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

cc/layers/picture_layer_impl.cc

Index: cc/layers/picture_layer_impl.cc
diff --git a/cc/layers/picture_layer_impl.cc b/cc/layers/picture_layer_impl.cc
index 8be30987e03be429d9c3e9919c4dc18c85dc6889..34a0ebb761d148427ffb3c768ed4c6375e39a33f 100644
--- a/cc/layers/picture_layer_impl.cc
+++ b/cc/layers/picture_layer_impl.cc
@@ -49,6 +49,26 @@ const int kMinHeightForGpuRasteredTile = 256;
 // of using the same tile size.
 const int kTileRoundUp = 64;
 
+// The precision value for rounding floating points values (of scale factors),
+// to given decimal places so that scale factors which are almost equal (differ
+// by some threshold magnitude of floating point epsilon) are considered as
+// same.
+const int kScalePrecision = 4;
+
+float RoundToFixedPrecision(float value) {
+  // Limiting precision 1 <= kScalePrecision <= 8, as floating point precision
+  // of >= 12 gives different values, 8 on a safer side.
+  DCHECK(kScalePrecision >= 1 && kScalePrecision <= 8);
+
+  const float factor = pow(10, kScalePrecision);
+  float integral;
+  float fractional = std::modf(value, &integral);

[prashant.n, 2015/09/01 16:55:25]

I tried checking standard way for doing this. Definitely there should be based
on logarithmic function. But I still did not get it. I'll try to work on it.

Meanwhile enne@/danakj, if you know this rounding can you pls. suggest here.

+  fractional *= factor;
+  fractional = std::round(fractional);
+  fractional /= factor;
+  return integral + fractional;
+}
+
 }  // namespace
 
 namespace cc {
@@ -902,10 +922,12 @@ void PictureLayerImpl::RecalculateRasterScales() {
   float old_raster_page_scale = raster_page_scale_;
   float old_raster_source_scale = raster_source_scale_;
 
+  // Get fixed precision values to avoid creating extra tilings for scales
+  // which are almost equal.
   raster_device_scale_ = ideal_device_scale_;
   raster_page_scale_ = ideal_page_scale_;
   raster_source_scale_ = ideal_source_scale_;
-  raster_contents_scale_ = ideal_contents_scale_;
+  raster_contents_scale_ = RoundToFixedPrecision(ideal_contents_scale_);

[prashant.n, 2015/09/01 16:55:25]

I thought of rounding all raster scales, so that all are in sync. When checked
by applying only for contents scale I did not see any issue.

 
   // If we're not animating, or leaving an animation, and the
   // ideal_source_scale_ changes, then things are unpredictable, and we fix