Write a CGAL mesh to wavefront OBJ

mesh_to_obj.cc

/* This takes a C2t3 from example 3.1 in CGAL manual on meshes.
   The `vertices_begin()` method returns non-const iterator, so
   we pass non-const reference. This should be fixed somehow.
 */
void write_to_obj(std::ostream &out, C2t3 &mesh)
{
  // create a map from Vertex_handle to the vertex index as it is
  // written into the OBJ file. Remember that OBJ indices start from 1.
  std::map<C2t3::Vertex_handle, unsigned> vertex_map;
  unsigned count = 1;

  // Loop over vertices
  for (auto v  = mesh.vertices_begin();
            v != mesh.vertices_end();
            ++v) {
    // create entry into vertex_map
    vertex_map[v] = count;
    ++count;
    
    // write vertex
    auto point = v->point();
    out << "v " << point << " 0.0\n";
  }

  out << "\n";

  // Map over facets. Each facet is a cell of the underlying
  // Delaunay triangulation, and the vertex that is not part of
  // this facet. We iterate all vertices of the cell except the one
  // that is opposite.
  for (auto f  = mesh.facets_begin();
            f != mesh.facets_end();
            ++f) {
    out << "f";
    for (unsigned j = 0; j < 4; ++j) {
      if (j != f->second) {
        out << " " << vertex_map[f->first->vertex(j)];
      }
    }
    out << std::endl;
  }
}