From f82fb79e6b3520cb7340b8701af12596cbce1891 Mon Sep 17 00:00:00 2001
From: Paul Melnikow <email@paulmelnikow.com>
Date: Mon, 30 Sep 2019 16:27:11 -0400
Subject: [PATCH] Plane.line_xsections: Document the behavior in a degenerate
 case

---
 polliwog/plane/plane.py      |  1 +
 polliwog/plane/test_plane.py | 20 ++++++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/polliwog/plane/plane.py b/polliwog/plane/plane.py
index 0c99d97..089b97d 100644
--- a/polliwog/plane/plane.py
+++ b/polliwog/plane/plane.py
@@ -248,6 +248,7 @@ def _line_segment_xsection(self, a, b):
     def line_xsections(self, pts, rays):
         k = vg.shape.check(locals(), "pts", (-1, 3))
         vg.shape.check(locals(), "rays", (k, 3))
+
         denoms = np.dot(rays, self.normal)
         denom_is_zero = denoms == 0
         denoms[denom_is_zero] = np.nan
diff --git a/polliwog/plane/test_plane.py b/polliwog/plane/test_plane.py
index 8e31463..19549aa 100644
--- a/polliwog/plane/test_plane.py
+++ b/polliwog/plane/test_plane.py
@@ -299,6 +299,26 @@ def test_line_plane_intersections():
     np.testing.assert_array_equal(is_intersecting, [False, False, True, True])
 
 
+def test_line_plane_intersections_coplanar_input():
+    # TODO Intersecting the x-z plane with a line that lies in the x-z plane
+    # returns non-intersecting. Is this desirable?
+
+    # x-z plane
+    normal = np.array([0.0, 1.0, 0.0])
+    sample = np.array([0.0, 0.0, 0.0])
+
+    plane = Plane(sample, normal)
+
+    # A line that lies in the x-z plane.
+    pts = np.array([[1.0, 0.0, -1.0]])
+    rays = np.array([[1.0, 0.0, 2.0]])
+
+    expected = np.array([[np.nan, np.nan, np.nan]])
+    intersections, is_intersecting = plane.line_xsections(pts, rays)
+    np.testing.assert_array_equal(intersections, expected)
+    np.testing.assert_array_equal(is_intersecting, [False])
+
+
 def test_line_segment_plane_intersection():
     # x-z plane
     normal = np.array([0.0, 1.0, 0.0])