From 5c59b8e0509b0c5d94ce758f80c0f017e5c4969a Mon Sep 17 00:00:00 2001
From: wmayer <wmayer@users.sourceforge.net>
Date: Tue, 19 Oct 2021 20:31:39 +0200
Subject: [PATCH] Mesh: [skip ci] add unit tests for coplanar and intersecting
 triangles

---
 src/Mod/Mesh/App/Core/Elements.cpp |  7 ++++---
 src/Mod/Mesh/App/MeshTestsApp.py   | 25 +++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/src/Mod/Mesh/App/Core/Elements.cpp b/src/Mod/Mesh/App/Core/Elements.cpp
index b3d41952bc..9366fa1415 100644
--- a/src/Mod/Mesh/App/Core/Elements.cpp
+++ b/src/Mod/Mesh/App/Core/Elements.cpp
@@ -326,8 +326,7 @@ bool MeshGeomEdge::IntersectWithEdge (const MeshGeomEdge &edge, Base::Vector3f &
         };
 
         if (is_in_range(t) && is_in_range(u)) {
-            res = p + t * r;
-            res = q + u * s;
+            res = p + t * r; // equal to q + u * s
             return true;
         }
 
@@ -1088,7 +1087,9 @@ int MeshGeomFacet::IntersectWithFacet (const MeshGeomFacet& rclFacet,
             }
         }
 
-        if (intersections.size() == 2) {
+        // If triangles overlap there can be more than two intersection points
+        // In that case use any two of them.
+        if (intersections.size() >= 2) {
             rclPt0 = intersections[0];
             rclPt1 = intersections[1];
         }
diff --git a/src/Mod/Mesh/App/MeshTestsApp.py b/src/Mod/Mesh/App/MeshTestsApp.py
index c7304fcabf..01712ef519 100644
--- a/src/Mod/Mesh/App/MeshTestsApp.py
+++ b/src/Mod/Mesh/App/MeshTestsApp.py
@@ -226,6 +226,31 @@ class MeshGeoTestCases(unittest.TestCase):
         res=f1.intersect(f2)
         self.assertTrue(len(res) == 0)
 
+    def testIntersectionCoplanar(self):
+        self.planarMesh.append( [0.,10.,10.] )
+        self.planarMesh.append( [10.,0.,10.] )
+        self.planarMesh.append( [10.,10.,10.] )
+        self.planarMesh.append( [6.,8.,10.] )
+        self.planarMesh.append( [16.,8.,10.] )
+        self.planarMesh.append( [6.,18.,10.] )
+        planarMeshObject = Mesh.Mesh(self.planarMesh)
+        f1 = planarMeshObject.Facets[0]
+        f2 = planarMeshObject.Facets[1]
+        res = f1.intersect(f2)
+        self.assertTrue(len(res) == 2)
+
+    def testIntersectionOverlap(self):
+        self.planarMesh.append( [0.,0.,0.] )
+        self.planarMesh.append( [5.,0.,0.] )
+        self.planarMesh.append( [8.,5.,0.] )
+        self.planarMesh.append( [4.,0.,0.] )
+        self.planarMesh.append( [10.,0.,0.] )
+        self.planarMesh.append( [9.,5.,0.] )
+        planarMeshObject = Mesh.Mesh(self.planarMesh)
+        f1 = planarMeshObject.Facets[0]
+        f2 = planarMeshObject.Facets[1]
+        res = f1.intersect(f2)
+        self.assertTrue(len(res) == 2)
 
     def testSelfIntersection(self):
         s = b"""solid Simple