fix zero-length tangent

src/core/SkGeometry.cpp

 /*
  * Copyright 2006 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 "SkGeometry.h"
 #include "SkMatrix.h"
 #include "SkNx.h"
@@ skipping 138 matching lines @@
     coeff[2] = pts[0];                          // C
 }
 
 void SkEvalQuadAt(const SkPoint src[3], SkScalar t, SkPoint* pt, SkVector* tangent) {
     SkASSERT(src);
     SkASSERT(t >= 0 && t <= SK_Scalar1);
 
     if (pt) {
         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));
+        }
     }
 }
 
 SkPoint SkEvalQuadAt(const SkPoint src[3], SkScalar t) {
     SkASSERT(src);
     SkASSERT(t >= 0 && t <= SK_Scalar1);
 
     const Sk2s t2(t);
 
     Sk2s P0 = from_point(src[0]);
@@ skipping 219 matching lines @@
 
 void SkEvalCubicAt(const SkPoint src[4], SkScalar t, SkPoint* loc,
                    SkVector* tangent, SkVector* curvature) {
     SkASSERT(src);
     SkASSERT(t >= 0 && t <= SK_Scalar1);
 
     if (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),
                        eval_cubic_2ndDerivative(&src[0].fY, t));
     }
 }
 
 /** Cubic'(t) = At^2 + Bt + C, where
     A = 3(-a + 3(b - c) + d)
     B = 6(a - 2b + c)
@@ skipping 812 matching lines @@
 }
 
 void SkConic::evalAt(SkScalar t, SkPoint* pt, SkVector* tangent) const {
     SkASSERT(t >= 0 && t <= SK_Scalar1);
 
     if (pt) {
         pt->set(conic_eval_pos(&fPts[0].fX, fW, t),
                 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));
+        }
     }
 }
 
 void SkConic::chopAt(SkScalar t, SkConic dst[2]) const {
     SkP3D tmp[3], tmp2[3];
 
     ratquad_mapTo3D(fPts, fW, tmp);
 
     p3d_interp(&tmp[0].fX, &tmp2[0].fX, t);
     p3d_interp(&tmp[0].fY, &tmp2[0].fY, t);
@@ skipping 332 matching lines @@
         matrix.preScale(SK_Scalar1, -SK_Scalar1);
     }
     if (userMatrix) {
         matrix.postConcat(*userMatrix);
     }
     for (int i = 0; i < conicCount; ++i) {
         matrix.mapPoints(dst[i].fPts, 3);
     }
     return conicCount;
 }