Write out a triangulation result to an OBJ file in CGAL - cgal

Question

I'm trying to make use of a 2D triangulation using CGAL, and create an obj file. I'm able to create a 2D triangulation. I now want to make the 3rd coordinate 0, ie z=0, and create a obj file out of the result of the triangulation. The samples of CGAL seem quite confusing, and I'm not sure how to go about this.

Answer 1

Here is how I did it. Hope it helps someone.
// A modifier creating a triangle with the incremental builder.
template<class HDS>
class polyhedron_builder : public CGAL::Modifier_base<HDS> {
public:
std::vector<Triangulation>& t_;
polyhedron_builder(std::vector<Triangulation>& t) : t_(t) {}
void operator()(HDS& hds) {
typedef typename HDS::Vertex Vertex;
typedef typename Vertex::Point Point3;
// create a cgal incremental builder
CGAL::Polyhedron_incremental_builder_3<HDS> B(hds, true);
// calculte total vertices
int face_num = 0;
int vertice_num = 0;
for (auto& tri : t_) {
face_num += tri.number_of_faces();
vertice_num += tri.number_of_vertices();
}
std::cout << face_num << ", " << vertice_num << ", " << t_.size() << "\n";
B.begin_surface(face_num, vertice_num);
// add the polyhedron vertices
for (auto& tri : t_) {
for (auto itr = tri.finite_vertices_begin(); itr != tri.finite_vertices_end(); ++itr) {
B.add_vertex(Point3(itr->point().x(), itr->point().y(), 0));
}
}
// add the polyhedron triangles
for (auto& tri : t_) {
for (auto itr = tri.finite_faces_begin(); itr != tri.finite_faces_end(); ++itr) {
B.begin_facet();
B.add_vertex_to_facet(itr->vertex(0)->info());
B.add_vertex_to_facet(itr->vertex(1)->info());
B.add_vertex_to_facet(itr->vertex(2)->info());
B.end_facet();
}
}
// finish up the surface
B.end_surface();
}
};
void OBJfile::write_obj_file(const std::string& filename) {
CGAL::Polyhedron_3<CGAL::Simple_cartesian<double>> polyhedron;
unsigned index = 0;
std::vector<Triangulation> t_vector;
// here, contours is an internal object that tracks the polygon outlines
for (auto& contour : contours_) {
Triangulation t;
std::vector < std::pair<Point, unsigned> > polygon;
for (auto& pt : contour) {
Point point(pt.x(), pt.y());
polygon.push_back(std::make_pair(point, index++));
}
triangulate(polygon, t);
t_vector.push_back(t);
}
polyhedron_builder<HalfedgeDS> builder(t_vector);
polyhedron.delegate(builder);
// write the polyhedron out as a .OFF file
std::ofstream os("test.obj");
CGAL::File_writer_wavefront writer;
CGAL::generic_print_polyhedron(os, polyhedron, writer);
os.close();
}
void OBJfile::triangulate(const std::vector<std::pair<Point, unsigned>>& polygon_points, Triangulation& t) {
auto begin = polygon_points.begin();
auto end = polygon_points.end();
//std::istream_iterator<Point> end;
t.insert(begin, end);
}