Fix precision issues in SkRRectsGaussianEdgeShader.cpp

src/effects/SkRRectsGaussianEdgeShader.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkRRectsGaussianEdgeShader.h"
 #include "SkReadBuffer.h"
 #include "SkWriteBuffer.h"
@@ skipping 99 matching lines @@
                           const char* radName,
                           const char* outputName,
                           const char  indices[2]) { // how to access the params for the 2 rrects
 
             // positive distance is towards the center of the circle
             fragBuilder->codeAppendf("vec2 delta = %s.xy - %s.%s;",
                                      fragBuilder->fragmentPosition(), posName, indices);
 
             switch (mode) {
             case kCircle_Mode:
-                fragBuilder->codeAppendf("%s = clamp((%s.%c - length(delta))/%s, 0.0, 1.0);",
+                // When a shadow circle gets large we can have some precision issues if
+                // we do "length(delta)/radius". The scaleDist temporary cuts the
+                // delta vector down a bit before invoking length.
+                fragBuilder->codeAppendf("float scaledDist = length(delta/%s);", radName);
+                fragBuilder->codeAppendf("%s = clamp((%s.%c/%s - scaledDist), 0.0, 1.0);",
                                          outputName, sizesName, indices[0], radName);
                 break;
             case kRect_Mode:
                 fragBuilder->codeAppendf(
                     "vec2 rectDist = vec2(1.0 - clamp((%s.%c - abs(delta.x))/%s, 0.0, 1.0),"
                                          "1.0 - clamp((%s.%c - abs(delta.y))/%s, 0.0, 1.0));",
                     sizesName, indices[0], radName,
                     sizesName, indices[1], radName);
                 fragBuilder->codeAppendf("%s = clamp(1.0 - length(rectDist), 0.0, 1.0);",
                                          outputName);
                 break;
             case kSimpleCircular_Mode:
                 // For the circular round rect we first compute the distance
                 // to the rect. Then we compute a multiplier that is 1 if the
                 // point is in one of the circular corners. We then compute the
                 // distance from the corner and then use the multiplier to mask
                 // between the two distances.
                 fragBuilder->codeAppendf("float xDist = clamp((%s.%c - abs(delta.x))/%s,"
-                                                             " 0.0, 1.0);",
+                                                             "0.0, 1.0);",
                                          sizesName, indices[0], radName);
                 fragBuilder->codeAppendf("float yDist = clamp((%s.%c - abs(delta.y))/%s,"
                                                              "0.0, 1.0);",
                                          sizesName, indices[1], radName);
                 fragBuilder->codeAppend("float rectDist = min(xDist, yDist);");
 
                 fragBuilder->codeAppendf("vec2 cornerCenter = %s.%s - %s.%s;",
                                          sizesName, indices, radiiName, indices);
                 fragBuilder->codeAppend("delta = vec2(abs(delta.x) - cornerCenter.x,"
                                                      "abs(delta.y) - cornerCenter.y);");
@@ skipping 270 matching lines @@
     return sk_make_sp<SkRRectsGaussianEdgeShaderImpl>(first, second, radius);
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_START(SkRRectsGaussianEdgeShader)
 SK_DEFINE_FLATTENABLE_REGISTRAR_ENTRY(SkRRectsGaussianEdgeShaderImpl)
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_END
 
 ///////////////////////////////////////////////////////////////////////////////