Fixed rounding issue on scrollbar rasterization.

cc/layers/painted_scrollbar_layer.cc

 // Copyright 2013 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 "cc/layers/painted_scrollbar_layer.h"
 
 #include "base/auto_reset.h"
 #include "base/basictypes.h"
 #include "base/debug/trace_event.h"
 #include "cc/layers/painted_scrollbar_layer_impl.h"
@@ skipping 152 matching lines @@
   }
 
   ContentsScalingLayer::SetLayerTreeHost(host);
 }
 
 gfx::Rect PaintedScrollbarLayer::ScrollbarLayerRectToContentRect(
     const gfx::Rect& layer_rect) const {
   // Don't intersect with the bounds as in LayerRectToContentRect() because
   // layer_rect here might be in coordinates of the containing layer.
   gfx::Rect expanded_rect = gfx::ScaleToEnclosingRect(
-      layer_rect, contents_scale_y(), contents_scale_y());
+      layer_rect, contents_scale_x(), contents_scale_y());
   // We should never return a rect bigger than the content_bounds().
   gfx::Size clamped_size = expanded_rect.size();
   clamped_size.SetToMin(content_bounds());
   expanded_rect.set_size(clamped_size);
   return expanded_rect;
 }
 
 gfx::Rect PaintedScrollbarLayer::OriginThumbRect() const {
   gfx::Size thumb_size;
   if (orientation() == HORIZONTAL) {
     thumb_size =
         gfx::Size(scrollbar_->ThumbLength(), scrollbar_->ThumbThickness());
   } else {
     thumb_size =
         gfx::Size(scrollbar_->ThumbThickness(), scrollbar_->ThumbLength());
   }
-  return ScrollbarLayerRectToContentRect(gfx::Rect(thumb_size));
+  return gfx::Rect(thumb_size);
 }
 
 void PaintedScrollbarLayer::UpdateThumbAndTrackGeometry() {
   UpdateProperty(scrollbar_->TrackRect(), &track_rect_);
   UpdateProperty(scrollbar_->Location(), &location_);
   UpdateProperty(scrollbar_->IsOverlay(), &is_overlay_);
   UpdateProperty(scrollbar_->HasThumb(), &has_thumb_);
   if (has_thumb_) {
     UpdateProperty(scrollbar_->ThumbThickness(), &thumb_thickness_);
     UpdateProperty(scrollbar_->ThumbLength(), &thumb_length_);
   }
 }
 
 bool PaintedScrollbarLayer::Update(ResourceUpdateQueue* queue,
                                    const OcclusionTracker* occlusion) {
   UpdateThumbAndTrackGeometry();
 
+  gfx::Rect track_layer_rect = gfx::Rect(location_, bounds());
   gfx::Rect scaled_track_rect = ScrollbarLayerRectToContentRect(
-      gfx::Rect(location_, bounds()));
+      track_layer_rect);
 
   if (track_rect_.IsEmpty() || scaled_track_rect.IsEmpty())
     return false;
 
   {
     base::AutoReset<bool> ignore_set_needs_commit(&ignore_set_needs_commit_,
                                                   true);
     ContentsScalingLayer::Update(queue, occlusion);
   }
 
   if (update_rect_.IsEmpty() && track_resource_)
     return false;
 
   track_resource_ = ScopedUIResource::Create(
-      layer_tree_host(), RasterizeScrollbarPart(scaled_track_rect, TRACK));
+      layer_tree_host(),
+      RasterizeScrollbarPart(track_layer_rect, scaled_track_rect, TRACK));
 
-  gfx::Rect thumb_rect = OriginThumbRect();
-  if (has_thumb_ && !thumb_rect.IsEmpty()) {
+  gfx::Rect thumb_layer_rect = OriginThumbRect();
+  gfx::Rect scaled_thumb_rect =
+      ScrollbarLayerRectToContentRect(thumb_layer_rect);
+  if (has_thumb_ && !scaled_thumb_rect.IsEmpty()) {
     thumb_resource_ = ScopedUIResource::Create(
-        layer_tree_host(), RasterizeScrollbarPart(thumb_rect, THUMB));
+        layer_tree_host(),
+        RasterizeScrollbarPart(thumb_layer_rect, scaled_thumb_rect, THUMB));
   }
 
   // UI resources changed so push properties is needed.
   SetNeedsPushProperties();
   return true;
 }
 
 UIResourceBitmap PaintedScrollbarLayer::RasterizeScrollbarPart(
+    const gfx::Rect& layer_rect,
     const gfx::Rect& rect,
     ScrollbarPart part) {
   DCHECK(!rect.size().IsEmpty());
+  DCHECK(!layer_rect.size().IsEmpty());
 
   SkBitmap skbitmap;
   skbitmap.setConfig(SkBitmap::kARGB_8888_Config, rect.width(), rect.height());
   skbitmap.allocPixels();
 
   SkCanvas skcanvas(skbitmap);
-  skcanvas.translate(SkFloatToScalar(-rect.x()), SkFloatToScalar(-rect.y()));
+
+  // Skia will round to the nearest pixel so we need to translate by the
+  // rounded origin coordinates
+  float roundedX = round(layer_rect.x() * contents_scale_x());
+  float roundedY = round(layer_rect.y() * contents_scale_y());
+  skcanvas.translate(SkFloatToScalar(-roundedX), SkFloatToScalar(-roundedY));
   skcanvas.scale(SkFloatToScalar(contents_scale_x()),
                  SkFloatToScalar(contents_scale_y()));
 
-  gfx::Rect layer_rect = gfx::ScaleToEnclosingRect(
-      rect, 1.f / contents_scale_x(), 1.f / contents_scale_y());
   SkRect layer_skrect = RectToSkRect(layer_rect);
   SkPaint paint;
   paint.setAntiAlias(false);
   paint.setXfermodeMode(SkXfermode::kClear_Mode);
   skcanvas.drawRect(layer_skrect, paint);
   skcanvas.clipRect(layer_skrect);
 
   scrollbar_->PaintPart(&skcanvas, part, layer_rect);
   // Make sure that the pixels are no longer mutable to unavoid unnecessary
   // allocation and copying.
   skbitmap.setImmutable();
 
   return UIResourceBitmap(skbitmap);
 }
 
 }  // namespace cc