sRGB support for distance field text.

src/gpu/effects/GrDistanceFieldGeoProc.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrDistanceFieldGeoProc.h"
 #include "GrInvariantOutput.h"
 #include "GrTexture.h"
@@ skipping 64 matching lines @@
                              dfTexEffect.inPosition()->fName,
                              args.fTransformsIn,
                              args.fTransformsOut);
 
         // add varyings
         GrGLSLVertToFrag recipScale(kFloat_GrSLType);
         GrGLSLVertToFrag uv(kVec2f_GrSLType);
         bool isUniformScale = (dfTexEffect.getFlags() & kUniformScale_DistanceFieldEffectMask) ==
                               kUniformScale_DistanceFieldEffectMask;
         bool isSimilarity = SkToBool(dfTexEffect.getFlags() & kSimilarity_DistanceFieldEffectFlag);
+        bool srgbOutput = SkToBool(dfTexEffect.getFlags() & kSRGB_DistanceFieldEffectFlag);
         varyingHandler->addVarying("TextureCoords", &uv, kHigh_GrSLPrecision);
         vertBuilder->codeAppendf("%s = %s;", uv.vsOut(), dfTexEffect.inTextureCoords()->fName);
 
         // compute numbers to be hardcoded to convert texture coordinates from float to int
         SkASSERT(dfTexEffect.numTextures() == 1);
         GrTexture* atlas = dfTexEffect.textureAccess(0).getTexture();
         SkASSERT(atlas && SkIsPow2(atlas->width()) && SkIsPow2(atlas->height()));
 
         GrGLSLVertToFrag st(kVec2f_GrSLType);
         varyingHandler->addVarying("IntTextureCoords", &st, kHigh_GrSLPrecision);
@@ skipping 51 matching lines @@
             fragBuilder->codeAppend("}");
 
             fragBuilder->codeAppendf("vec2 Jdx = dFdx(%s);", st.fsIn());
             fragBuilder->codeAppendf("vec2 Jdy = dFdy(%s);", st.fsIn());
             fragBuilder->codeAppend("vec2 grad = vec2(dist_grad.x*Jdx.x + dist_grad.y*Jdy.x,");
             fragBuilder->codeAppend("                 dist_grad.x*Jdx.y + dist_grad.y*Jdy.y);");
 
             // this gives us a smooth step across approximately one fragment
             fragBuilder->codeAppend("afwidth = " SK_DistanceFieldAAFactor "*length(grad);");
         }
-        fragBuilder->codeAppend("float val = smoothstep(-afwidth, afwidth, distance);");
+
+        // The smoothstep falloff compensates for the non-linear sRGB response curve. If we are
+        // doing gamma-correct rendering (to an sRGB or F16 buffer), then we actually want distance
+        // mapped linearly to coverage, so use a linear step:
+        if (srgbOutput) {
+            fragBuilder->codeAppend(
+                "float val = clamp(distance + afwidth / (2.0 * afwidth), 0.0, 1.0);");
+        } else {
+            fragBuilder->codeAppend("float val = smoothstep(-afwidth, afwidth, distance);");
+        }
 
         fragBuilder->codeAppendf("%s = vec4(val);", args.fOutputCoverage);
     }
 
     void setData(const GrGLSLProgramDataManager& pdman, const GrPrimitiveProcessor& proc) override {
 #ifdef SK_GAMMA_APPLY_TO_A8
         const GrDistanceFieldA8TextGeoProc& dfTexEffect = proc.cast<GrDistanceFieldA8TextGeoProc>();
         float distanceAdjust = dfTexEffect.getDistanceAdjust();
         if (distanceAdjust != fDistanceAdjust) {
             pdman.set1f(fDistanceAdjustUni, distanceAdjust);
@@ skipping 217 matching lines @@
             fragBuilder->codeAppend("}");
 
             fragBuilder->codeAppend("vec2 Jdx = dFdx(st);");
             fragBuilder->codeAppend("vec2 Jdy = dFdy(st);");
             fragBuilder->codeAppend("vec2 grad = vec2(dist_grad.x*Jdx.x + dist_grad.y*Jdy.x,");
             fragBuilder->codeAppend("                 dist_grad.x*Jdx.y + dist_grad.y*Jdy.y);");
 
             // this gives us a smooth step across approximately one fragment
             fragBuilder->codeAppend("afwidth = " SK_DistanceFieldAAFactor "*length(grad);");
         }
         fragBuilder->codeAppend("float val = smoothstep(-afwidth, afwidth, distance);");

[jvanverth1, 2016/04/12 19:34:46]

The SDF path renderer doesn't do any fake gamma, so it's probably worth seeing
how it looks when you use a linear ramp instead of smoothstep.

[Brian Osman, 2016/04/12 19:45:59]

Yeah, I was planning to make that change after testing - seemed like the logic
would still apply.

 
         fragBuilder->codeAppendf("%s = vec4(val);", args.fOutputCoverage);
     }
 
     void setData(const GrGLSLProgramDataManager& pdman, const GrPrimitiveProcessor& proc) override {
         SkASSERT(fTextureSizeUni.isValid());
 
         GrTexture* texture = proc.texture(0);
         if (texture->width() != fTextureSize.width() ||
             texture->height() != fTextureSize.height()) {
@@ skipping 141 matching lines @@
                              uniformHandler,
                              gpArgs->fPositionVar,
                              dfTexEffect.inPosition()->fName,
                              args.fTransformsIn,
                              args.fTransformsOut);
 
         // set up varyings
         bool isUniformScale = (dfTexEffect.getFlags() & kUniformScale_DistanceFieldEffectMask) ==
                               kUniformScale_DistanceFieldEffectMask;
         bool isSimilarity = SkToBool(dfTexEffect.getFlags() & kSimilarity_DistanceFieldEffectFlag);
+        bool srgbOutput = SkToBool(dfTexEffect.getFlags() & kSRGB_DistanceFieldEffectFlag);
         GrGLSLVertToFrag recipScale(kFloat_GrSLType);
         GrGLSLVertToFrag uv(kVec2f_GrSLType);
         varyingHandler->addVarying("TextureCoords", &uv, kHigh_GrSLPrecision);
         vertBuilder->codeAppendf("%s = %s;", uv.vsOut(), dfTexEffect.inTextureCoords()->fName);
 
         // compute numbers to be hardcoded to convert texture coordinates from float to int
         SkASSERT(dfTexEffect.numTextures() == 1);
         GrTexture* atlas = dfTexEffect.textureAccess(0).getTexture();
         SkASSERT(atlas && SkIsPow2(atlas->width()) && SkIsPow2(atlas->height()));
 
@@ skipping 94 matching lines @@
             fragBuilder->codeAppend("} else {");
             fragBuilder->codeAppend("dist_grad = dist_grad*inversesqrt(dg_len2);");
             fragBuilder->codeAppend("}");
             fragBuilder->codeAppend("vec2 grad = vec2(dist_grad.x*Jdx.x + dist_grad.y*Jdy.x,");
             fragBuilder->codeAppend("                 dist_grad.x*Jdx.y + dist_grad.y*Jdy.y);");
 
             // this gives us a smooth step across approximately one fragment
             fragBuilder->codeAppend("afwidth = " SK_DistanceFieldAAFactor "*length(grad);");
         }
 
-        fragBuilder->codeAppend(
-                      "vec4 val = vec4(smoothstep(vec3(-afwidth), vec3(afwidth), distance), 1.0);");
+        // The smoothstep falloff compensates for the non-linear sRGB response curve. If we are
+        // doing gamma-correct rendering (to an sRGB or F16 buffer), then we actually want distance
+        // mapped linearly to coverage, so use a linear step:
+        if (srgbOutput) {
+            fragBuilder->codeAppend("vec4 val = "
+                "vec4(clamp(distance + vec3(afwidth) / vec3(2.0 * afwidth), 0.0, 1.0), 1.0f);");
+        } else {
+            fragBuilder->codeAppend(
+                "vec4 val = vec4(smoothstep(vec3(-afwidth), vec3(afwidth), distance), 1.0);");
+        }
+
         // set alpha to be max of rgb coverage
         fragBuilder->codeAppend("val.a = max(max(val.r, val.g), val.b);");
 
         fragBuilder->codeAppendf("%s = val;", args.fOutputCoverage);
     }
 
     void setData(const GrGLSLProgramDataManager& pdman,
                  const GrPrimitiveProcessor& processor) override {
         SkASSERT(fDistanceAdjustUni.isValid());
 
@@ skipping 101 matching lines @@
         flags |= d->fRandom->nextBool() ? kScaleOnly_DistanceFieldEffectFlag : 0;
     }
     flags |= d->fRandom->nextBool() ? kBGR_DistanceFieldEffectFlag : 0;
     return GrDistanceFieldLCDTextGeoProc::Create(GrRandomColor(d->fRandom),
                                                  GrTest::TestMatrix(d->fRandom),
                                                  d->fTextures[texIdx], params,
                                                  wa,
                                                  flags,
                                                  d->fRandom->nextBool());
 }