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

cc/output/gl_renderer.cc

Index: cc/output/gl_renderer.cc
diff --git a/cc/output/gl_renderer.cc b/cc/output/gl_renderer.cc
index e35b6975f1ea79f457e87f76229dea64e9414fed..9a636f0d4efa1b7feb0bb606a935f2afc9f16232 100644
--- a/cc/output/gl_renderer.cc
+++ b/cc/output/gl_renderer.cc
@@ -513,6 +513,7 @@ static inline SkBitmap ApplyFilters(GLRenderer* renderer,
 
 static SkBitmap ApplyImageFilter(GLRenderer* renderer,
                                  ContextProvider* offscreen_contexts,
+                                 gfx::Point origin,
                                  SkImageFilter* filter,
                                  ScopedResource* source_texture_resource) {
   if (!filter)
@@ -577,6 +578,7 @@ static SkBitmap ApplyImageFilter(GLRenderer* renderer,
   SkPaint paint;
   paint.setImageFilter(filter);
   canvas.clear(SK_ColorTRANSPARENT);
+  canvas.translate(SkFloatToScalar(-origin.x()), SkFloatToScalar(-origin.y()));

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

nit: I think you want SkIntToScalar?

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

Done. Thanks!

   canvas.drawSprite(source, 0, 0, &paint);

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

Could you just pass the origin here instead of 0,0 and do the same thing?

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

Nope. We're drawing the bitmap in the correct place, it's just the filter
parameters we want to affect. If we applied the origin here, the bitmap would be
offset.

 
   // Flush skia context so that all the rendered stuff appears on the
@@ -778,6 +780,7 @@ void GLRenderer::DrawRenderPassQuad(DrawingFrame* frame,
     } else {
       filter_bitmap = ApplyImageFilter(this,
                                        frame->offscreen_context_provider,
+                                       quad->rect.origin(),
                                        quad->filter.get(),
                                        contents_texture);
     }