GrFP can express distance vector field req., program builder declares variable for it

src/gpu/GrPrimitiveProcessor.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 GrPrimitiveProcessor_DEFINED
 #define GrPrimitiveProcessor_DEFINED
 
@@ skipping 203 matching lines @@
 
     /**
      * If non-null, overrides the dest color returned by GrGLSLFragmentShaderBuilder::dstColor().
      */
     virtual const char* getDestColorOverride() const { return nullptr; }
 
     virtual float getSampleShading() const {
         return 0.0;
     }
 
+    /* Sub-class should override and return true if this primitive processor implements the distance
+     * vector field, a field of vectors to the nearest point in the edge of the shape.  */
+    virtual bool implementsDistanceVector() const { return false; }
+
 protected:
     GrPrimitiveProcessor() : fVertexStride(0) {}
 
     enum { kPreallocAttribCnt = 8 };
     SkSTArray<kPreallocAttribCnt, Attribute> fAttribs;
     size_t fVertexStride;
 
 private:
     void notifyRefCntIsZero() const final {};
     virtual bool hasExplicitLocalCoords() const = 0;
 
     typedef GrProcessor INHERITED;
 };
 
 #endif