From 33766800b6da78d53bd65b0f963ad9afcde15fbe Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mael=20Rouxel-Labb=C3=A9?=
 <mael.rouxel.labbe@geometryfactory.com>
Date: Mon, 3 Feb 2020 18:10:41 +0100
Subject: [PATCH] Fix some corner cases with the new compute normals and
 degenerate faces

---
 .../Polygon_mesh_processing/compute_normal.h  | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/compute_normal.h b/Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/compute_normal.h
index a149ef1155f..0c254332e78 100644
--- a/Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/compute_normal.h
+++ b/Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/compute_normal.h
@@ -311,18 +311,18 @@ typename GT::Vector_3 compute_normals_bisector(const typename GT::Vector_3& ni,
 
   Vector_3 nb = cv_3(CGAL::NULL_VECTOR);
 
-  if(almost_equal(ni, nj, traits))
+  if(almost_equal(ni, nj, traits) || nk == CGAL::NULL_VECTOR)
   {
     if(almost_equal(nj, nk, traits))
       nb = ni;
     else // ni == nj, but nij != nk
       nb = compute_normals_bisector(nj, nk, traits);
   }
-  else if(almost_equal(ni, nk, traits)) // ni != nj
+  else if(almost_equal(ni, nk, traits) || nj == CGAL::NULL_VECTOR) // ni != nj
   {
     nb = compute_normals_bisector(nj, nk, traits);
   }
-  else if(almost_equal(nj, nk, traits)) // ni != nj, ni != nk
+  else if(almost_equal(nj, nk, traits) || ni == CGAL::NULL_VECTOR) // ni != nj, ni != nk
   {
     nb = compute_normals_bisector(ni, nk, traits);
   }
@@ -345,7 +345,7 @@ typename GT::Vector_3 compute_normals_bisector(const typename GT::Vector_3& ni,
       return csv_3(csv_3(cslv_3(ni, third), cslv_3(nj, third)), cslv_3(nk, third));
     }
 
-    nb = cv_3(CGAL::ORIGIN, c); // note that this isn't normalized
+    nb = cv_3(CGAL::ORIGIN, c); // not normalized
   }
 
   return nb;
@@ -432,6 +432,10 @@ compute_most_visible_normal_3_points(const std::vector<typename boost::graph_tra
         const Vector_ref nj = get(face_normals, incident_faces[j]);
         const Vector_ref nk = get(face_normals, incident_faces[k]);
 
+        CGAL_warning(ni != CGAL::NULL_VECTOR);
+        CGAL_warning(nj != CGAL::NULL_VECTOR);
+        CGAL_warning(nk != CGAL::NULL_VECTOR);
+
         Vector_3 nb = compute_normals_bisector(ni, nj, nk, traits);
         if(traits.equal_3_object()(nb, CGAL::NULL_VECTOR))
           return nb;
@@ -491,7 +495,12 @@ compute_vertex_normal_most_visible_min_circle(typename boost::graph_traits<Polyg
   if(res != CGAL::NULL_VECTOR) // found a valid normal through 2 point min circle
     return res;
 
-  CGAL_assertion(incident_faces.size() > 2);
+  // The vertex has only two incident faces with opposite normals (fold)...
+  // @todo devise something based on the directions of the 2/3/4 incident edges?
+  if(incident_faces.size() == 2 && res == CGAL::NULL_VECTOR)
+    return res;
+
+  CGAL_assertion(incident_faces.size() >= 2);
 
   return compute_most_visible_normal_3_points<PolygonMesh>(incident_faces, face_normals, traits);
 }