Use fuzzier definition of zero-length segments in compute_intersection.

When working with segments in compute_intersection, we're often dividing
by the length squared. If that's 0, we'll end up dividing by 0. This
effectively puts a lower bound on the size of segment we can determine
intersections with other than treating it like a single point. We
arbitrarily choose the first endpoint as this point.

Bug: oss-fuzz:20009
Change-Id: I7581922f704ff8cef77456a80159cdb8ba3b716f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/282160
Auto-Submit: Jim Van Verth <[email protected]>
Commit-Queue: Kevin Lubick <[email protected]>
Reviewed-by: Kevin Lubick <[email protected]>

Author: jvanverth

src/utils/SkPolyUtils.cpp

@@ -79,9 +79,16 @@ static inline bool outside_interval(SkScalar numer, SkScalar denom, bool denomPo
            (!denomPositive && (numer > 0 || numer < denom));
 }
 
+// special zero-length test when we're using vdotv as a denominator
+static inline bool zero_length(const SkPoint& v, SkScalar vdotv) {
+    return !(SkScalarsAreFinite(v.fX, v.fY) && vdotv);
+}
+
 // Compute the intersection 'p' between segments s0 and s1, if any.
 // 's' is the parametric value for the intersection along 's0' & 't' is the same for 's1'.
 // Returns false if there is no intersection.
+// If the length squared of a segment is 0, then we treat the segment as degenerate
+// and use only the first endpoint for tests.
 static bool compute_intersection(const OffsetSegment& s0, const OffsetSegment& s1,
                                  SkPoint* p, SkScalar* s, SkScalar* t) {
     const SkVector& v0 = s0.fV;
@@ -98,35 +105,39 @@ static bool compute_intersection(const OffsetSegment& s0, const OffsetSegment& s
         }
 
         // Check for zero-length segments
-        if (!SkPointPriv::CanNormalize(v0.fX, v0.fY)) {
+        SkScalar v0dotv0 = v0.dot(v0);
+        if (zero_length(v0, v0dotv0)) {
             // Both are zero-length
-            if (!SkPointPriv::CanNormalize(v1.fX, v1.fY)) {
+            SkScalar v1dotv1 = v1.dot(v1);
+            if (zero_length(v1, v1dotv1)) {
                 // Check if they're the same point
                 if (!SkPointPriv::CanNormalize(w.fX, w.fY)) {
                     *p = s0.fP0;
                     *s = 0;
                     *t = 0;
                     return true;
                 } else {
+                    // Intersection is indeterminate
                     return false;
                 }
             }
             // Otherwise project segment0's origin onto segment1
             tNumer = v1.dot(-w);
-            denom = v1.dot(v1);
+            denom = v1dotv1;
             if (outside_interval(tNumer, denom, true)) {
                 return false;
             }
             sNumer = 0;
         } else {
             // Project segment1's endpoints onto segment0
             sNumer = v0.dot(w);
-            denom = v0.dot(v0);
+            denom = v0dotv0;
             tNumer = 0;
             if (outside_interval(sNumer, denom, true)) {
                 // The first endpoint doesn't lie on segment0
                 // If segment1 is degenerate, then there's no collision
-                if (!SkPointPriv::CanNormalize(v1.fX, v1.fY)) {
+                SkScalar v1dotv1 = v1.dot(v1);
+                if (zero_length(v1, v1dotv1)) {
                     return false;
                 }
 
@@ -143,7 +154,7 @@ static bool compute_intersection(const OffsetSegment& s0, const OffsetSegment& s
                     // otherwise project segment0's endpoint onto segment1 instead
                     sNumer = 0;
                     tNumer = v1.dot(-w);
-                    denom = v1.dot(v1);
+                    denom = v1dotv1;
                 }
             }
         }