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..9d6b57134fdc1d567c6d37c2b9497db8b5a04966 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();
+  raw_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 (raw_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.

[Wez, 2014/06/08 00:55:53]

Should we add an SkRegion scaler? Or pass SkPaths down to here so we avoid too
much conversion back-and-forth?

[garykac, 2014/06/09 21:29:59]

My first approach was to add a SkRegion scaler. See crrev.com/315083004 for the
discussion of why Skia didn't want that.
We only convert when the Shape is set or the DSF changes, so we don't have lots
of back-and-forth conversions.

Note that if we expand the SetShape API to accept paths, then we'll need to
re-think this.

+    SkPath path;
+    raw_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);

[Wez, 2014/06/08 00:55:54]

This won't work for |raw_alpha_shape_|s that are non-contiguous (e.g. if the
region consists of two non-adjacent rects) - IIRC setPath doesn't currently cope
with that case.

[garykac, 2014/06/09 21:29:59]

It worked when I tried it (with 3 non-overlapping rects).

+  }
+}
+
 }  // namespace ui