When reading an off-file with cgal it appears that the vertex order of a face decides whether or not it is read in by read_OFF. But the off-file definition does not say anything about the vertex order of a face.
I am reading in self generated off-files using the read_OFF method of cgal:
using Kernel = CGAL::Exact_predicates_inexact_constructions_kernel;
using Point_3 = typename Kernel::Point_3;
...
CGAL::Surface_mesh<Point_3> test_mash;
CGAL::IO::read_OFF(file_name, test_mash);
std::cout << "Number of vertices: " << test_mash.vertices().size()
<< ", Number of faces: " << test_mash.faces().size() << std::endl;
two_faces_read.off:
OFF
4 2 0
1 1 1
2 -2 2
3 3 -3
-4 4 4
3 0 1 2
3 0 3 1
one_face_read.off:
OFF
4 2 0
1 1 1
2 -2 2
3 3 -3
-4 4 4
3 0 1 2
3 0 1 3
Reading two_faces_read.off works as expected, printing:
Number of vertices: 4, Number of faces: 2.
But when i read one_face_read.off i get Number of vertices: 4, Number of faces: 1. The only difference between these two files is the last line, the vertex order of the second face is different. After trying all possible combinations it seems that with 031, 103, 310 2 faces are read in, while with 013, 130, 301 only 1 face is read in.
The off-file specification referenced by cgal does not mention any rules concerning the vertex order of a face.
Why does this happen and how can i ensure that all faces are read in?
one_face_read.off does not define a valid surface mesh has the orientation of the two faces are not compatible. You can use the following function to read points and faces and call CGAL::Polygon_mesh_processing::is_polygon_soup_a_polygon_mesh() to check if the input is a valid surface mesh. The function CGAL::Polygon_mesh_processing::orient_polygon_soup() can be used to fix orientations. CGAL::Polygon_mesh_processing::polygon_soup_to_polygon_mesh() can be used to create the mesh.