cc: Do not create separate tilings for almost equal scale factors.

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 <algorithm>
 #include <cmath>
 #include <limits>
 #include <set>
@@ skipping 30 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;
 
+// The precision value for rounding floating points values of scale factors.
+// With this scale factors will be having 3 digits fractional part, which will
+// make almost equal (having smaller difference by some magnitude of floating
+// point epsilon) scale factors to be considered as same.
+const int kScalePrecision = 4;
+
 }  // namespace
 
 namespace cc {
 
 PictureLayerImpl::PictureLayerImpl(
     LayerTreeImpl* tree_impl,
     int id,
     bool is_mask,
     scoped_refptr<SyncedScrollOffset> scroll_offset)
     : LayerImpl(tree_impl, id, scroll_offset),
@@ skipping 1119 matching lines @@
           : settings.skewport_target_time_in_seconds,
       settings.skewport_extrapolation_limit_in_content_pixels);
 }
 
 void PictureLayerImpl::UpdateIdealScales() {
   DCHECK(CanHaveTilings());
 
   float min_contents_scale = MinimumContentsScale();
   DCHECK_GT(min_contents_scale, 0.f);
 
-  ideal_page_scale_ = IsAffectedByPageScale()
-                          ? layer_tree_impl()->current_page_scale_factor()
-                          : 1.f;
+  ideal_page_scale_ =
+      IsAffectedByPageScale()
+          ? MathUtil::RoundToFixedPrecision(
+                layer_tree_impl()->current_page_scale_factor(), kScalePrecision)
+          : 1.f;
   ideal_device_scale_ = layer_tree_impl()->device_scale_factor();
-  ideal_contents_scale_ = std::max(GetIdealContentsScale(), min_contents_scale);
+  ideal_contents_scale_ = MathUtil::RoundToFixedPrecision(
+      std::max(GetIdealContentsScale(), min_contents_scale), kScalePrecision);

[prashant.n, 2015/10/26 16:35:04]

Modifying ideal page and contents scales are needed as they are checked against
in ShouldAdjustRasterScale() and RecalculateRasterScales() functions.

   ideal_source_scale_ =
       ideal_contents_scale_ / ideal_page_scale_ / ideal_device_scale_;
 }
 
 void PictureLayerImpl::GetDebugBorderProperties(
     SkColor* color,
     float* width) const {
   *color = DebugColors::TiledContentLayerBorderColor();
   *width = DebugColors::TiledContentLayerBorderWidth(layer_tree_impl());
 }
@@ skipping 58 matching lines @@
 
 bool PictureLayerImpl::IsOnActiveOrPendingTree() const {
   return !layer_tree_impl()->IsRecycleTree();
 }
 
 bool PictureLayerImpl::HasValidTilePriorities() const {
   return IsOnActiveOrPendingTree() && IsDrawnRenderSurfaceLayerListMember();
 }
 
 }  // namespace cc