Add alternative spot shadow to Android shadow sample

src/effects/SkGaussianEdgeShader.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkGaussianEdgeShader.h"
 #include "SkReadBuffer.h"
 #include "SkWriteBuffer.h"
 
  /** \class SkGaussianEdgeShaderImpl
  This subclass of shader applies a Gaussian to shadow edge
+
+ The radius of the Gaussian blur is specified by the g and b values of the color,
+ where g is the integer component and b is the fractional component. The r value
+ represents the max final alpha.
  */
 class SkGaussianEdgeShaderImpl : public SkShader {
 public:
     SkGaussianEdgeShaderImpl() {}
 
     bool isOpaque() const override;
 
 #if SK_SUPPORT_GPU
     sk_sp<GrFragmentProcessor> asFragmentProcessor(const AsFPArgs&) const override;
 #endif
@@ skipping 34 matching lines @@
     }
 
     class GLSLGaussianEdgeFP : public GrGLSLFragmentProcessor {
     public:
         GLSLGaussianEdgeFP() {}
 
         void emitCode(EmitArgs& args) override {
 
             GrGLSLFPFragmentBuilder* fragBuilder = args.fFragBuilder;
 
-            fragBuilder->codeAppendf("vec4 output = %s;", args.fInputColor);
-            // outside the outer edge
-            fragBuilder->codeAppendf("if (%s.z <= 0) {", fragBuilder->distanceVectorName());
-            fragBuilder->codeAppend("output *= 0.0;");
-            // inside the stroke
-            fragBuilder->codeAppendf("} else if (%s.w > 0) {", fragBuilder->distanceVectorName());
-            fragBuilder->codeAppendf("float factor = %s.w/(%s.z + %s.w);",
-                                     fragBuilder->distanceVectorName(),
-                                     fragBuilder->distanceVectorName(),
+            fragBuilder->codeAppendf("vec4 color = %s;", args.fInputColor);
+            fragBuilder->codeAppend("float radius = color.g*255.0 + color.b;");
+
+            fragBuilder->codeAppendf("float factor = 1.0 - clamp(%s.z/radius, 0.0, 1.0);",
                                      fragBuilder->distanceVectorName());
             fragBuilder->codeAppend("factor = exp(-factor * factor * 4.0) - 0.018;");
-            fragBuilder->codeAppend("output *= factor;");
-            fragBuilder->codeAppend("}");
-            fragBuilder->codeAppendf("%s = output;", args.fOutputColor);
+            fragBuilder->codeAppendf("%s = factor*vec4(0.0, 0.0, 0.0, color.r);", args.fOutputColor);
         }
 
         static void GenKey(const GrProcessor& proc, const GrGLSLCaps&,
                            GrProcessorKeyBuilder* b) {
             // only one shader generated currently
             b->add32(0x0);
         }
 
     protected:
         void onSetData(const GrGLSLProgramDataManager& pdman, const GrProcessor& proc) override {}
@@ skipping 51 matching lines @@
     return sk_make_sp<SkGaussianEdgeShaderImpl>();
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_START(SkGaussianEdgeShader)
 SK_DEFINE_FLATTENABLE_REGISTRAR_ENTRY(SkGaussianEdgeShaderImpl)
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_END
 
 ///////////////////////////////////////////////////////////////////////////////