fix zero-length tangent

src/core/SkGeometry.cpp

Index: src/core/SkGeometry.cpp
diff --git a/src/core/SkGeometry.cpp b/src/core/SkGeometry.cpp
index 6afd9d7ffb0ed1478b6afd0062a76ce93f2db02a..e35bc8cb3ddfd1384a27275a9837c8f6ade94a03 100644
--- a/src/core/SkGeometry.cpp
+++ b/src/core/SkGeometry.cpp
@@ -157,8 +157,12 @@ void SkEvalQuadAt(const SkPoint src[3], SkScalar t, SkPoint* pt, SkVector* tange
         pt->set(eval_quad(&src[0].fX, t), eval_quad(&src[0].fY, t));
     }
     if (tangent) {

[reed1, 2015/08/24 20:15:46]

// The derivative equation is just 2(b - a +(a - 2b +c)t), and this can return a
zero tangent vector at
// these two T values if the control-point (b) is collinear with the end-point,
hence the need to test
// for those explicitly.

[caryclark, 2015/08/25 12:04:48]

Done.

-        tangent->set(eval_quad_derivative(&src[0].fX, t),
-                     eval_quad_derivative(&src[0].fY, t));
+        if ((t == 0 && src[0] == src[1]) || (t == 1 && src[1] == src[2])) {
+            *tangent = src[2] - src[0];
+        } else {
+            tangent->set(eval_quad_derivative(&src[0].fX, t),
+                         eval_quad_derivative(&src[0].fY, t));
+        }
     }
 }
 
@@ -398,8 +402,19 @@ void SkEvalCubicAt(const SkPoint src[4], SkScalar t, SkPoint* loc,
         loc->set(eval_cubic(&src[0].fX, t), eval_cubic(&src[0].fY, t));
     }
     if (tangent) {
-        tangent->set(eval_cubic_derivative(&src[0].fX, t),
-                     eval_cubic_derivative(&src[0].fY, t));
+        if ((t == 0 && src[0] == src[1]) || (t == 1 && src[2] == src[3])) {
+            if (t == 0) {
+                *tangent = src[2] - src[0];
+            } else {
+                *tangent = src[3] - src[1];
+            }
+            if (!tangent->fX && !tangent->fY) {
+                *tangent = src[3] - src[0];
+            }
+        } else {
+            tangent->set(eval_cubic_derivative(&src[0].fX, t),
+                         eval_cubic_derivative(&src[0].fY, t));
+        }
     }
     if (curvature) {
         curvature->set(eval_cubic_2ndDerivative(&src[0].fX, t),
@@ -1232,8 +1247,12 @@ void SkConic::evalAt(SkScalar t, SkPoint* pt, SkVector* tangent) const {
                 conic_eval_pos(&fPts[0].fY, fW, t));
     }
     if (tangent) {
-        tangent->set(conic_eval_tan(&fPts[0].fX, fW, t),
-                     conic_eval_tan(&fPts[0].fY, fW, t));
+        if ((t == 0 && fPts[0] == fPts[1]) || (t == 1 && fPts[1] == fPts[2])) {
+            *tangent = fPts[2] - fPts[0];
+        } else {
+            tangent->set(conic_eval_tan(&fPts[0].fX, fW, t),
+                         conic_eval_tan(&fPts[0].fY, fW, t));
+        }
     }
 }