Use LCD text if the transform IsAlmostIdentityAndIntegerTranslation

cc/layer_tree_host_common.cc

Index: cc/layer_tree_host_common.cc
diff --git a/cc/layer_tree_host_common.cc b/cc/layer_tree_host_common.cc
index c28db4087d3f31f8d5c671f1042c957d8e43b441..f7c38a0a2563c78eaa53349f6632dad5b4e2bd34 100644
--- a/cc/layer_tree_host_common.cc
+++ b/cc/layer_tree_host_common.cc
@@ -731,7 +731,7 @@ static void calculateDrawPropertiesInternal(LayerType* layer, const gfx::Transfo
     // To avoid color fringing, LCD text should only be used on opaque layers with just integral translation.
     bool layerCanUseLCDText = subtreeCanUseLCDText &&
                               (accumulatedDrawOpacity == 1.0) &&
-                              layerDrawProperties.target_space_transform.IsIdentityOrIntegerTranslation();
+                              layerDrawProperties.target_space_transform.IsAlmostIdentityOrIntegerTranslation();

[alokp, 2013/03/08 19:40:06]

If you make a decision based on rounded transform, but use the actual transform
to do the drawing, I am not sure if the pixels will line up as expected. I also
do not know how to compute the epsilon that lets you do this.

I will be more comfortable if we were also modifying the target-space-transform,
but I am sure it will cause other problems.

[Xianzhu, 2013/03/08 23:36:56]

Do you have experience about how much pixel displacement would cause color
fringing? For example, would 1/1000 displacement cause it? 1/1000 is the
displacement of the bottom-right-most pixel when we use 1e-6 epsilon for a
1000x1000 screen.

About blurriness, based on my tests, >1/10 pixel displacement will cause visible
blurriness. Given a 1000x1000 screen, a scale error >1e-4 (1/10 * 1/1000) will
cause the bottom-right-most pixel to get a displacement >1/10 pixels.
Patch Set 1 modifies target_space_transform, which got also many concerns.

 
     gfx::RectF contentRect(gfx::PointF(), layer->contentBounds());