Implement SkXfermode image filter

include/core/SkXfermode.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 SkXfermode_DEFINED
 #define SkXfermode_DEFINED
 
 #include "SkFlattenable.h"
 #include "SkColor.h"
 
 class GrContext;
 class GrEffectRef;
+class GrTexture;
 class SkString;
 
 /** \class SkXfermode
 
     SkXfermode is the base class for objects that are called to implement custom
     "transfer-modes" in the drawing pipeline. The static function Create(Modes)
     can be called to return an instance of any of the predefined subclasses as
     specified in the Modes enum. When an SkXfermode is assigned to an SkPaint,
     then objects drawn with that paint have the xfermode applied.
 */
@@ skipping 162 matching lines @@
         return AsMode(xfer, mode);
     }
 
     /** A subclass may implement this factory function to work with the GPU backend. It is legal
         to call this with all but the context param NULL to simply test the return value. effect,
         src, and dst must all be NULL or all non-NULL. If effect is non-NULL then the xfermode may
         optionally allocate an effect to return and the caller as *effect. The caller will install
         it and own a ref to it. Since the xfermode may or may not assign *effect, the caller should
         set *effect to NULL beforehand. If the function returns true and *effect is NULL then the
         src and dst coeffs will be applied to the draw. When *effect is non-NULL the coeffs are
-        ignored.
+        ignored. background specifies the texture to use as the background for compositing, and
+        should be accessed in the effect's fragment shader. If NULL, the effect should request
+        access to destination color (setWillReadDstColor()), and use that in the fragment shader
+        (builder->dstColor()).
      */
     virtual bool asNewEffectOrCoeff(GrContext*,
                                     GrEffectRef** effect,
                                     Coeff* src,
-                                    Coeff* dst) const;
+                                    Coeff* dst,
+                                    GrTexture* background = NULL) const;
 
     /**
      *  The same as calling xfermode->asNewEffect(...), except that this also checks if the xfermode
      *  is NULL, and if so, treats it as kSrcOver_Mode.
      */
     static bool AsNewEffectOrCoeff(SkXfermode*,
                                    GrContext*,
                                    GrEffectRef** effect,
                                    Coeff* src,
-                                   Coeff* dst);
+                                   Coeff* dst,
+                                   GrTexture* background = NULL);
 
     SkDEVCODE(virtual void toString(SkString* str) const = 0;)
     SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP()
 protected:
     SkXfermode(SkFlattenableReadBuffer& rb) : SkFlattenable(rb) {}
 
     /** The default implementation of xfer32/xfer16/xferA8 in turn call this
         method, 1 color at a time (upscaled to a SkPMColor). The default
         implmentation of this method just returns dst. If performance is
         important, your subclass should override xfer32/xfer16/xferA8 directly.
@@ skipping 43 matching lines @@
         fProc = proc;
     }
 
 private:
     SkXfermodeProc  fProc;
 
     typedef SkXfermode INHERITED;
 };
 
 #endif