Don't create render passes for transparent images.

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 "cc/layer.h"
 #include "cc/layer_impl.h"
 #include "cc/layer_iterator.h"
 #include "cc/layer_sorter.h"
@@ skipping 241 matching lines @@
     // 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() && descendantDrawsContent)
         return true;
 
     // If the layer clips its descendants but it is not axis-aligned with respect to its parent.
     if (layerClipsSubtree(layer) && !axisAlignedWithRespectToParent && descendantDrawsContent)
         return true;
 
     // If the layer has opacity != 1 and does not have a preserves-3d transform style.
-    if (layer->opacity() != 1 && !layer->preserves3D() && descendantDrawsContent)
+    if (layer->opacity() != 1 && !layer->preserves3D() && layer->drawsContent() && descendantDrawsContent)
         return true;
 
     return false;
 }
 
 WebTransformationMatrix computeScrollCompensationForThisLayer(LayerImpl* scrollingLayer, const WebTransformationMatrix& parentMatrix)
 {
     // For every layer that has non-zero scrollDelta, we have to compute a transform that can undo the
     // scrollDelta translation. In particular, we want this matrix to premultiply a fixed-position layer's
     // parentMatrix, so we design this transform in three steps as follows. The steps described here apply
@@ skipping 204 matching lines @@
 
     // If we early-exit anywhere in this function, the drawableContentRect of this subtree should be considered empty.
     drawableContentRectOfSubtree = gfx::Rect();
 
     // The root layer cannot skip calcDrawTransforms.
     if (!isRootLayer(layer) && subtreeShouldBeSkipped(layer))
         return;
 
     gfx::Rect clipRectForSubtree;
     bool subtreeShouldBeClipped = false;
-    
+
     float drawOpacity = layer->opacity();
     bool drawOpacityIsAnimating = layer->opacityIsAnimating();
-    if (layer->parent() && layer->parent()->preserves3D()) {
+    if (layer->parent()) {

[danakj, 2012/11/26 18:35:26]

This should be replaced with a check for something else.. like:

if (layer->parent() && !layer->parent()->renderSurface())

When the parent owns a surface, the opacity will be applied at draw time of the
surface, and we don't need to inherit it.

[shawnsingh, 2012/11/26 18:52:57]

danakj@ is this true?  we're using the parent layer's drawOpacity, which already
accounts for whether or not its own opacity was re-parented by its
renderSurface.   I think the additional if-statement portion you're proposing
would be a no-op, wouldn't it?   

If what I said is correct, I'm still OK with whichever way seems to make the
semantics more obvious.  But a comment is definitely needed here, and maybe that
makes it clear enough the situation without the additional if-statement.

[danakj, 2012/11/26 18:57:34]

Oh that is true.

I thought we had the preserves3D here because then you know the parent didn't
own a surface. This check is from so long ago and has just been bumped around
over time.

Since we use the drawOpacity, this change seems fine as is to me also.

         drawOpacity *= layer->parent()->drawOpacity();
         drawOpacityIsAnimating |= layer->parent()->drawOpacityIsAnimating();
     }
 
     bool animatingTransformToTarget = layer->transformIsAnimating();
     bool animatingTransformToScreen = animatingTransformToTarget;
     if (layer->parent()) {
         animatingTransformToTarget |= layer->parent()->drawTransformIsAnimating();
         animatingTransformToScreen |= layer->parent()->screenSpaceTransformIsAnimating();
     }
@@ skipping 486 matching lines @@
 
         foundLayer = currentLayer;
         break;
     }
 
     // This can potentially return 0, which means the screenSpacePoint did not successfully hit test any layers, not even the root layer.
     return foundLayer;
 }
 
 }  // namespace cc