From b8bb65aeb44ac17d375a2f66bf622d995c1cc4c6 Mon Sep 17 00:00:00 2001
From: Dori <111175613+Doribelove@users.noreply.github.com>
Date: Tue, 3 Nov 2026 15:20:12 +0800
Subject: [PATCH] Normalize finite homogeneous endpoints in line-segment
 intersection

---
 spatialmath/geom2d.py |  5 +++++
 tests/test_geom2d.py  | 12 ++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/spatialmath/geom2d.py b/spatialmath/geom2d.py
index ee42c08f..7462e1f4 100755
--- a/spatialmath/geom2d.py
+++ b/spatialmath/geom2d.py
@@ -178,12 +178,17 @@ def intersect_segment(
         Tests whether the line intersects the line segment defined by endpoints
         ``p1`` and ``p2`` which are given in Euclidean or homogeneous form.
         """
+        # Use unit weight for finite endpoints in the sign and tolerance tests.
         p1 = smb.getvector(p1)
         if len(p1) == 2:
             p1 = np.r_[p1, 1]
+        elif p1[2] != 0:
+            p1 = p1 / p1[2]
         p2 = smb.getvector(p2)
         if len(p2) == 2:
             p2 = np.r_[p2, 1]
+        elif p2[2] != 0:
+            p2 = p2 / p2[2]
 
         z1 = np.dot(self.line, p1)
         z2 = np.dot(self.line, p2)
diff --git a/tests/test_geom2d.py b/tests/test_geom2d.py
index 49aa1d8b..faeb5335 100755
--- a/tests/test_geom2d.py
+++ b/tests/test_geom2d.py
@@ -140,6 +140,18 @@ def test_intersect_segment(self):
         self.assertFalse(l1.intersect_segment((2, 1), (2, 3)))
         self.assertTrue(l1.intersect_segment((2, 1), (2, -1)))
 
+    def test_intersect_segment_homogeneous_endpoints(self):
+        line = Line2([0, 1, 0])  # y = 0
+        above = np.array([1, 2, 1])
+        below = np.array([1, -2, 1])
+        for p1, p2, expected in (
+            ((0, 1), -above, False),
+            (-below, (0, 1), True),
+            (1e-16 * above, (0, 1), False),
+        ):
+            with self.subTest(p1=p1, p2=p2):
+                self.assertEqual(line.intersect_segment(p1, p2), expected)
+
 
 class EllipseTest(unittest.TestCase):
     def test_constructor(self):