add compose-colorfilter

include/core/SkColorFilter.h

 
 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #ifndef SkColorFilter_DEFINED
@@ skipping 105 matching lines @@
     */
     static SkColorFilter* CreateModeFilter(SkColor c, SkXfermode::Mode mode);
 
     /** Create a colorfilter that multiplies the RGB channels by one color, and
         then adds a second color, pinning the result for each component to
         [0..255]. The alpha components of the mul and add arguments
         are ignored.
     */
     static SkColorFilter* CreateLightingFilter(SkColor mul, SkColor add);
 
+    /** Construct a colorfilter whose effect is to first apply the inner filter and then apply
+     *  the outer filter to the result of the inner's.
+     *  The reference counts for outer and inner are incremented.
+     */
+    static SkColorFilter* CreateComposeFilter(SkColorFilter* outer, SkColorFilter* inner);

[f(malita), 2015/03/02 00:47:07]

I was expecting the arg order to correspond to the order in which the filters
are applied, and found the current order slightly surprising. I presume it's due
to C' = outer(inner(C)), so from that angle it does make sense.

+
     /** A subclass may implement this factory function to work with the GPU backend. If the return
         is non-NULL then the caller owns a ref on the returned object.
      */
     virtual GrFragmentProcessor* asFragmentProcessor(GrContext*) const;
 
     SK_TO_STRING_PUREVIRT()
 
     SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP()
     SK_DEFINE_FLATTENABLE_TYPE(SkColorFilter)
 
 protected:
     SkColorFilter() {}
 
 private:
     typedef SkFlattenable INHERITED;
 };
 
 #endif