Add alternative ambient shadow method to Android shadow sample

src/effects/SkGaussianEdgeShader.cpp

Index: src/effects/SkGaussianEdgeShader.cpp
diff --git a/src/effects/SkGaussianEdgeShader.cpp b/src/effects/SkGaussianEdgeShader.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..e519598f1bba8ca5088f8001ec4aa90d72820482
--- /dev/null
+++ b/src/effects/SkGaussianEdgeShader.cpp
@@ -0,0 +1,152 @@
+/*
+ * 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
+ */
+class SkGaussianEdgeShaderImpl : public SkShader {
+public:
+    SkGaussianEdgeShaderImpl() {}
+
+    bool isOpaque() const override;
+
+#if SK_SUPPORT_GPU
+    sk_sp<GrFragmentProcessor> asFragmentProcessor(const AsFPArgs&) const override;
+#endif
+
+    SK_TO_STRING_OVERRIDE()
+    SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkGaussianEdgeShaderImpl)
+
+protected:
+    void flatten(SkWriteBuffer&) const override;
+
+private:
+    friend class SkGaussianEdgeShader;
+
+    typedef SkShader INHERITED;
+};
+
+////////////////////////////////////////////////////////////////////////////
+
+#if SK_SUPPORT_GPU
+
+#include "GrCoordTransform.h"
+#include "GrFragmentProcessor.h"
+#include "GrInvariantOutput.h"
+#include "glsl/GrGLSLFragmentProcessor.h"
+#include "glsl/GrGLSLFragmentShaderBuilder.h"
+#include "glsl/GrGLSLProgramDataManager.h"
+#include "glsl/GrGLSLUniformHandler.h"
+#include "SkGr.h"
+#include "SkGrPriv.h"
+
+class GaussianEdgeFP : public GrFragmentProcessor {
+public:
+    GaussianEdgeFP() {
+        this->initClassID<GaussianEdgeFP>();
+
+        // enable output of distance information for shape
+        fUsesDistanceVectorField = true;
+    }
+
+    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->distanceVectorName());
+            fragBuilder->codeAppend("factor = exp(-factor * factor * 4.0) - 0.018;");
+            fragBuilder->codeAppend("output *= factor;");
+            fragBuilder->codeAppend("}");
+            fragBuilder->codeAppendf("%s = output;", 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 {}
+    };
+
+    void onGetGLSLProcessorKey(const GrGLSLCaps& caps, GrProcessorKeyBuilder* b) const override {
+        GLSLGaussianEdgeFP::GenKey(*this, caps, b);
+    }
+
+    const char* name() const override { return "GaussianEdgeFP"; }
+
+    void onComputeInvariantOutput(GrInvariantOutput* inout) const override {
+        inout->mulByUnknownFourComponents();
+    }
+
+private:
+    GrGLSLFragmentProcessor* onCreateGLSLInstance() const override { return new GLSLGaussianEdgeFP; }
+
+    bool onIsEqual(const GrFragmentProcessor& proc) const override { return true; }
+};
+
+////////////////////////////////////////////////////////////////////////////
+
+sk_sp<GrFragmentProcessor> SkGaussianEdgeShaderImpl::asFragmentProcessor(const AsFPArgs& args) const {
+    return sk_make_sp<GaussianEdgeFP>();
+}
+
+#endif
+
+////////////////////////////////////////////////////////////////////////////
+
+bool SkGaussianEdgeShaderImpl::isOpaque() const {
+    return false;
+}
+
+////////////////////////////////////////////////////////////////////////////
+
+#ifndef SK_IGNORE_TO_STRING
+void SkGaussianEdgeShaderImpl::toString(SkString* str) const {
+    str->appendf("GaussianEdgeShader: ()");
+}
+#endif
+
+sk_sp<SkFlattenable> SkGaussianEdgeShaderImpl::CreateProc(SkReadBuffer& buf) {
+    return sk_make_sp<SkGaussianEdgeShaderImpl>();
+}
+
+void SkGaussianEdgeShaderImpl::flatten(SkWriteBuffer& buf) const {
+    this->INHERITED::flatten(buf);
+}
+
+///////////////////////////////////////////////////////////////////////////////
+
+sk_sp<SkShader> SkGaussianEdgeShader::Make() {
+    return sk_make_sp<SkGaussianEdgeShaderImpl>();
+}
+
+///////////////////////////////////////////////////////////////////////////////
+
+SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_START(SkGaussianEdgeShader)
+SK_DEFINE_FLATTENABLE_REGISTRAR_ENTRY(SkGaussianEdgeShaderImpl)
+SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_END
+
+///////////////////////////////////////////////////////////////////////////////