Remove the 0.5 bias from the ellipse edge function. This makes coverage drop to zero before we get …

src/gpu/GrOvalRenderer.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 "GrOvalRenderer.h"
 
 #include "GrBatchFlushState.h"
@@ skipping 285 matching lines @@
             // for outer curve
             fragBuilder->codeAppendf("vec2 scaledOffset = %s*%s.xy;", ellipseOffsets.fsIn(),
                                      ellipseRadii.fsIn());
             fragBuilder->codeAppend("float test = dot(scaledOffset, scaledOffset) - 1.0;");
             fragBuilder->codeAppendf("vec2 grad = 2.0*scaledOffset*%s.xy;", ellipseRadii.fsIn());
             fragBuilder->codeAppend("float grad_dot = dot(grad, grad);");
 
             // avoid calling inversesqrt on zero.
             fragBuilder->codeAppend("grad_dot = max(grad_dot, 1.0e-4);");
             fragBuilder->codeAppend("float invlen = inversesqrt(grad_dot);");
-            fragBuilder->codeAppend("float edgeAlpha = clamp(0.5-test*invlen, 0.0, 1.0);");
+            fragBuilder->codeAppend("float edgeAlpha = clamp(-test*invlen, 0.0, 1.0);");
 
             // for inner curve
             if (ee.isStroked()) {
                 fragBuilder->codeAppendf("scaledOffset = %s*%s.zw;",
                                          ellipseOffsets.fsIn(), ellipseRadii.fsIn());
                 fragBuilder->codeAppend("test = dot(scaledOffset, scaledOffset) - 1.0;");
                 fragBuilder->codeAppendf("grad = 2.0*scaledOffset*%s.zw;",
                                          ellipseRadii.fsIn());
                 fragBuilder->codeAppend("invlen = inversesqrt(dot(grad, grad));");
                 fragBuilder->codeAppend("edgeAlpha *= clamp(0.5+test*invlen, 0.0, 1.0);");
@@ skipping 1761 matching lines @@
 }
 
 DRAW_BATCH_TEST_DEFINE(RRectBatch) {
     SkMatrix viewMatrix = GrTest::TestMatrixRectStaysRect(random);
     GrColor color = GrRandomColor(random);
     const SkRRect& rrect = GrTest::TestRRectSimple(random);
     return create_rrect_batch(color, viewMatrix, rrect, GrTest::TestStrokeRec(random));
 }
 
 #endif