cc: Fix low-res impl-side painting artifacts

cc/picture_pile.cc

Index: cc/picture_pile.cc
diff --git a/cc/picture_pile.cc b/cc/picture_pile.cc
index eab92c66e39ab46a072adca036e55fcfc5a9f8fe..565c6c2d8eb14a9b09b64dfb82208b3ed6c4b986 100644
--- a/cc/picture_pile.cc
+++ b/cc/picture_pile.cc
@@ -18,7 +18,10 @@ const float kResetThreshold = 0.7f;
 
 namespace cc {
 
-PicturePile::PicturePile() {
+PicturePile::PicturePile()
+    : min_contents_scale_(0),
+      buffer_pixels_(0) {
+  SetMinContentsScale(1);
 }
 
 PicturePile::~PicturePile() {
@@ -32,6 +35,28 @@ void PicturePile::Resize(gfx::Size size) {
   size_ = size;
 }
 
+void PicturePile::SetMinContentsScale(float min_contents_scale) {
+  DCHECK(min_contents_scale);
+  if (min_contents_scale_ == min_contents_scale)
+    return;
+
+  pile_.clear();
+  min_contents_scale_ = min_contents_scale;
+
+  // Picture contents are played back scaled. When the final contents scale is
+  // less than 1 (i.e. low res), then multiple recorded pixels will be used
+  // to raster one final pixel.  To avoid splitting a final pixel across
+  // pictures (which would result in incorrect rasterization due to blending), a
+  // buffer margin is added so that any picture can be snapped to integral
+  // final pixels.
+  //
+  // For example, if a 1/4 contents scale is used, then that would be 3 buffer
+  // pixels, since that's the minimum number of pixels to add so that resulting
+  // content can be snapped to a four pixel aligned grid.
+  buffer_pixels_ = static_cast<int>(ceil(1 / min_contents_scale_) - 1);
+  buffer_pixels_ = std::max(0, buffer_pixels_);
+}
+
 void PicturePile::Update(
     ContentLayerClient* painter,
     const Region& invalidation,
@@ -63,6 +88,16 @@ void PicturePile::InvalidateRect(gfx::Rect invalidation) {
   if (invalidation.IsEmpty())
     return;
 
+  // Inflate all recordings from invalidations with a margin so that when
+  // scaled down to at least min_contents_scale, any final pixel touched by an
+  // invalidation can be fully rasterized by this picture.
+  invalidation.Inset(
+      -buffer_pixels_,
+      -buffer_pixels_,
+      -buffer_pixels_,
+      -buffer_pixels_);
+  invalidation.Intersect(gfx::Rect(size_));
+
   std::vector<Pile::iterator> overlaps;
   for (Pile::iterator i = pile_.begin(); i != pile_.end(); ++i) {
     if ((*i)->LayerRect().Contains(invalidation) && !(*i)->HasRecording())
@@ -98,6 +133,7 @@ void PicturePile::ResetPile(ContentLayerClient* painter,
 
 void PicturePile::PushPropertiesTo(PicturePileImpl* other) {
   other->pile_ = pile_;
+  other->min_contents_scale_ = min_contents_scale_;
   // Remove all old clones.
   other->clones_.clear();
 }