlags & PaintLayerPaintingOverflowContents) ? IgnoreOverflowClip : RespectOverflowClip, &offsetOfPaginationLayerFromRoot, paintingInfo.subPixelAccumulation, &transformedExtent);
@@ -2351,7 +2354,7 @@ void RenderLayer::paintBackgroundForFragments(const LayerFragments& layerFragmen
 
         // Begin transparency layers lazily now that we know we have to paint something.
         if (haveTransparency || paintsWithBlendMode())
-            beginTransparencyLayers(transparencyLayerContext, localPaintingInfo.rootLayer, transparencyPaintDirtyRect, localPaintingInfo.paintBehavior);
+            beginTransparencyLayers(transparencyLayerContext, localPaintingInfo.rootLayer, transparencyPaintDirtyRect, localPaintingInfo.subPixelAccumulation, localPaintingInfo.paintBehavior);
 
         if (localPaintingInfo.clipToDirtyRect) {
             // Paint our background first, before painting any child layers.
@@ -2378,7 +2381,7 @@ void RenderLayer::paintForegroundForFragments(const LayerFragments& layerFragmen
         for (size_t i = 0; i < layerFragments.size(); ++i) {
             const LayerFragment& fragment = layerFragments.at(i);
             if (fragment.shouldPaintContent && !fragment.foregroundRect.isEmpty()) {
-                beginTransparencyLayers(transparencyLayerContext, localPaintingInfo.rootLayer, transparencyPaintDirtyRect, localPaintingInfo.paintBehavior);
+                beginTransparencyLayers(transparencyLayerContext, localPaintingInfo.rootLayer, transparencyPaintDirtyRect, localPaintingInfo.subPixelAccumulation, localPaintingInfo.paintBehavior);
                 break;
             }
         }
@@ -2951,7 +2954,8 @@ RenderLayer* RenderLayer::hitTestTransformedLayerInFragments(RenderLayer* rootLa
 {
     LayerFragments enclosingPaginationFragments;
     LayoutPoint offsetOfPaginationLayerFromRoot;
-    LayoutRect transformedExtent = transparencyClipBox(this, enclosingPaginationLayer(), HitTestingTransparencyClipBox, RootOfTransparencyClipBox);
+    // FIXME: We're missing a sub-pixel offset here crbug.com/348728
+    LayoutRect transformedExtent = transparencyClipBox(this, enclosingPaginationLayer(), HitTestingTransparencyClipBox, RootOfTransparencyClipBox, LayoutSize());
     enclosingPaginationLayer()->collectFragments(enclosingPaginationFragments, rootLayer, hitTestRect,
         RootRelativeClipRects, IncludeOverlayScrollbarSize, RespectOverflowClip, &offsetOfPaginationLayerFromRoot, LayoutSize(), &transformedExtent);