Check for degenerate edges in cubic->quad conversion called by convex path renderer.

Cubics that are nearly degenerate now make it down here via the convex path renderer. They used to get filtered out by the path iterator but that was problematic. We wound up producing NaN vertices.

BUG=2677
R=jvanverth@google.com

Author: bsalomon@google.com

Review URL: https://codereview.chromium.org/338633007

1 files changed, 44 insertions, 37 deletions

diff --git a/src/gpu/GrPathUtils.cpp b/src/gpu/GrPathUtils.cpp
index 8e4eeb0a31..464231a8dc 100644
--- a/src/gpu/GrPathUtils.cpp
+++ b/src/gpu/GrPathUtils.cpp
@@ -405,49 +405,56 @@ void convert_noninflect_cubic_to_quads(const SkPoint p[4],
         dc = p[1] - p[3];
     }
 
-    // When the ab and cd tangents are nearly parallel with vector from d to a the constraint that
-    // the quad point falls between the tangents becomes hard to enforce and we are likely to hit
-    // the max subdivision count. However, in this case the cubic is approaching a line and the
-    // accuracy of the quad point isn't so important. We check if the two middle cubic control
-    // points are very close to the baseline vector. If so then we just pick quadratic points on the
-    // control polygon.
+    // When the ab and cd tangents are degenerate or nearly parallel with vector from d to a the
+    // constraint that the quad point falls between the tangents becomes hard to enforce and we are
+    // likely to hit the max subdivision count. However, in this case the cubic is approaching a
+    // line and the accuracy of the quad point isn't so important. We check if the two middle cubic
+    // control points are very close to the baseline vector. If so then we just pick quadratic
+    // points on the control polygon.
 
     if (constrainWithinTangents) {
         SkVector da = p[0] - p[3];
-        SkScalar invDALengthSqd = da.lengthSqd();
-        if (invDALengthSqd > SK_ScalarNearlyZero) {
-            invDALengthSqd = SkScalarInvert(invDALengthSqd);
-            // cross(ab, da)^2/length(da)^2 == sqd distance from b to line from d to a.
-            // same goed for point c using vector cd.
-            SkScalar detABSqd = ab.cross(da);
-            detABSqd = SkScalarSquare(detABSqd);
-            SkScalar detDCSqd = dc.cross(da);
-            detDCSqd = SkScalarSquare(detDCSqd);
-            if (SkScalarMul(detABSqd, invDALengthSqd) < toleranceSqd &&
-                SkScalarMul(detDCSqd, invDALengthSqd) < toleranceSqd) {
-                SkPoint b = p[0] + ab;
-                SkPoint c = p[3] + dc;
-                SkPoint mid = b + c;
-                mid.scale(SK_ScalarHalf);
-                // Insert two quadratics to cover the case when ab points away from d and/or dc
-                // points away from a.
-                if (SkVector::DotProduct(da, dc) < 0 || SkVector::DotProduct(ab,da) > 0) {
-                    SkPoint* qpts = quads->push_back_n(6);
-                    qpts[0] = p[0];
-                    qpts[1] = b;
-                    qpts[2] = mid;
-                    qpts[3] = mid;
-                    qpts[4] = c;
-                    qpts[5] = p[3];
-                } else {
-                    SkPoint* qpts = quads->push_back_n(3);
-                    qpts[0] = p[0];
-                    qpts[1] = mid;
-                    qpts[2] = p[3];
+        bool doQuads = dc.lengthSqd() < SK_ScalarNearlyZero ||
+                       ab.lengthSqd() < SK_ScalarNearlyZero;
+        if (!doQuads) {
+            SkScalar invDALengthSqd = da.lengthSqd();
+            if (invDALengthSqd > SK_ScalarNearlyZero) {
+                invDALengthSqd = SkScalarInvert(invDALengthSqd);
+                // cross(ab, da)^2/length(da)^2 == sqd distance from b to line from d to a.
+                // same goes for point c using vector cd.
+                SkScalar detABSqd = ab.cross(da);
+                detABSqd = SkScalarSquare(detABSqd);
+                SkScalar detDCSqd = dc.cross(da);
+                detDCSqd = SkScalarSquare(detDCSqd);
+                if (SkScalarMul(detABSqd, invDALengthSqd) < toleranceSqd &&
+                    SkScalarMul(detDCSqd, invDALengthSqd) < toleranceSqd) {
+                    doQuads = true;
                 }
-                return;
             }
         }
+        if (doQuads) {
+            SkPoint b = p[0] + ab;
+            SkPoint c = p[3] + dc;
+            SkPoint mid = b + c;
+            mid.scale(SK_ScalarHalf);
+            // Insert two quadratics to cover the case when ab points away from d and/or dc
+            // points away from a.
+            if (SkVector::DotProduct(da, dc) < 0 || SkVector::DotProduct(ab,da) > 0) {
+                SkPoint* qpts = quads->push_back_n(6);
+                qpts[0] = p[0];
+                qpts[1] = b;
+                qpts[2] = mid;
+                qpts[3] = mid;
+                qpts[4] = c;
+                qpts[5] = p[3];
+            } else {
+                SkPoint* qpts = quads->push_back_n(3);
+                qpts[0] = p[0];
+                qpts[1] = mid;
+                qpts[2] = p[3];
+            }
+            return;
+        }
     }
 
     static const SkScalar kLengthScale = 3 * SK_Scalar1 / 2;