Reland of: cc: Add occlusion checker as a fixed view of occlusion tracker.

cc/trees/occlusion.cc

Index: cc/trees/occlusion.cc
diff --git a/cc/trees/occlusion.cc b/cc/trees/occlusion.cc
new file mode 100644
index 0000000000000000000000000000000000000000..c91c0ca25d586b13eb2638c404b71d450ec10b42
--- /dev/null
+++ b/cc/trees/occlusion.cc
@@ -0,0 +1,48 @@
+// Copyright 2014 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.h"
+
+#include "cc/base/math_util.h"
+#include "ui/gfx/rect.h"
+
+namespace cc {
+
+Occlusion::Occlusion() {
+}
+
+Occlusion::Occlusion(const gfx::Transform& draw_transform,
+                     const SimpleEnclosedRegion& occlusion_from_outside_target,
+                     const SimpleEnclosedRegion& occlusion_from_inside_target)
+    : draw_transform_(draw_transform),
+      occlusion_from_outside_target_(occlusion_from_outside_target),
+      occlusion_from_inside_target_(occlusion_from_inside_target) {
+}
+
+bool Occlusion::IsOccluded(const gfx::Rect& content_rect) const {
+  if (content_rect.IsEmpty())
+    return true;
+
+  if (occlusion_from_inside_target_.IsEmpty() &&
+      occlusion_from_outside_target_.IsEmpty()) {
+    return false;
+  }
+
+  // Take the ToEnclosingRect at each step, as we want to contain any unoccluded
+  // partial pixels in the resulting Rect.
+  gfx::Rect unoccluded_rect_in_target_surface =
+      MathUtil::MapEnclosingClippedRect(draw_transform_, content_rect);
+  DCHECK_LE(occlusion_from_inside_target_.GetRegionComplexity(), 1u);
+  DCHECK_LE(occlusion_from_outside_target_.GetRegionComplexity(), 1u);
+  // These subtract operations are more lossy than if we did both operations at
+  // once.
+  unoccluded_rect_in_target_surface.Subtract(
+      occlusion_from_inside_target_.bounds());
+  unoccluded_rect_in_target_surface.Subtract(
+      occlusion_from_outside_target_.bounds());
+
+  return unoccluded_rect_in_target_surface.IsEmpty();
+}
+
+}  // namespace cc