From 342fc2b18f4db0beba8159a197fa4e072a17dccd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?S=C3=A9bastien=20Loriot?= <sebastien.loriot@cgal.org>
Date: Fri, 21 Nov 2025 11:16:23 +0100
Subject: [PATCH] one more tangency fix

---
 .../Polygon_mesh_processing/refine_with_plane.h | 17 +++++++++++++----
 .../Polygon_mesh_processing/test_pmp_clip.cpp   | 15 ++++++---------
 2 files changed, 19 insertions(+), 13 deletions(-)

diff --git a/Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/refine_with_plane.h b/Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/refine_with_plane.h
index 1561cc1a2ba..5334d89daa0 100644
--- a/Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/refine_with_plane.h
+++ b/Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/refine_with_plane.h
@@ -484,7 +484,6 @@ void refine_with_plane(PolygonMesh& pm,
     else
     {
       // sort hedges to make them match
-      CGAL_assertion(!triangulate);
       // TODO: need mechanism to make it robust even with EPICK
       auto less_hedge = [&pm, vpm](halfedge_descriptor h1, halfedge_descriptor h2)
       {
@@ -492,13 +491,23 @@ void refine_with_plane(PolygonMesh& pm,
       };
       std::sort(f_and_hs.second.begin(), f_and_hs.second.end(), less_hedge);
 
-      if (f_and_hs.second.front()==*std::next(f_and_hs.second.begin()))
-        f_and_hs.second.erase(f_and_hs.second.begin());
+      if (f_and_hs.second[0]==f_and_hs.second[1])
+      {
+        halfedge_descriptor h = f_and_hs.second[0];
+        if ( get(vertex_os, source(h, pm))==ON_ORIENTED_BOUNDARY || get(vertex_os, target(next(h, pm), pm))==ON_ORIENTED_BOUNDARY)
+          f_and_hs.second.erase(f_and_hs.second.begin());
+      }
       if (f_and_hs.second.back()==*std::prev(f_and_hs.second.end(),2))
-        f_and_hs.second.pop_back();
+      {
+        halfedge_descriptor h = f_and_hs.second.back();
+        if ( get(vertex_os, source(h, pm))==ON_ORIENTED_BOUNDARY || get(vertex_os, target(next(h, pm), pm))==ON_ORIENTED_BOUNDARY)
+          f_and_hs.second.pop_back();
+      }
+
 
       nb_hedges = f_and_hs.second.size();
       CGAL_assertion(nb_hedges%2==0);
+      CGAL_assertion(!triangulate || nb_hedges==2);
 
       for (std::size_t i=0; i<nb_hedges; i+=2)
       {
diff --git a/Polygon_mesh_processing/test/Polygon_mesh_processing/test_pmp_clip.cpp b/Polygon_mesh_processing/test/Polygon_mesh_processing/test_pmp_clip.cpp
index 5a96ee0bc60..0180902791d 100644
--- a/Polygon_mesh_processing/test/Polygon_mesh_processing/test_pmp_clip.cpp
+++ b/Polygon_mesh_processing/test/Polygon_mesh_processing/test_pmp_clip.cpp
@@ -910,15 +910,19 @@ void test_new_clip()
     assert(faces(e).size()==5);
     assert(vertices(e).size()==24);
   }
-
   {
     TriangleMesh c;
     std::ifstream("data-clip/c.off") >> c;
     PMP::refine_with_plane(c, K::Plane_3(1,0,0,-2));
     assert(faces(c).size()==2);
     assert(vertices(c).size()==8);
+    c.clear();
+    std::ifstream("data-clip/c.off") >> c;
+    PMP::reverse_face_orientations(c);
+    PMP::refine_with_plane(c, K::Plane_3(1,0,0,-2));
+    assert(faces(c).size()==2);
+    assert(vertices(c).size()==8);
   }
-
   {
     TriangleMesh e;
     std::ifstream("data-clip/ee.off") >> e;
@@ -927,7 +931,6 @@ void test_new_clip()
     assert(faces(e).size()==30);
     assert(vertices(e).size()==28);
   }
-
   {
     TriangleMesh c;
     std::ifstream("data-clip/c.off") >> c;
@@ -980,7 +983,6 @@ void test_new_clip()
     TriangleMesh c;
     std::ifstream("data-clip/tgt_case_2.off") >> c;
     PMP::refine_with_plane(c, K::Plane_3(1,0,0,-384), CGAL::parameters::do_not_triangulate_faces(false));
-std::ofstream("/tmp/ref.off") << c;
     assert(faces(c).size()==1);
     assert(vertices(c).size()==13);
     c.clear();
@@ -1003,7 +1005,6 @@ std::ofstream("/tmp/ref.off") << c;
     assert(faces(c).size()==6);
     assert(vertices(c).size()==18);
   }
-
   {
     TriangleMesh ele;
     std::ifstream(CGAL::data_file_path("meshes/elephant.off")) >> ele;
@@ -1013,10 +1014,6 @@ std::ofstream("/tmp/ref.off") << c;
     assert(faces(ele).size()==1220);
     assert(vertices(ele).size()==691);
   }
-
-
-
-
 }
 
 struct Clip_and_split_visitor