Snap layers in views to physical pixel

ui/views/view.cc

Index: ui/views/view.cc
diff --git a/ui/views/view.cc b/ui/views/view.cc
index 3809c1e73250005d9607e065292d14316ff57e81..8972e0101bb1db934d65110417227ab3d9bb9617 100644
--- a/ui/views/view.cc
+++ b/ui/views/view.cc
@@ -20,6 +20,7 @@
 #include "ui/base/cursor/cursor.h"
 #include "ui/base/dragdrop/drag_drop_types.h"
 #include "ui/compositor/compositor.h"
+#include "ui/compositor/dip_util.h"
 #include "ui/compositor/layer.h"
 #include "ui/compositor/layer_animator.h"
 #include "ui/events/event_target_iterator.h"
@@ -112,6 +113,7 @@ View::View()
       root_bounds_dirty_(true),
       clip_insets_(0, 0, 0, 0),
       needs_layout_(true),
+      snap_layer_to_pixel_boundary_(false),
       flip_canvas_on_paint_for_rtl_ui_(false),
       paint_to_layer_(false),
       accelerator_focus_manager_(NULL),
@@ -572,6 +574,19 @@ void View::SetLayoutManager(LayoutManager* layout_manager) {
     layout_manager_->Installed(this);
 }
 
+void View::SnapLayerToPixelBoundary() {

[Ben Goodger (Google), 2014/07/08 16:07:34]

should this be implemented in Layer?

[oshima, 2014/07/08 17:39:18]

Layer currently does not know which layers are aligned (this CL assumes that
widgets are aligned). I can add a flag to layer (to enable or disable depending
on the default), that's one option. Or I can define
Layer::SnapToPixelBoundary(ui::Layer* aligned_layer) and let caller decide which
layers are aligned.

ben@, danakj@, what do you think? Let me know if you have a better idea, and I'm
happy to change.

+  if (!layer())
+    return;
+
+  if (snap_layer_to_pixel_boundary_ &&
+      layer()->parent() && layer()->GetCompositor()) {

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

why the check for GetCompositor?

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

Because view can exist without parent nor widget.
It gets recomputed when (re)attached in OnDeviceScaleFactorChanged.

+    ui::SnapLayerToPhysicalPixelBoundary(layer()->parent(), layer());
+  } else {
+    // Reset the offset.
+    layer()->SetSubpixelPositionOffset(gfx::Vector2dF());
+  }
+}
+
 // Attributes ------------------------------------------------------------------
 
 const char* View::GetClassName() const {
@@ -1542,6 +1557,9 @@ void View::OnPaintLayer(gfx::Canvas* canvas) {
 }
 
 void View::OnDeviceScaleFactorChanged(float device_scale_factor) {
+  snap_layer_to_pixel_boundary_ =

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

Didn't you want this to be false for some layers like shadows?

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

Yes, it'll be handled in a separate CL for ash. (shadow is in ash, not views)

+      (device_scale_factor - std::floor(device_scale_factor)) != 0.0f;
+  SnapLayerToPixelBoundary();
   // Repainting with new scale factor will paint the content at the right scale.
 }
 
@@ -2071,6 +2089,7 @@ void View::RemoveDescendantToNotify(View* view) {
 
 void View::SetLayerBounds(const gfx::Rect& bounds) {
   layer()->SetBounds(bounds);
+  SnapLayerToPixelBoundary();
 }
 
 void View::SetRootBoundsDirty(bool origin_changed) {