Tweak distance field path renderer behavior in gamma-correct mode

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 344 matching lines @@
         fragBuilder->codeAppend(".r;");
         fragBuilder->codeAppend("float distance = "
             SK_DistanceFieldMultiplier "*(texColor - " SK_DistanceFieldThreshold ");");
 
         fragBuilder->appendPrecisionModifier(kHigh_GrSLPrecision);
         fragBuilder->codeAppendf("vec2 st = uv*%s;", textureSizeUniName);
         fragBuilder->codeAppend("float afwidth;");
         bool isUniformScale = (dfTexEffect.getFlags() & kUniformScale_DistanceFieldEffectMask) ==
                                kUniformScale_DistanceFieldEffectMask;
         bool isSimilarity = SkToBool(dfTexEffect.getFlags() & kSimilarity_DistanceFieldEffectFlag);
+        bool isGammaCorrect =
+            SkToBool(dfTexEffect.getFlags() & kGammaCorrect_DistanceFieldEffectFlag);
         if (isUniformScale) {
             // For uniform scale, we adjust for the effect of the transformation on the distance
             // by using the length of the gradient of the t coordinate in the y direction.
             // We use st coordinates to ensure we're mapping 1:1 from texel space to pixel space.
             // We use the y gradient because there is a bug in the Mali 400 in the x direction.
 
             // this gives us a smooth step across approximately one fragment
             fragBuilder->codeAppend("afwidth = abs(" SK_DistanceFieldAAFactor "*dFdy(st.y));");
 
         } else if (isSimilarity) {
@@ skipping 20 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);");
+        // 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 (isGammaCorrect) {
+            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 {
         SkASSERT(fTextureSizeUni.isValid());
 
         GrTexture* texture = proc.texture(0);
         if (texture->width() != fTextureSize.width() ||
             texture->height() != fTextureSize.height()) {
@@ skipping 389 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());
 }