In order to use CSS luminance masking, we need to be able to create an instance of SkLumaXfermode w…

include/effects/SkLumaXfermode.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkLumaXfermode_DEFINED
 #define SkLumaXfermode_DEFINED
 
 #include "SkXfermode.h"
 
 /**
  *  Luminance mask transfer, as defined in
  *  http://www.w3.org/TR/SVG/masking.html#Masking
  *  http://www.w3.org/TR/css-masking/#MaskValues
  *
  *  The luminance-to-alpha function is applied before performing a standard
- *  SrcIn/DstIn xfer:
+ *  SrcIn/DstIn/SrcOver xfer:
  *
  *    luma(C) = (0.2125 * C.r + 0.7154 * C.g + 0.0721 * C.b) * C.a
  *
  *  (where C is un-premultiplied)
  */
 class SK_API SkLumaMaskXfermode : public SkXfermode {
 public:
     /** Return an SkLumaMaskXfermode object for the specified submode.
      *
-     *  Only kSrcIn_Mode and kDstIn_Mode are supported - for everything else,
+     *  Only kSrcIn_Mode, kDstIn_Mode kSrcOver_Mode are supported - for everything else,
      *  the factory returns NULL.
      */
     static SkXfermode* Create(SkXfermode::Mode);
 
     virtual SkPMColor xferColor(SkPMColor, SkPMColor) const SK_OVERRIDE;
     virtual void xfer32(SkPMColor dst[], const SkPMColor src[], int count,
                         const SkAlpha aa[]) const SK_OVERRIDE;
 
     SK_DEVELOPER_TO_STRING()
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkLumaMaskXfermode)
 
 #if SK_SUPPORT_GPU
     virtual bool asNewEffectOrCoeff(GrContext*, GrEffectRef**, Coeff*, Coeff*,
                                     GrTexture*) const SK_OVERRIDE;
 #endif
 
 protected:
     SkLumaMaskXfermode(SkFlattenableReadBuffer&);
     virtual void flatten(SkFlattenableWriteBuffer&) const SK_OVERRIDE;
 
-private:
     SkLumaMaskXfermode(SkXfermode::Mode);
 
+private:
     const SkXfermode::Mode fMode;
 
     typedef SkXfermode INHERITED;
+
+    virtual SkPMColor lumaProc(const SkPMColor a, const SkPMColor b) const;
 };
 
 #endif