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

cc/resources/picture_layer_tiling.cc

 // Copyright 2012 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/resources/picture_layer_tiling.h"
 
 #include <algorithm>
 #include <cmath>
 
 #include "base/debug/trace_event.h"
@@ skipping 357 matching lines @@
 
   gfx::Size tile_size = tiling_data_.max_texture_size();
   int64 prioritized_rect_area =
       max_tiles_for_interest_area *
       tile_size.width() * tile_size.height();
 
   gfx::Rect prioritized_rect = ExpandRectEquallyToAreaBoundedBy(
       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
   // not be live this frame, and mark them as being dead.
   for (TilingData::DifferenceIterator iter(&tiling_data_,
                                            last_prioritized_rect_,
                                            prioritized_rect);
        iter;
        ++iter) {
     TileMap::iterator find = tiles_.find(iter.index());
@@ skipping 136 matching lines @@
 
 gfx::Rect PictureLayerTiling::ExpandRectEquallyToAreaBoundedBy(
     gfx::Rect starting_rect,
     int64 target_area,
     gfx::Rect bounding_rect) {
   DCHECK(!starting_rect.IsEmpty());
   DCHECK(!bounding_rect.IsEmpty());
   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;
 
   // These values will be updated by the loop and uses as the output.
   int origin_x = rect.x();
   int origin_y = rect.y();
   int width = rect.width();
   int height = rect.height();
 
   // In the beginning we will consider 2 edges in each dimension.
   int num_y_edges = 2;
@@ skipping 62 matching lines @@
     }
 
     // If our delta is less then our event distance, we're done.
     if (delta < event.distance)
       break;
   }
 
   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