fine-tune tolerance for pinchy quads in stroker

BUG=
R=jvanverth@google.com

Review URL: https://codereview.chromium.org/22947005

git-svn-id: http://skia.googlecode.com/svn/trunk@10753 2bbb7eff-a529-9590-31e7-b0007b416f81

2 files changed, 33 insertions, 3 deletions

diff --git a/gm/aaclip.cpp b/gm/aaclip.cpp
index 48df3a480f..0725ea2d0f 100644
--- a/gm/aaclip.cpp
+++ b/gm/aaclip.cpp
@@ -9,6 +9,26 @@
 #include "SkCanvas.h"
 #include "SkPath.h"
 
+static void test_quadstroke(SkCanvas* canvas) {
+    SkPath path;
+    path.moveTo(6, 0);
+    path.quadTo(150, 150, 0, 6);
+
+    SkPaint paint;
+    
+    paint.setAntiAlias(true);
+    paint.setStyle(SkPaint::kStroke_Style);
+    canvas->translate(20, 20);
+
+#if 1
+    canvas->drawPath(path, paint);
+    canvas->translate(100, 0);
+#endif
+    
+    paint.setStrokeWidth(1.01f);
+    canvas->drawPath(path, paint);
+}
+
 static void draw_conic(SkCanvas* canvas, SkScalar weight, const SkPaint& paint) {
     SkPath path;
     path.moveTo(100, 100);
@@ -253,6 +273,7 @@ protected:
     }
 
     virtual void onDraw(SkCanvas* canvas) SK_OVERRIDE {
+        if (false) { test_quadstroke(canvas); return; }
         if (false) { test_conic(canvas); return; }
         if (false) {
             SkRect bounds;
diff --git a/src/core/SkStroke.cpp b/src/core/SkStroke.cpp
index d094ef65b7..2037224733 100644
--- a/src/core/SkStroke.cpp
+++ b/src/core/SkStroke.cpp
@@ -30,9 +30,18 @@ static inline bool normals_too_curvy(const SkVector& norm0, SkVector& norm1) {
 }
 
 static inline bool normals_too_pinchy(const SkVector& norm0, SkVector& norm1) {
-    static const SkScalar kTooPinchyNormalDotProd = -SK_Scalar1 * 999 / 1000;
-
-    return SkPoint::DotProduct(norm0, norm1) <= kTooPinchyNormalDotProd;
+    // 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));
+
+    SkScalar dot = SkPoint::DotProduct(norm0, norm1);
+    return dot <= kTooPinchyNormalDotProd;
 }
 
 static bool set_normal_unitnormal(const SkPoint& before, const SkPoint& after,