cc: Correct inverted directions in occlusion tracker background filter logic

cc/trees/occlusion_tracker.cc

 // Copyright 2012 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/trees/occlusion_tracker.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 
@@ skipping 203 matching lines @@
     return;
 
   gfx::Rect affected_area_in_target =
       MathUtil::MapEnclosingClippedRect(surface_transform, surface_rect);
   if (contributing_surface->is_clipped()) {
     affected_area_in_target.Intersect(contributing_surface->clip_rect());
   }
   if (affected_area_in_target.IsEmpty())
     return;
 
-  int outset_top, outset_right, outset_bottom, outset_left;
-  contributing_surface->BackgroundFilters().GetOutsets(
-      &outset_top, &outset_right, &outset_bottom, &outset_left);
-
   // The filter can move pixels from outside of the clip, so allow affected_area
   // to expand outside the clip. Notably the content we're concerned with here
   // is not the affected area, but rather stuff slightly outside it. Thus the

[danakj, 2017/05/19 19:28:54]

I think this comment doesn't explain what we have learnt very well. In my words
I'd say that the filter's bounds for a background filter are relative to the
target surface here rather than to the filtered surface, which means we have to
invert the outsets to apply them to the filtered surface's bounds. wdyt?

   // directions of the outsets are reversed from normal.
-  affected_area_in_target.Inset(-outset_right, -outset_bottom, -outset_left,
-                                -outset_top);
+  int outset_top, outset_right, outset_bottom, outset_left;
+  contributing_surface->BackgroundFilters().GetOutsets(
+      &outset_bottom, &outset_left, &outset_top, &outset_right);
+
+  affected_area_in_target.Inset(-outset_left, -outset_top, -outset_right,
+                                -outset_bottom);
   SimpleEnclosedRegion affected_occlusion = *occlusion_from_inside_target;
   affected_occlusion.Intersect(affected_area_in_target);
 
   occlusion_from_inside_target->Subtract(affected_area_in_target);
   for (size_t i = 0; i < affected_occlusion.GetRegionComplexity(); ++i) {
     gfx::Rect occlusion_rect = affected_occlusion.GetRect(i);
 
     // Shrink the rect by expanding the non-opaque pixels outside the rect.
 
     // The left outset of the filters moves pixels on the right side of
@@ skipping 130 matching lines @@
   DCHECK(layer_tree->RootRenderSurface() == stack_.back().target);
   const SimpleEnclosedRegion& occluded =
       stack_.back().occlusion_from_inside_target;
   Region visible_region(screen_space_clip_rect_);
   for (size_t i = 0; i < occluded.GetRegionComplexity(); ++i)
     visible_region.Subtract(occluded.GetRect(i));
   return visible_region;
 }
 
 }  // namespace cc