Extract subpixel offsets from composited layer transforms when possible.

cc/layers/layer.cc

Index: cc/layers/layer.cc
diff --git a/cc/layers/layer.cc b/cc/layers/layer.cc
index 35816ee56e78e1233886feb3d44afc1a628baf5d..8844b9f517ab07ff9f7ba5d2f9ae1098fc525d64 100644
--- a/cc/layers/layer.cc
+++ b/cc/layers/layer.cc
@@ -614,15 +614,32 @@ bool Are2dAxisAligned(const gfx::Transform& a,
 
 void Layer::SetTransform(const gfx::Transform& transform) {
   DCHECK(IsPropertyChangeAllowed());
-  if (transform_ == transform)
+  transform_ = transform;
+
+  if (transform.IsIdentityOr2DTranslation()) {
+    gfx::Vector2dF translate = transform.To2dTranslation();
+    // Am I correct? -vmpstr
+    subpixel_offset_.set_x(translate.x() - std::floor(translate.x()));
+    subpixel_offset_.set_y(translate.y() - std::floor(translate.y()));
+    transform_.Translate(-subpixel_offset_.x(), -subpixel_offset_.y());
+    LOG(ERROR) << "Layer::SetTransform with subpixel: " << this << ": " << subpixel_offset_.ToString() << ": " << transform_.To2dTranslation().ToString();
+  }
+
+  if (transform_ == transform) {
+    LOG(ERROR) << "early out";
     return;
+  }
 
   SetSubtreePropertyChanged();
   if (layer_tree_host_) {
+            LOG(ERROR) << "layer_tree_host_ found";
+
     if (TransformNode* transform_node =
             layer_tree_host_->property_trees()->transform_tree.Node(
                 transform_tree_index())) {
+        LOG(ERROR) << "node found";
       if (transform_node->owner_id == id()) {
+        LOG(ERROR) << "id found";
         // We need to trigger a rebuild if we could have affected 2d axis
         // alignment. We'll check to see if transform and transform_ are axis
         // align with respect to one another.
@@ -630,6 +647,7 @@ void Layer::SetTransform(const gfx::Transform& transform) {
         bool preserves_2d_axis_alignment =
             Are2dAxisAligned(transform_, transform, &invertible);
         transform_node->data.local = transform;
+
         transform_node->data.needs_local_transform_update = true;
         transform_node->data.transform_changed = true;
         layer_tree_host_->property_trees()->transform_tree.set_needs_update(
@@ -643,9 +661,10 @@ void Layer::SetTransform(const gfx::Transform& transform) {
         return;
       }
     }
+  } else {
+    LOG(ERROR) << "no layer tree host???";
   }
 
-  transform_ = transform;
   transform_is_invertible_ = transform.IsInvertible();
 
   SetNeedsCommit();
@@ -1135,7 +1154,7 @@ void Layer::PushPropertiesTo(LayerImpl* layer) {
 #else
   layer->SetDebugInfo(TakeDebugInfo());
 #endif
-
+  layer->SetSubpixelOffset(subpixel_offset_);
   layer->SetTransformTreeIndex(transform_tree_index());
   layer->SetEffectTreeIndex(effect_tree_index());
   layer->SetClipTreeIndex(clip_tree_index());