Change GPU Tile rounding to 32

cc/layers/picture_layer_impl.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/layers/picture_layer_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 33 matching lines @@
 
 // Even for really wide viewports, at some point GPU raster should use
 // less than 4 tiles to fill the viewport. This is set to 256 as a
 // sane minimum for now, but we might want to tune this for low-end.
 const int kMinHeightForGpuRasteredTile = 256;
 
 // When making odd-sized tiles, round them up to increase the chances
 // of using the same tile size.
 const int kTileRoundUp = 64;
 
+// Round GPU default tile sizes to a multiple of 32. This helps prevent
+// rounding errors during compositing.
+const int kGpuDefaultTileRoundUp = 32;
+
 // For performance reasons and to support compressed tile textures, tile
 // width and height should be an even multiple of 4 in size.
 const int kTileMinimalAlignment = 4;
 
 }  // namespace
 
 namespace cc {
 
 PictureLayerImpl::PictureLayerImpl(LayerTreeImpl* tree_impl,
                                    int id,
@@ skipping 668 matching lines @@
       divisor = 2;
     if (content_bounds.width() <= viewport_width / 4)
       divisor = 1;
     default_tile_height =
         MathUtil::UncheckedRoundUp(viewport_height, divisor) / divisor;
 
     // Grow default sizes to account for overlapping border texels.
     default_tile_width += 2 * PictureLayerTiling::kBorderTexels;
     default_tile_height += 2 * PictureLayerTiling::kBorderTexels;
 
+    // Round GPU default tile sizes to a multiple of kGpuDefaultTileAlignment.
+    // This helps prevent rounding errors in our CA path. crbug.com/632274
+    default_tile_width =
+        MathUtil::UncheckedRoundUp(default_tile_width, kGpuDefaultTileRoundUp);
+    default_tile_height =
+        MathUtil::UncheckedRoundUp(default_tile_height, kGpuDefaultTileRoundUp);
+
     default_tile_height =
         std::max(default_tile_height, kMinHeightForGpuRasteredTile);
   } else {
     // For CPU rasterization we use tile-size settings.
     const LayerTreeSettings& settings = layer_tree_impl()->settings();
     int max_untiled_content_width = settings.max_untiled_layer_size.width();
     int max_untiled_content_height = settings.max_untiled_layer_size.height();
     default_tile_width = settings.default_tile_size.width();
     default_tile_height = settings.default_tile_size.height();
 
@@ skipping 552 matching lines @@
 bool PictureLayerImpl::IsOnActiveOrPendingTree() const {
   return !layer_tree_impl()->IsRecycleTree();
 }
 
 bool PictureLayerImpl::HasValidTilePriorities() const {
   return IsOnActiveOrPendingTree() &&
          is_drawn_render_surface_layer_list_member();
 }
 
 }  // namespace cc