Pass the quad's rect (contents_rect) origin to skia image filters as an offset in the CTM.

cc/trees/layer_tree_host_pixeltest_filters.cc

Index: cc/trees/layer_tree_host_pixeltest_filters.cc
diff --git a/cc/trees/layer_tree_host_pixeltest_filters.cc b/cc/trees/layer_tree_host_pixeltest_filters.cc
index 553e66e8dba352e4acf121e0a2d040319d932523..8305d46ec299e50fe016dd224042f00ba147d5da 100644
--- a/cc/trees/layer_tree_host_pixeltest_filters.cc
+++ b/cc/trees/layer_tree_host_pixeltest_filters.cc
@@ -6,6 +6,8 @@
 #include "cc/layers/solid_color_layer.h"
 #include "cc/test/layer_tree_pixel_test.h"
 #include "cc/test/pixel_comparator.h"
+#include "third_party/skia/include/effects/SkColorFilterImageFilter.h"
+#include "third_party/skia/include/effects/SkColorMatrixFilter.h"
 
 #if !defined(OS_ANDROID)
 
@@ -148,6 +150,49 @@ TEST_F(LayerTreeHostFiltersPixelTest, BackgroundFilterBlurOffAxis) {
                    "background_filter_blur_off_axis.png")));
 }
 
+TEST_F(LayerTreeHostFiltersPixelTest, ImageFilterClipped) {
+  scoped_refptr<SolidColorLayer> root = CreateSolidColorLayer(
+      gfx::Rect(200, 200), SK_ColorBLACK);
+
+  scoped_refptr<SolidColorLayer> background = CreateSolidColorLayer(
+      gfx::Rect(200, 200), SK_ColorYELLOW);
+  root->AddChild(background);
+
+  scoped_refptr<SolidColorLayer> foreground = CreateSolidColorLayer(
+      gfx::Rect(200, 200), SK_ColorRED);
+  background->AddChild(foreground);
+
+  SkScalar matrix[20];
+  memset(matrix, 0, 20 * sizeof(matrix[0]));
+  // This filter does a red-blue swap, so the foreground becomes blue.
+  matrix[2] = matrix[6] = matrix[10] = matrix[18] = SK_Scalar1;
+  skia::RefPtr<SkColorFilter> colorFilter(skia::AdoptRef(
+      new SkColorMatrixFilter(matrix)));
+  // We filter only the bottom 200x100 pixels of the foreground.
+  SkIRect cropRect = SkIRect::MakeXYWH(0, 100, 200, 100);
+  skia::RefPtr<SkImageFilter> filter =
+      skia::AdoptRef(SkColorFilterImageFilter::Create(
+          colorFilter.get(),
+          NULL,
+          &cropRect));
+

[danakj, 2013/08/29 02:07:16]

// Make the foreground layer's render surface be clipped by the background
layer.

[Stephen White, 2013/08/29 17:07:00]

Done.

+  background->SetMasksToBounds(true);
+  foreground->SetFilter(filter);
+
+  // Then we translate the foreground up by 100 pixels in Y, so the cropped
+  // region scrolls to the top. This ensures that the crop rect is being

[danakj, 2013/08/29 02:07:16]

nit: s/scrolls/is moved to/

[Stephen White, 2013/08/29 17:07:00]

Done.

+  // correctly transformed in skia by the amount of clipping that the
+  // compositor performs.
+  gfx::Transform transform;
+  transform.Translate(0.0, -100.0);
+  foreground->SetTransform(transform);
+
+  RunPixelTest(GL_WITH_BITMAP,
+               background,
+               base::FilePath(FILE_PATH_LITERAL(
+                   "blue_yellow.png")));
+}
+
 }  // namespace
 }  // namespace cc