Fix zero length 2d segment raycast (#357)

* fix zero length segment always reporting intersection

* add changelog

* add test for segments with very close points

* chore: remove unused import

---------

Co-authored-by: Sébastien Crozet <sebcrozet@dimforge.com>

Author: ThierryBerger

CHANGELOG.md

@@ -3,6 +3,7 @@
 ### Fixed
 
 - Fix `clip_aabb_line` crashing when given incorrect inputs (zero length direction or NAN).
+- Fix `Segment::intersects_ray` returning false-positive when the segment is zero-length. ([#31](https://github.com/dimforge/parry/issues/31)).
 
 ## 0.21.1
 

src/query/ray/ray_support_map.rs

@@ -229,10 +229,15 @@ impl RayCast for Segment {
             } else if s >= 0.0 && s <= max_time_of_impact && t >= 0.0 && t <= 1.0 {
                 let normal = self.normal().map(|n| *n).unwrap_or_else(Vector::zeros);
 
-                if normal.dot(&ray.dir) > 0.0 {
+                let dot = normal.dot(&ray.dir);
+                if dot > 0.0 {
                     Some(RayIntersection::new(s, -normal, FeatureId::Face(1)))
-                } else {
+                } else if dot < 0.0 {
                     Some(RayIntersection::new(s, normal, FeatureId::Face(0)))
+                } else {
+                    // dot == 0 happens when lines are parallel, which is normally handled before,
+                    // but this may happen if segment is zero length, as the ray is not considered parallel.
+                    None
                 }
             } else {
                 // The closest points are outside of

src/shape/segment.rs

@@ -359,3 +359,86 @@ impl ConvexPolyhedron for Segment {
     }
 }
 */
+
+#[cfg(test)]
+mod test {
+    use crate::query::{Ray, RayCast};
+
+    pub use super::*;
+    #[test]
+    fn segment_intersect_zero_length_issue_31() {
+        // never intersect each other
+        let ray = Ray::new(Point::origin(), Vector::x());
+        let segment = Segment {
+            a: Point::new(
+                10.0,
+                10.0,
+                #[cfg(feature = "dim3")]
+                10.0,
+            ),
+            b: Point::new(
+                10.0,
+                10.0,
+                #[cfg(feature = "dim3")]
+                10.0,
+            ),
+        };
+
+        let hit = segment.intersects_ray(&Isometry::identity(), &ray, Real::MAX);
+        assert_eq!(hit, false);
+    }
+    #[test]
+    fn segment_very_close_points_hit() {
+        let epsilon = 1.1920929e-7;
+        // intersect each other
+        let ray = Ray::new(
+            Point::new(
+                epsilon * 0.5,
+                0.3,
+                #[cfg(feature = "dim3")]
+                0.0,
+            ),
+            -Vector::y(),
+        );
+        let segment = Segment {
+            a: Point::origin(),
+            b: Point::new(
+                // Theoretically, epsilon would suffice but imprecisions force us to add some more offset.
+                epsilon * 1.01,
+                0.0,
+                #[cfg(feature = "dim3")]
+                0.0,
+            ),
+        };
+
+        let hit = segment.intersects_ray(&Isometry::identity(), &ray, Real::MAX);
+        assert_eq!(hit, true);
+    }
+    #[test]
+    fn segment_very_close_points_no_hit() {
+        let epsilon = 1.1920929e-7;
+        // never intersect each other
+        let ray = Ray::new(
+            Point::new(
+                // Theoretically, epsilon would suffice  but imprecisions force us to add some more offset.
+                epsilon * 11.0,
+                0.1,
+                #[cfg(feature = "dim3")]
+                0.0,
+            ),
+            -Vector::y(),
+        );
+        let segment = Segment {
+            a: Point::origin(),
+            b: Point::new(
+                epsilon * 0.9,
+                0.0,
+                #[cfg(feature = "dim3")]
+                0.0,
+            ),
+        };
+
+        let hit = segment.intersects_ray(&Isometry::identity(), &ray, Real::MAX);
+        assert_eq!(hit, false);
+    }
+}