Snap layers in views to physical pixel

ui/compositor/dip_util.cc

 // Copyright (c) 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 "ui/compositor/dip_util.h"
 
 #include "base/command_line.h"
 #include "ui/compositor/compositor.h"
 #include "ui/compositor/compositor_switches.h"
 #include "ui/compositor/layer.h"
@@ skipping 52 matching lines @@
   // Use ToEnclosingRect() to ensure we paint all the possible pixels
   // touched. ToEnclosingRect() floors the origin, and ceils the max
   // coordinate. To do otherwise (such as flooring the size) potentially
   // results in rounding down and not drawing all the pixels that are
   // touched.
   return gfx::ToEnclosingRect(
       gfx::RectF(gfx::ScalePoint(rect_in_dip.origin(), scale),
                  gfx::ScaleSize(rect_in_dip.size(), scale)));
 }
 
+void SnapLayerToPhysicalPixelBoundary(ui::Layer* snapped_layer,
+                                      ui::Layer* layer_to_snap) {
+  DCHECK_NE(snapped_layer, layer_to_snap);
+  if (!snapped_layer || !snapped_layer->Contains(layer_to_snap))

[danakj, 2014/07/15 21:11:02]

Could these be DCHECKs instead?
Also can we DCHECK that the snapped layer is in fact snapped?

[oshima, 2014/07/15 22:28:42]

Done.

+    return;
+
+  gfx::Point view_offset_dips = layer_to_snap->GetTargetBounds().origin();
+  ui::Layer::ConvertPointToLayer(
+      layer_to_snap->parent(), snapped_layer, &view_offset_dips);
+  gfx::PointF view_offset = view_offset_dips;
+
+  float scale_factor = GetDeviceScaleFactor(layer_to_snap);
+  view_offset.Scale(scale_factor);
+  gfx::PointF view_offset_snapped(std::round(view_offset.x()),
+                                  std::round(view_offset.y()));

[jamesr, 2014/07/08 01:00:59]

this moves the layer towards (0, 0).  If this is used by nested layers, won't
each one move up a bit causing large accumulated error?

This 'works' for RWHVAura since there can't be multiple layers of embedding
there, but there can be on an arbitrary layer.

+
+  gfx::Vector2dF fudge = view_offset_snapped - view_offset;
+  fudge.Scale(1.0 / scale_factor);
+  layer_to_snap->SetSubpixelPositionOffset(fudge);
+}
+
 }  // namespace ui