Prioritize tiles near the viewport in out-of-viewport layers

cc/resources/picture_layer_tiling.cc

Index: cc/resources/picture_layer_tiling.cc
diff --git a/cc/resources/picture_layer_tiling.cc b/cc/resources/picture_layer_tiling.cc
index c3a2ffe17d6a604b94cab4b2f09199202b9115cd..01f933ec0c48f1bf5316e6543adad05c0cd316be 100644
--- a/cc/resources/picture_layer_tiling.cc
+++ b/cc/resources/picture_layer_tiling.cc
@@ -375,6 +375,17 @@ void PictureLayerTiling::UpdateTilePriorities(
       viewport_in_content_space,
       prioritized_rect_area,
       ContentRect());
+
+  if (prioritized_rect.IsEmpty()) {
+    // For a layer out of the viewport, prioritize its tiles near to the
+    // viewport border so that they could be available when scrolled in.
+    prioritized_rect = ExpandRectBySizeBoundedBy(

[danakj, 2013/04/03 23:08:34]

NAK: Using the viewport rect (or something at most a constant size away from the
viewport rect) gives you a quadratic number of tiles relative to the page scale,
which is not desirable.

+        viewport_in_content_space,
+        tile_size.width() * 2,
+        tile_size.height() * 2,
+        ContentRect());
+  }
+
   DCHECK(ContentRect().Contains(prioritized_rect));
 
   // Iterate through all of the tiles that were live last frame but will
@@ -531,6 +542,7 @@ gfx::Rect PictureLayerTiling::ExpandRectEquallyToAreaBoundedBy(
   DCHECK_GT(target_area, 0);
 
   gfx::Rect rect = IntersectRects(starting_rect, bounding_rect);
+
   if (rect.IsEmpty())

[enne (OOO), 2013/04/03 23:41:49]

I think this patch should really just remove this conditional.

     return rect;
 
@@ -614,4 +626,13 @@ gfx::Rect PictureLayerTiling::ExpandRectEquallyToAreaBoundedBy(
   return gfx::Rect(origin_x, origin_y, width, height);
 }
 
+gfx::Rect PictureLayerTiling::ExpandRectBySizeBoundedBy(
+    gfx::Rect starting_rect,
+    int horizontal_expansion,
+    int vertical_expansion,
+    gfx::Rect bounding_rect) {
+  starting_rect.Inset(-horizontal_expansion, -vertical_expansion);
+  return IntersectRects(starting_rect, bounding_rect);
+}
+
 }  // namespace cc