cc: Fix skewport math

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 <limits>
 #include <set>
@@ skipping 546 matching lines @@
   gfx::Rect max_skewport = skewport;
   max_skewport.Inset(-skewport_extrapolation_limit_in_content_pixels_,
                      -skewport_extrapolation_limit_in_content_pixels_);
 
   // Inset the skewport by the needed adjustment.
   skewport.Inset(extrapolation_multiplier * (new_x - old_x),
                  extrapolation_multiplier * (new_y - old_y),
                  extrapolation_multiplier * (old_right - new_right),
                  extrapolation_multiplier * (old_bottom - new_bottom));
 
-  // Clip the skewport to |max_skewport|.
+  // Ensure that visible rect is contained in the skewport.
+  skewport.Union(visible_rect_in_content_space);
+
+  // Clip the skewport to |max_skewport|. This needs to happen after the
+  // union in case intersecting would have left the empty rect.
   skewport.Intersect(max_skewport);
 
-  // Finally, ensure that visible rect is contained in the skewport.
-  skewport.Union(visible_rect_in_content_space);
+  // The max_skewport should always contain the visible_rect_in_content_space
+  // so this should still be true.
+  DCHECK(skewport.Contains(visible_rect_in_content_space));
+
   return skewport;
 }
 
 bool PictureLayerTiling::ComputeTilePriorityRects(
     const gfx::Rect& viewport_in_layer_space,
     float ideal_contents_scale,
     double current_frame_time_in_seconds,
     const Occlusion& occlusion_in_layer_space) {
   if (!NeedsUpdateForFrameAtTimeAndViewport(current_frame_time_in_seconds,
                                             viewport_in_layer_space)) {
     // This should never be zero for the purposes of has_ever_been_updated().
     DCHECK_NE(current_frame_time_in_seconds, 0.0);
     return false;
   }
 
   gfx::Rect visible_rect_in_content_space =
       gfx::ScaleToEnclosingRect(viewport_in_layer_space, contents_scale_);
 
   if (tiling_size().IsEmpty()) {
     UpdateVisibleRectHistory(current_frame_time_in_seconds,
                              visible_rect_in_content_space);
     last_viewport_in_layer_space_ = viewport_in_layer_space;
     return false;
   }
 
   // Calculate the skewport.
   gfx::Rect skewport = ComputeSkewport(current_frame_time_in_seconds,
                                        visible_rect_in_content_space);
   DCHECK(skewport.Contains(visible_rect_in_content_space));

[vmpstr, 2015/03/18 23:11:51]

<_<

 
   // Calculate the eventually/live tiles rect.
   gfx::Size tile_size = tiling_data_.max_texture_size();
   int64 eventually_rect_area =
       max_tiles_for_interest_area_ * tile_size.width() * tile_size.height();
 
   gfx::Rect eventually_rect =
       ExpandRectEquallyToAreaBoundedBy(visible_rect_in_content_space,
                                        eventually_rect_area,
                                        gfx::Rect(tiling_size()),
@@ skipping 430 matching lines @@
       break;
   }
 
   gfx::Rect result(origin_x, origin_y, width, height);
   if (cache)
     cache->previous_result = result;
   return result;
 }
 
 }  // namespace cc