add SkPoint::CanNormalize to unify decisions about when a vector is degenerate

git-svn-id: http://skia.googlecode.com/svn/trunk@2236 2bbb7eff-a529-9590-31e7-b0007b416f81

include/core/SkPoint.h

@@ -199,6 +199,16 @@ struct SK_API SkPoint {
     SkScalar length() const { return SkPoint::Length(fX, fY); }
     SkScalar distanceToOrigin() const { return this->length(); }
 
+    /**
+     *  Return true if the computed length of the vector is >= the internal
+     *  tolerance (used to avoid dividing by tiny values).
+     */
+    static bool CanNormalize(SkScalar dx, SkScalar dy);
+
+    bool canNormalize() const {
+        return CanNormalize(fX, fY);
+    }
+
     /** Set the point (vector) to be unit-length in the same direction as it
         already points.  If the point has a degenerate length (i.e. nearly 0)
         then return false and do nothing; otherwise return true.
@@ -317,7 +327,8 @@ struct SK_API SkPoint {
     static SkScalar Length(SkScalar x, SkScalar y);
 
     /** Normalize pt, returning its previous length. If the prev length is too
-        small (degenerate), return 0 and leave pt unchanged.
+        small (degenerate), return 0 and leave pt unchanged. This uses the same
+        tolerance as CanNormalize.
 
         Note that this method may be significantly more expensive than
         the non-static normalize(), because it has to return the previous length

src/core/SkPoint.cpp

@@ -100,16 +100,21 @@ SkScalar SkPoint::Normalize(SkPoint* pt) {
 
 #ifdef SK_SCALAR_IS_FLOAT
 
+bool SkPoint::CanNormalize(SkScalar dx, SkScalar dy) {
+    float mag2 = dx * dx + dy * dy;
+    return mag2 > SK_ScalarNearlyZero * SK_ScalarNearlyZero;
+}
+
 SkScalar SkPoint::Length(SkScalar dx, SkScalar dy) {
     return sk_float_sqrt(dx * dx + dy * dy);
 }
 
 bool SkPoint::setLength(float x, float y, float length) {
-    float mag = sk_float_sqrt(x * x + y * y);
-    if (mag > SK_ScalarNearlyZero) {
-        length /= mag;
-        fX = x * length;
-        fY = y * length;
+    float mag2 = x * x + y * y;
+    if (mag2 > SK_ScalarNearlyZero * SK_ScalarNearlyZero) {
+        float scale = length / sk_float_sqrt(mag2);
+        fX = x * scale;
+        fY = y * scale;
         return true;
     }
     return false;
@@ -119,6 +124,23 @@ bool SkPoint::setLength(float x, float y, float length) {
 
 #include "Sk64.h"
 
+bool SkPoint::CanNormalize(SkScalar dx, SkScalar dy) {
+    Sk64    tmp1, tmp2, tolSqr;
+    
+    tmp1.setMul(dx, dx);
+    tmp2.setMul(dy, dy);
+    tmp1.add(tmp2);
+
+    // we want nearlyzero^2, but to compute it fast we want to just do a
+    // 32bit multiply, so we require that it not exceed 31bits. That is true
+    // if nearlyzero is <= 0xB504, which should be trivial, since usually
+    // nearlyzero is a very small fixed-point value.
+    SkASSERT(SK_ScalarNearlyZero <= 0xB504);
+
+    tolSqr.set(0, SK_ScalarNearlyZero * SK_ScalarNearlyZero);
+    return tmp1 > tolSqr;
+}
+
 SkScalar SkPoint::Length(SkScalar dx, SkScalar dy) {
     Sk64    tmp1, tmp2;
 

src/core/SkStroke.cpp

@@ -15,7 +15,7 @@
 #define kMaxCubicSubdivide  4
 
 static inline bool degenerate_vector(const SkVector& v) {
-    return SkScalarNearlyZero(v.fX) && SkScalarNearlyZero(v.fY);
+    return !SkPoint::CanNormalize(v.fX, v.fY);
 }
 
 static inline bool degenerate_line(const SkPoint& a, const SkPoint& b,

tests/PathTest.cpp

@@ -6,10 +6,47 @@
  * found in the LICENSE file.
  */
 #include "Test.h"
+#include "SkPaint.h"
 #include "SkPath.h"
 #include "SkParse.h"
 #include "SkSize.h"
 
+static void stroke_cubic(const SkPoint pts[4]) {
+    SkPath path;
+    path.moveTo(pts[0]);
+    path.cubicTo(pts[1], pts[2], pts[3]);
+    
+    SkPaint paint;
+    paint.setStyle(SkPaint::kStroke_Style);
+    paint.setStrokeWidth(SK_Scalar1 * 2);
+    
+    SkPath fill;
+    paint.getFillPath(path, &fill);
+}
+
+// just ensure this can run w/o any SkASSERTS firing in the debug build
+// we used to assert due to differences in how we determine a degenerate vector
+// but that was fixed with the introduction of SkPoint::CanNormalize
+static void stroke_tiny_cubic() {
+    SkPoint p0[] = {
+        { 372.0f,   92.0f },
+        { 372.0f,   92.0f },
+        { 372.0f,   92.0f },
+        { 372.0f,   92.0f },
+    };
+    
+    stroke_cubic(p0);
+    
+    SkPoint p1[] = {
+        { 372.0f,       92.0f },
+        { 372.0007f,    92.000755f },
+        { 371.99927f,   92.003922f },
+        { 371.99826f,   92.003899f },
+    };
+    
+    stroke_cubic(p1);
+}
+
 static void check_close(skiatest::Reporter* reporter, const SkPath& path) {
     for (int i = 0; i < 2; ++i) {
         SkPath::Iter iter(path, (bool)i);
@@ -102,6 +139,8 @@ static void test_close(skiatest::Reporter* reporter) {
     moves.moveTo(SK_Scalar1, 10 * SK_Scalar1);
     moves.moveTo(10 *SK_Scalar1, SK_Scalar1);
     check_close(reporter, moves);
+
+    stroke_tiny_cubic();
 }
 
 static void check_convexity(skiatest::Reporter* reporter, const SkPath& path,