add virtuals to optimize composing colorfilters

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 80 matching lines @@
         */
         kHasFilter16_Flag    = 0x02
     };
 
     /** Returns the flags for this filter. Override in subclasses to return
         custom flags.
     */
     virtual uint32_t getFlags() const { return 0; }
 
     /**
+     *  If this subclass can optimally createa composition with the inner filter, return it as
+     *  a new filter (which the caller must unref() when it is done). If no such optimization
+     *  is known, return NULL.
+     *
+     *  e.g. result(color) == this_filter(inner(color))
+     */
+    virtual SkColorFilter* newComposed(const SkColorFilter* /*inner*/) const { return NULL; }
+
+    /**
      *  Apply this colorfilter to the specified SkColor. This routine handles
      *  converting to SkPMColor, calling the filter, and then converting back
      *  to SkColor. This method is not virtual, but will call filterSpan()
      *   which is virtual.
      */
     SkColor filterColor(SkColor) const;
 
     /** Create a colorfilter that uses the specified color and mode.
         If the Mode is DST, this function will return NULL (since that
         mode will have no effect on the result).
@@ skipping 29 matching lines @@
     SK_DEFINE_FLATTENABLE_TYPE(SkColorFilter)
 
 protected:
     SkColorFilter() {}
 
 private:
     typedef SkFlattenable INHERITED;
 };
 
 #endif