fine-tune tolerance for pinchy quads in stroker

src/core/SkStroke.cpp

 /*
  * Copyright 2008 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkStrokerPriv.h"
 #include "SkGeometry.h"
 #include "SkPath.h"
@@ skipping 12 matching lines @@
     static const SkScalar kFlatEnoughNormalDotProd =
                                             SK_ScalarSqrt2/2 + SK_Scalar1/10;
 
     SkASSERT(kFlatEnoughNormalDotProd > 0 &&
              kFlatEnoughNormalDotProd < SK_Scalar1);
 
     return SkPoint::DotProduct(norm0, norm1) <= kFlatEnoughNormalDotProd;
 }
 
 static inline bool normals_too_pinchy(const SkVector& norm0, SkVector& norm1) {
-    static const SkScalar kTooPinchyNormalDotProd = -SK_Scalar1 * 999 / 1000;
+    // if the dot-product is -1, then we are definitely too pinchy. We tweak
+    // that by an epsilon to ensure we have significant bits in our test
+    static const int kMinSigBitsForDot = 8;
+    static const SkScalar kDotEpsilon = FLT_EPSILON * (1 << kMinSigBitsForDot);
+    static const SkScalar kTooPinchyNormalDotProd = kDotEpsilon - 1;
+    
+    // just some sanity asserts to help document the expected range
+    SkASSERT(kTooPinchyNormalDotProd >= -1);
+    SkASSERT(kTooPinchyNormalDotProd < SkDoubleToScalar(-0.999));
 
-    return SkPoint::DotProduct(norm0, norm1) <= kTooPinchyNormalDotProd;
+    SkScalar dot = SkPoint::DotProduct(norm0, norm1);
+    return dot <= kTooPinchyNormalDotProd;
 }
 
 static bool set_normal_unitnormal(const SkPoint& before, const SkPoint& after,
                                   SkScalar radius,
                                   SkVector* normal, SkVector* unitNormal) {
     if (!unitNormal->setNormalize(after.fX - before.fX, after.fY - before.fY)) {
         return false;
     }
     unitNormal->rotateCCW();
     unitNormal->scale(radius, normal);
@@ skipping 670 matching lines @@
         default:
             break;
     }
 
     if (fWidth < SkMinScalar(rw, rh) && !fDoFill) {
         r = rect;
         r.inset(radius, radius);
         dst->addRect(r, reverse_direction(dir));
     }
 }