mix-blend-mode implementation for accelerated layers

cc/trees/layer_tree_host_common.cc

Index: cc/trees/layer_tree_host_common.cc
diff --git a/cc/trees/layer_tree_host_common.cc b/cc/trees/layer_tree_host_common.cc
index 0f31038f4990f5920fea4d320e691bfacdb978af..44b13b0f738f903f735cdd90e86ba0709784538e 100644
--- a/cc/trees/layer_tree_host_common.cc
+++ b/cc/trees/layer_tree_host_common.cc
@@ -587,6 +587,33 @@ static bool SubtreeShouldRenderToSeparateSurface(
     return true;
   }
 
+  // If the layer has blending.
+  // TODO(rosca): this is temporary, until blending is implemented for other
+  // types of quads than RenderPassQuad. Layers having descendants that draw
+  // content will still create a separate rendering surface.
+  if (!layer->uses_default_blend_mode()) {
+    TRACE_EVENT_INSTANT0(
+        "cc",
+        "LayerTreeHostCommon::SubtreeShouldRenderToSeparateSurface blending",
+        TRACE_EVENT_SCOPE_THREAD);
+    DCHECK(!is_root);
+    return true;
+  }
+
+  // If the layer has isolation.
+  // TODO(rosca): to be optimized - create separate rendering surface only when
+  // the blending descendants might have access to the content behind this layer
+  // (layer has transparent background or descendants overflow)
+  // https://code.google.com/p/chromium/issues/detail?id=301738
+  if (layer->is_root_for_isolated_group()) {
+    TRACE_EVENT_INSTANT0(
+        "cc",
+        "LayerTreeHostCommon::SubtreeShouldRenderToSeparateSurface isolation",
+        TRACE_EVENT_SCOPE_THREAD);
+    DCHECK(!is_root);

[enne (OOO), 2013/11/06 22:40:19]

Why can't this be the root? In the cases above, we really don't create a
separate surface for the root for masks or replicas and so those features can't
be used.  In this case, it seems like blending with the root "surface" as the
isolated group would work just fine.

[rosca, 2013/11/07 01:58:16]

Agree. Done.

+    return true;
+  }
+
   // If the layer clips its descendants but it is not axis-aligned with respect
   // to its parent.
   bool layer_clips_external_content =
@@ -2048,6 +2075,16 @@ static void CalculateDrawPropertiesInternal(
       return;
     }
 
+    // Layers having a non-default blend mode will blend with the content
+    // inside its parent's render target. This render target should be
+    // either root_for_isolated_group, or the root of the layer tree.
+    // Otherwise, this layer will use an incomplete backdrop, limited to its
+    // render target and the blending result will be incorrect.
+    DCHECK(layer->uses_default_blend_mode() || !layer->parent() ||
+           !layer->parent()->render_target() ||
+           IsRootLayer(layer->parent()->render_target()) ||
+           layer->parent()->render_target()->is_root_for_isolated_group());
+
     render_surface->SetContentRect(clipped_content_rect);
 
     // The owning layer's screen_space_transform has a scale from content to