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

cc/output/render_surface_filters.cc

 // Copyright 2012 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/output/render_surface_filters.h"
 
 #include <algorithm>
 
 #include "base/logging.h"
 #include "cc/output/filter_operation.h"
@@ skipping 245 matching lines @@
           image_filter = CreateMatrixImageFilter(matrix, image_filter);
         } else if (image_filter) {
           image_filter = skia::AdoptRef(SkComposeImageFilter::Create(
               op.image_filter().get(), image_filter.get()));
         } else {
           image_filter = op.image_filter();
         }
         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()));
         } else {
           image_filter = alpha_filter;
         }
         break;
       }
     }
   }
   return image_filter;
 }
 
 }  // namespace cc