extend SkShader to report a luminance-color to be used for gamma correction

include/core/SkShader.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 SkShader_DEFINED
 #define SkShader_DEFINED
@@ skipping 368 matching lines @@
      *  paintColor should not be modified. It is not recommended to specialize the effect to
      *  the paint's color as then many GPU shaders may be generated.
      *
      *  The GrContext may be used by the effect to create textures. The GPU device does not
      *  call createContext. Instead we pass the SkPaint here in case the shader needs paint info.
      */
     virtual bool asNewEffect(GrContext* context, const SkPaint& paint,
                              const SkMatrix* localMatrixOrNull, GrColor* paintColor,
                              GrEffect** effect) const;
 
+    /**
+     *  If the shader can represent its "average" luminance in a single color, return true and
+     *  if color is not NULL, return that color. If it cannot, return false and ignore the color
+     *  parameter.
+     *
+     *  Note: if this returns true, the returned color will always be opaque, as only the RGB
+     *  components are used to compute luminance.
+     */
+    bool asLuminanceColor(SkColor*) const;
+
 #ifdef SK_BUILD_FOR_ANDROID_FRAMEWORK
     /**
      *  If the shader is a custom shader which has data the caller might want, call this function
      *  to get that data.
      */
     virtual bool asACustomShader(void** customData) const { return false; }
 #endif
 
     //////////////////////////////////////////////////////////////////////////
     //  Factory methods for stock shaders
@@ skipping 66 matching lines @@
     virtual void flatten(SkWriteBuffer&) const SK_OVERRIDE;
 
     bool computeTotalInverse(const ContextRec&, SkMatrix* totalInverse) const;
 
     /**
      *  Your subclass must also override contextSize() if it overrides onCreateContext().
      *  Base class impl returns NULL.
      */
     virtual Context* onCreateContext(const ContextRec&, void* storage) const;
 
+    virtual bool onAsLuminanceColor(SkColor*) const {
+        return false;
+    }
 private:
     // This is essentially const, but not officially so it can be modified in
     // constructors.
     SkMatrix fLocalMatrix;
 
     // So the SkLocalMatrixShader can whack fLocalMatrix in its SkReadBuffer constructor.
     friend class SkLocalMatrixShader;
 
     typedef SkFlattenable INHERITED;
 };
 
 #endif