When tracking regions, treat sub-pixel areas as belonging to the region.

Source/platform/graphics/RegionTracker.cpp

 /*
  * Copyright (c) 2012, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 35 matching lines @@
 
 void RegionTracker::reset()
 {
     ASSERT(m_canvasLayerStack.isEmpty());
     m_opaqueRect = SkRect::MakeEmpty();
 }
 
 IntRect RegionTracker::asRect() const
 {
     // Returns the largest enclosed rect.
-    // TODO: actually, this logic looks like its returning the smallest.
-    //       to return largest, shouldn't we take floor of left/top
-    //       and the ceil of right/bottom?
-    int left = SkScalarCeilToInt(m_opaqueRect.fLeft);
-    int top = SkScalarCeilToInt(m_opaqueRect.fTop);
-    int right = SkScalarFloorToInt(m_opaqueRect.fRight);
-    int bottom = SkScalarFloorToInt(m_opaqueRect.fBottom);
+
+    // epsilon is large enough to accommodate machine precision issues and
+    // small enough to have a negligible effect on rendered results.
+    const SkScalar epsilon = 1.0f / 512.0f;
+
+    int left = SkScalarCeilToInt(m_opaqueRect.fLeft - epsilon);
+    int top = SkScalarCeilToInt(m_opaqueRect.fTop - epsilon);
+    int right = SkScalarFloorToInt(m_opaqueRect.fRight + epsilon);
+    int bottom = SkScalarFloorToInt(m_opaqueRect.fBottom + epsilon);
     return IntRect(left, top, right-left, bottom-top);
 }
 
 // Returns true if the xfermode will force the dst to be opaque, regardless of the current dst.
 static inline bool xfermodeIsOpaque(const SkPaint& paint, bool srcIsOpaque)
 {
     if (!srcIsOpaque)
         return false;
 
     SkXfermode* xfermode = paint.getXfermode();
@@ skipping 388 matching lines @@
     opaqueRect.setEmpty();
 }
 
 SkRect& RegionTracker::currentTrackingOpaqueRect()
 {
     // If we are drawing into a canvas layer, then track the opaque rect in that layer.
     return m_canvasLayerStack.isEmpty() ? m_opaqueRect : m_canvasLayerStack.last().opaqueRect;
 }
 
 } // namespace blink