Changes subtreeShouldRenderToSeparateSurface logic to account for explicit clipping

cc/layer_tree_host_common.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/layer_tree_host_common.h"
 
 #include <algorithm>
 
 #include "base/debug/trace_event.h"
 #include "cc/layer.h"
@@ skipping 256 matching lines @@
         return true;
 
     int numDescendantsThatDrawContent = layer->drawProperties().num_descendants_that_draw_content;
 
     // If the layer flattens its subtree (i.e. the layer doesn't preserve-3d), but it is
     // treated as a 3D object by its parent (i.e. parent does preserve-3d).
     if (layerIsInExisting3DRenderingContext(layer) && !layer->preserves3D() && numDescendantsThatDrawContent > 0)
         return true;
 
     // If the layer clips its descendants but it is not axis-aligned with respect to its parent.
-    if (layerClipsSubtree(layer) && !axisAlignedWithRespectToParent && numDescendantsThatDrawContent > 0)
+    if (layerClipsSubtree(layer) && !axisAlignedWithRespectToParent && !layer->drawProperties().can_clip_self)

[danakj, 2012/12/15 18:49:12]

Why doesn't a layer need to know that all layers in its entire subtree can each
clip themselves to their respective parents in order to decide it doesn't need
create and render the subtree into a render surface? Don't you need to check
subtreeCanEachClipToParent?

         return true;
 
     // If the layer has some translucency and does not have a preserves-3d transform style.
     // This condition only needs a render surface if two or more layers in the
     // subtree overlap. But checking layer overlaps is unnecessarily costly so
     // instead we conservatively create a surface whenever at least two layers
     // draw content for this subtree.
     bool atLeastTwoLayersInSubtreeDrawContent = layer->hasDelegatedContent() ||
         (numDescendantsThatDrawContent > 0 && (layer->drawsContent() || numDescendantsThatDrawContent > 1));
 
@@ skipping 140 matching lines @@
     renderSurfaceLayerList.pop_back();
     layerToRemove->clearRenderSurface();
 }
 
 // Recursively walks the layer tree to compute any information that is needed
 // before doing the main recursion.
 template<typename LayerType>
 static void preCalculateMetaInformation(LayerType* layer)
 {
     int numDescendantsThatDrawContent = 0;
+    bool canClipSelf = true;

[danakj, 2012/12/15 18:49:12]

Why isn't this initialized to the return for the current Layer::canClipSelf()
function?

Does that function actually mean allowClippingToParent?

+    bool sublayerXformPreventsClip = !layer->sublayerTransform().IsPositiveScaleOrTranslation();
 
     for (size_t i = 0; i < layer->children().size(); ++i) {
         LayerType* childLayer = layer->children()[i];
         preCalculateMetaInformation<LayerType>(childLayer);
+
         numDescendantsThatDrawContent += childLayer->drawsContent() ? 1 : 0;
         numDescendantsThatDrawContent += childLayer->drawProperties().num_descendants_that_draw_content;
+
+        if ((childLayer->drawsContent() && !childLayer->canClipSelf()) ||
+            !childLayer->drawProperties().can_clip_self ||
+            sublayerXformPreventsClip ||
+            !childLayer->transform().IsPositiveScaleOrTranslation())
+            canClipSelf = false;
     }
 
     layer->drawProperties().num_descendants_that_draw_content = numDescendantsThatDrawContent;
+    layer->drawProperties().can_clip_self = canClipSelf;
 }
 
 // Recursively walks the layer tree starting at the given node and computes all the
 // necessary transformations, clipRects, render surfaces, etc.
 template<typename LayerType, typename LayerList, typename RenderSurfaceType>
 static void calculateDrawPropertiesInternal(LayerType* layer, const gfx::Transform& parentMatrix,
     const gfx::Transform& fullHierarchyMatrix, const gfx::Transform& currentScrollCompensationMatrix,
     const gfx::Rect& clipRectFromAncestor, const gfx::Rect& clipRectFromAncestorInDescendantSpace, bool ancestorClipsSubtree,
     RenderSurfaceType* nearestAncestorThatMovesPixels, LayerList& renderSurfaceLayerList, LayerList& layerList,
     LayerSorter* layerSorter, int maxTextureSize, float deviceScaleFactor, float pageScaleFactor, bool subtreeCanUseLCDText,
@@ skipping 638 matching lines @@
 
   // At this point, we think the point does hit the touch event handler region on the layer, but we need to walk up
   // the parents to ensure that the layer was not clipped in such a way that the
   // hit point actually should not hit the layer.
   if (pointIsClippedBySurfaceOrClipRect(screenSpacePoint, layerImpl))
      return false;
 
   return true;
 }
 }  // namespace cc