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..88a36fdc8d6718948e15b2ded06606f3fba87bb8 100755
--- a/src/gpu/effects/GrDistanceFieldTextureEffect.cpp
+++ b/src/gpu/effects/GrDistanceFieldTextureEffect.cpp
@@ -13,14 +13,17 @@
 #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)
-        : INHERITED (factory) {}
+    GrGLDistanceFieldTextureEffect(const GrBackendEffectFactory& factory, 
+                                   const GrDrawEffect& drawEffect)
+        : INHERITED (factory)
+        , fTextureSize(SkSize::Make(-1.f,-1.f)) {}
 
     virtual void emitCode(GrGLFullShaderBuilder* builder,
                           const GrDrawEffect& drawEffect,
@@ -31,43 +34,77 @@ public:
                           const TextureSamplerArray& samplers) SK_OVERRIDE {
         SkASSERT(1 == drawEffect.castEffect<GrDistanceFieldTextureEffect>().numVertexAttribs());
 
+        SkAssertResult(builder->enableFeature(GrGLShaderBuilder::kStandardDerivatives_GLSLFeature));
+
         SkString fsCoordName;
-        const char* vsVaryingName;
-        const char* fsVaryingNamePtr;
-        builder->addVarying(kVec2f_GrSLType, "textureCoords", &vsVaryingName, &fsVaryingNamePtr);
-        fsCoordName = fsVaryingNamePtr;
+        const char* vsCoordName;
+        const char* fsCoordNamePtr;
+        builder->addVarying(kVec2f_GrSLType, "textureCoords", &vsCoordName, &fsCoordNamePtr);
+        fsCoordName = fsCoordNamePtr;
 
-        const char* attrName =
+        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* textureSizeUniName = NULL;
+        fTextureSizeUni = builder->addUniform(GrGLShaderBuilder::kFragment_Visibility,
+                                              kVec2f_GrSLType, "TextureSize",
+                                              &textureSizeUniName);
 
         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");
+
+        // we adjust for the effect of the transformation on the distance by using
+        // the length of the gradient of the texture coordinates. We use st coordinates
+        // to ensure we're mapping 1:1 from texel space to pixel space.
+        builder->fsCodeAppendf("\tvec2 st = %s*%s;\n", fsCoordName.c_str(), textureSizeUniName);
+        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());
     }
 
     virtual void setData(const GrGLUniformManager& uman,
-                         const GrDrawEffect& drawEffect) SK_OVERRIDE {}
+                         const GrDrawEffect& drawEffect) SK_OVERRIDE {
+        SkASSERT(fTextureSizeUni.isValid());
+        const GrDistanceFieldTextureEffect& distanceFieldEffect = 
+                                              drawEffect.castEffect<GrDistanceFieldTextureEffect>();
+        if (distanceFieldEffect.getSize().width() != fTextureSize.width() ||
+            distanceFieldEffect.getSize().height() != fTextureSize.height()) {
+            fTextureSize = distanceFieldEffect.getSize();
+            uman.set2f(fTextureSizeUni, 
+                       distanceFieldEffect.getSize().width(),
+                       distanceFieldEffect.getSize().height());
+        }
+    }
 
 private:
+    GrGLUniformManager::UniformHandle fTextureSizeUni;
+    SkSize                            fTextureSize;
+
     typedef GrGLVertexEffect INHERITED;
 };
 
 ///////////////////////////////////////////////////////////////////////////////
 
 GrDistanceFieldTextureEffect::GrDistanceFieldTextureEffect(GrTexture* texture,
-                                                         const GrTextureParams& params)
-    : fTextureAccess(texture, params) {
+                                                           const GrTextureParams& params,
+                                                           const SkISize& size)
+    : fTextureAccess(texture, params)
+    , fSize(SkSize::Make(SkIntToScalar(size.width()), SkIntToScalar(size.height()))) {
     this->addTextureAccess(&fTextureAccess);
     this->addVertexAttrib(kVec2f_GrSLType);
 }
@@ -112,6 +149,7 @@ GrEffectRef* GrDistanceFieldTextureEffect::TestCreate(SkRandom* random,
     };
     GrTextureParams params(tileModes, random->nextBool() ? GrTextureParams::kBilerp_FilterMode :
                                                            GrTextureParams::kNone_FilterMode);
+    SkISize size = SkISize::Make(1024, 2048);
 
-    return GrDistanceFieldTextureEffect::Create(textures[texIdx], params);
+    return GrDistanceFieldTextureEffect::Create(textures[texIdx], params, size);
 }