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..3369a996ba7427c86d002748ae11d4d955e85f74 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 +150,48 @@ TEST_F(LayerTreeHostFiltersPixelTest, BackgroundFilterBlurOffAxis) {
                    "background_filter_blur_off_axis.png")));
 }
 
+TEST_F(LayerTreeHostFiltersPixelTest, ImageFilterClipped) {
+  scoped_refptr<SolidColorLayer> root = CreateSolidColorLayer(
+      gfx::Rect(200, 200), SK_ColorBLUE);
+
+  scoped_refptr<SolidColorLayer> background = CreateSolidColorLayer(
+      gfx::Rect(200, 200), SK_ColorGREEN);
+  root->AddChild(background);
+
+  scoped_refptr<SolidColorLayer> foreground = CreateSolidColorLayer(
+      gfx::Rect(200, 200), SK_ColorGREEN);
+  background->AddChild(foreground);
+
+  SkScalar matrix[20];
+  memset(matrix, 0, 20 * sizeof(matrix[0]));
+  matrix[1] = matrix[5] = matrix[12] = matrix[18] = SK_Scalar1;
+  skia::RefPtr<SkColorFilter> colorFilter(skia::AdoptRef(
+      new SkColorMatrixFilter(matrix)));
+  // We filter only the top 200x100 pixels of the foreground.
+  SkIRect cropRect = SkIRect::MakeWH(200, 100);
+  skia::RefPtr<SkImageFilter> filter =
+      skia::AdoptRef(SkColorFilterImageFilter::Create(
+          colorFilter.get(),
+          NULL,
+          &cropRect));
+
+  background->SetMasksToBounds(true);
+  foreground->SetFilter(filter);
+
+  // Then we translate the foreground up by 100 pixels in Y, so the cropped
+  // region scrolls off. This ensures that the crop rect is being correctly

[danakj, 2013/08/28 22:26:58]

If you translated by the origin twice in the renderer, this test would still
pass, right? Moving something entirely out of the viewport is going to give less
confidence than moving it inside the viewport and seeing it is in the right
spot. Can we do the latter? Maybe using the blue_yellow.png?

[Stephen White, 2013/08/29 00:36:34]

Yes, it would still pass. But the layout tests I've got ready to go in my WebKit
patch would all fail. Still let's make sure both the belt and suspenders work.
:)
OK, I'll give that a shot.

+  // 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(
+                   "green.png")));
+}
+
 }  // namespace
 }  // namespace cc