alpha threshold bitmap shader

include/core/SkShader.h

Index: include/core/SkShader.h
diff --git a/include/core/SkShader.h b/include/core/SkShader.h
index e1cab967958e36ccfa64b5589cc33f59d37cfced..2632830ef0019aeffd0f433387684f5c0bee9e04 100644
--- a/include/core/SkShader.h
+++ b/include/core/SkShader.h
@@ -318,9 +318,11 @@ public:
     virtual GradientType asAGradient(GradientInfo* info) const;
 
     /**
-     *  If the shader subclass has a GrEffect implementation, this installs an effect on the stage.
-     *  The GrContext may be used by the effect to create textures. The GPU device does not call
-     *  setContext. Instead we pass the paint here in case the shader needs paint info.
+     *  If the shader subclass has a GrEffect implementation, this resturns the effect to install.
+     *  The incoming color to the effect has r=g=b=a all extracted from the SkPaint's alpha.
+     *  The output color should be the computed SkShader premul color modulated by the incoming
+     *  color. The GrContext may be used by the effect to create textures. The GPU device does not
+     *  call setContext. Instead we pass the SkPaint here in case the shader needs paint info.
      */
     virtual GrEffectRef* asNewEffect(GrContext* context, const SkPaint& paint) const;