ccpr: Use Manhattan distance for curve AA

Manhattan distance has the nice property that every pixel crossed
anywhere by the curve gets a fractional coverage value, whereas pixels
not touched by the curve will have zero coverage. This arguably looks
better on its own merits, but it also helps the curves blend more
seamlessly with their adjoining edges, which are already using Manhattan
distance.

Bug: skia:
Change-Id: I31cf28171d0b74512c74dca1088e50f0f442b924
Reviewed-on: https://skia-review.googlesource.com/115781
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>

2 files changed, 5 insertions, 3 deletions

diff --git a/src/gpu/ccpr/GrCCCubicShader.cpp b/src/gpu/ccpr/GrCCCubicShader.cpp
index edff5ff563..d0228d59ad 100644
--- a/src/gpu/ccpr/GrCCCubicShader.cpp
+++ b/src/gpu/ccpr/GrCCCubicShader.cpp
@@ -105,8 +105,9 @@ void GrCCCubicShader::onEmitFragmentCode(GrGLSLFPFragmentBuilder* f,
                                          const char* outputCoverage) const {
     f->codeAppendf("float k = %s.x, l = %s.y, m = %s.z;", fKLMD.fsIn(), fKLMD.fsIn(), fKLMD.fsIn());
     f->codeAppend ("float f = k*k*k - l*m;");
-    f->codeAppendf("float2 grad_f = %s * float2(k, 1);", fGradMatrix.fsIn());
-    f->codeAppendf("%s = clamp(0.5 - f * inversesqrt(dot(grad_f, grad_f)), 0, 1);", outputCoverage);
+    f->codeAppendf("float2 grad = %s * float2(k, 1);", fGradMatrix.fsIn());
+    f->codeAppend ("float fwidth = abs(grad.x) + abs(grad.y);");
+    f->codeAppendf("%s = clamp(0.5 - f/fwidth, 0, 1);", outputCoverage);
 
     f->codeAppendf("half d = min(%s.w, 0);", fKLMD.fsIn()); // Flat edge opposite the curve.
     // Wind is the sign of both L and/or M. Take the sign of whichever has the larger magnitude.
diff --git a/src/gpu/ccpr/GrCCQuadraticShader.cpp b/src/gpu/ccpr/GrCCQuadraticShader.cpp
index 287e63e8bf..236c407ade 100644
--- a/src/gpu/ccpr/GrCCQuadraticShader.cpp
+++ b/src/gpu/ccpr/GrCCQuadraticShader.cpp
@@ -73,7 +73,8 @@ void GrCCQuadraticShader::onEmitFragmentCode(GrGLSLFPFragmentBuilder* f,
     f->codeAppendf("float x = %s.x, y = %s.y;", fCoord.fsIn(), fCoord.fsIn());
     f->codeAppend ("float f = x*x - y;");
     f->codeAppendf("float2 grad = %s.zw;", fCoord.fsIn());
-    f->codeAppendf("%s = clamp(0.5 - f * inversesqrt(dot(grad, grad)), 0, 1);", outputCoverage);
+    f->codeAppend ("float fwidth = abs(grad.x) + abs(grad.y);");
+    f->codeAppendf("%s = clamp(0.5 - f/fwidth, 0, 1);", outputCoverage);
 
     f->codeAppendf("half d = min(%s.x, 0);", fCoverages.fsIn()); // Flat edge opposite the curve.
     f->codeAppendf("half wind = %s.y;", fCoverages.fsIn());