Improved distance field sampling

src/gpu/effects/GrDistanceFieldTextureEffect.cpp

Index: src/gpu/effects/GrDistanceFieldTextureEffect.cpp
diff --git a/src/gpu/effects/GrDistanceFieldTextureEffect.cpp b/src/gpu/effects/GrDistanceFieldTextureEffect.cpp
index 8c38f9bebf919071af1c7a079deef5adbca983d2..8512cca78b0d89f45cdadb2332a5709123b43b32 100755
--- a/src/gpu/effects/GrDistanceFieldTextureEffect.cpp
+++ b/src/gpu/effects/GrDistanceFieldTextureEffect.cpp
@@ -13,13 +13,15 @@
 #include "GrTBackendEffectFactory.h"
 #include "GrTexture.h"
 
-// The distance field is constructed as unsigned char values, so that the zero value is at 128.
-// Hence our zero threshold is 128/255.
+// The distance field is constructed as unsigned char values, so that the zero value is at 128, 
+// and the range is [-4, 4 - 1/255). Hence our multiplier is 8 - 1/32 and zero threshold is 128/255.  
+#define MULTIPLIER "7.96875"
 #define THRESHOLD "0.50196078431"
 
 class GrGLDistanceFieldTextureEffect : public GrGLVertexEffect {
 public:
-    GrGLDistanceFieldTextureEffect(const GrBackendEffectFactory& factory, const GrDrawEffect& drawEffect)
+    GrGLDistanceFieldTextureEffect(const GrBackendEffectFactory& factory, 
+                                   const GrDrawEffect& drawEffect)
         : INHERITED (factory) {}
 
     virtual void emitCode(GrGLFullShaderBuilder* builder,
@@ -29,28 +31,45 @@ public:
                           const char* inputColor,
                           const TransformedCoordsArray&,
                           const TextureSamplerArray& samplers) SK_OVERRIDE {
-        SkASSERT(1 == drawEffect.castEffect<GrDistanceFieldTextureEffect>().numVertexAttribs());
+        SkASSERT(2 == drawEffect.castEffect<GrDistanceFieldTextureEffect>().numVertexAttribs());
 
         SkString fsCoordName;
-        const char* vsVaryingName;
-        const char* fsVaryingNamePtr;
-        builder->addVarying(kVec2f_GrSLType, "textureCoords", &vsVaryingName, &fsVaryingNamePtr);
-        fsCoordName = fsVaryingNamePtr;
-
-        const char* attrName =
+        const char* vsCoordName;
+        const char* fsCoordNamePtr;
+        builder->addVarying(kVec2f_GrSLType, "textureCoords", &vsCoordName, &fsCoordNamePtr);
+        fsCoordName = fsCoordNamePtr;
+
+        SkString fsSizeName;
+        const char* vsSizeName;
+        const char* fsSizeNamePtr;
+        builder->addVarying(kVec2f_GrSLType, "size", &vsSizeName, &fsSizeNamePtr);
+        fsSizeName = fsSizeNamePtr;
+
+        const char* attrName0 =
             builder->getEffectAttributeName(drawEffect.getVertexAttribIndices()[0])->c_str();
-        builder->vsCodeAppendf("\t%s = %s;\n", vsVaryingName, attrName);
+        builder->vsCodeAppendf("\t%s = %s;\n", vsCoordName, attrName0);
+        const char* attrName1 =
+            builder->getEffectAttributeName(drawEffect.getVertexAttribIndices()[1])->c_str();
+        builder->vsCodeAppendf("\t%s = %s;\n", vsSizeName, attrName1);
 
         builder->fsCodeAppend("\tvec4 texColor = ");
         builder->fsAppendTextureLookup(samplers[0],
                                        fsCoordName.c_str(),
                                        kVec2f_GrSLType);
         builder->fsCodeAppend(";\n");
-        builder->fsCodeAppend("\tfloat distance = texColor.r;\n");
+        builder->fsCodeAppend("\tfloat distance = "MULTIPLIER"*(texColor.r - "THRESHOLD");\n");
+
+        builder->fsCodeAppendf("\tvec2 st = %s*%s;\n", fsCoordName.c_str(), fsSizeName.c_str());
+        builder->fsCodeAppend("\tvec2 Jdx = dFdx(st);\n");
+        builder->fsCodeAppend("\tvec2 Jdy = dFdy(st);\n");
+        builder->fsCodeAppend("\tvec2 st_grad = normalize(st);\n");
+        builder->fsCodeAppend("\tvec2 grad = vec2(st_grad.x*Jdx.x + st_grad.y*Jdy.x,\n");
+        builder->fsCodeAppend("\t                 st_grad.x*Jdx.y + st_grad.y*Jdy.y);\n");
+
         // this gives us a smooth step across approximately one fragment
         // (assuming a radius of the diagonal of the fragment, hence a factor of sqrt(2)/2)
-        builder->fsCodeAppend("\tfloat afwidth = 0.7071*length(vec2(dFdx(distance), dFdy(distance)));\n");
-        builder->fsCodeAppend("\tfloat val = smoothstep("THRESHOLD"-afwidth, "THRESHOLD"+afwidth, distance);\n");
+        builder->fsCodeAppend("\tfloat afwidth = 0.7071*length(grad);\n");
+        builder->fsCodeAppend("\tfloat val = smoothstep(-afwidth, afwidth, distance);\n");
 
         builder->fsCodeAppendf("\t%s = %s;\n", outputColor,
                                    (GrGLSLExpr4(inputColor) * GrGLSLExpr1("val")).c_str());
@@ -70,6 +89,7 @@ GrDistanceFieldTextureEffect::GrDistanceFieldTextureEffect(GrTexture* texture,
     : fTextureAccess(texture, params) {
     this->addTextureAccess(&fTextureAccess);
     this->addVertexAttrib(kVec2f_GrSLType);
+    this->addVertexAttrib(kVec2f_GrSLType);
 }
 
 bool GrDistanceFieldTextureEffect::onIsEqual(const GrEffect& other) const {