Scale the ALPHA_THRESHOLD filter's region based on the device scale factor.

cc/output/render_surface_filters.cc

Index: cc/output/render_surface_filters.cc
diff --git a/cc/output/render_surface_filters.cc b/cc/output/render_surface_filters.cc
index 34e056dad521be2fa8a3260c904790aceee3fa3d..581a6717c6ec4a5824734cf6d6fe239cdb25a4e8 100644
--- a/cc/output/render_surface_filters.cc
+++ b/cc/output/render_surface_filters.cc
@@ -263,9 +263,22 @@ skia::RefPtr<SkImageFilter> RenderSurfaceFilters::BuildImageFilter(
         break;
       }
       case FilterOperation::ALPHA_THRESHOLD: {
-        skia::RefPtr<SkImageFilter> alpha_filter = skia::AdoptRef(
-            SkAlphaThresholdFilter::Create(
-                op.region(), op.amount(), op.outer_threshold()));
+        // Scale the region by the device_scale_factor to properly handle
+        // HiDPI devices. Since there is no SkRegion scaling, we convert to
+        // a path, scale the path and then convert back to a SkRegion.
+        SkPath path;
+        op.region().getBoundaryPath(&path);
+        SkMatrix scale;
+        scale.setScale(op.device_scale_factor(), op.device_scale_factor());
+        path.transform(scale);
+        SkRegion clip;
+        clip.setRect(0, 0, size.width(), size.height());
+        SkRegion scaled_shape;
+        scaled_shape.setPath(path, clip);
+
+        skia::RefPtr<SkImageFilter> alpha_filter =
+            skia::AdoptRef(SkAlphaThresholdFilter::Create(

[danakj, 2014/06/17 21:35:28]

Can we just add the scale to this SkAlphaThresholdFilter? That would be a lot
cheaper, to just multiply values inside the filter instead of doing this
Region->Path->Region thing.

[garykac, 2014/07/10 23:02:24]

Done.

+                scaled_shape, op.amount(), op.outer_threshold()));

[danakj, 2014/06/17 21:42:01]

If you put a transform on the ui::Layer with a scale in it, will you get the
right result just using the DSF? Can you test that case?

[garykac, 2014/07/10 23:02:24]

I tested this and we need to pass the layer scale as well as use the DSF. Code
has been updated to handle this.

         if (image_filter.get()) {
           image_filter = skia::AdoptRef(SkComposeImageFilter::Create(
               alpha_filter.get(), image_filter.get()));