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

ui/compositor/layer.cc

Index: ui/compositor/layer.cc
diff --git a/ui/compositor/layer.cc b/ui/compositor/layer.cc
index ca3ff288ee293a43b49d39bee0f6f97311e6a0cd..d3bc3a4576f8227b7d34b33ae18b48459cadd9af 100644
--- a/ui/compositor/layer.cc
+++ b/ui/compositor/layer.cc
@@ -334,7 +334,8 @@ void Layer::SetBackgroundZoom(float zoom, int inset) {
 }
 
 void Layer::SetAlphaShape(scoped_ptr<SkRegion> region) {
-  alpha_shape_ = region.Pass();
+  dips_alpha_shape_ = region.Pass();
+  RecomputeAlphaShape();
 
   SetLayerFilters();
 }
@@ -621,6 +622,7 @@ void Layer::OnDeviceScaleFactorChanged(float device_scale_factor) {
   device_scale_factor_ = device_scale_factor;
   RecomputeDrawsContentAndUVRect();
   RecomputePosition();
+  RecomputeAlphaShape();
   SchedulePaint(gfx::Rect(bounds_.size()));
   if (delegate_)
     delegate_->OnDeviceScaleFactorChanged(device_scale_factor);
@@ -930,4 +932,21 @@ void Layer::RecomputePosition() {
   cc_layer_->SetPosition(gfx::PointF(bounds_.x(), bounds_.y()));
 }
 
+void Layer::RecomputeAlphaShape() {
+  if (dips_alpha_shape_) {
+    // 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.

[danakj, 2014/06/09 22:21:39]

:/ Storing 2 regions now, regions are costly, so I'm not excited about that.
Would it work to just add a scale to the FilterOperation, and make use of that
scale when applying the region to the alpha channel?

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

Acknowledged.

+    SkPath path;
+    dips_alpha_shape_->getBoundaryPath(&path);
+    SkMatrix scale;
+    scale.setScale(device_scale_factor_, device_scale_factor_);
+    path.transform(scale);
+    SkRegion clip;
+    clip.setRect(0, 0, bounds_.width(), bounds_.height());
+
+    alpha_shape_->setPath(path, clip);
+  }
+}
+
 }  // namespace ui