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Hi!

I am trying to convert a triangle mesh to a polyhedron, i.e. I want to join 
all faces which are adjacent and have the same normal

As an easy example I chose a flat patch consisting of triangles in .off 
format. As a result I want to have a polyhedron consisting of a single 
polygon.

I was able to load the file, but the follwoing did not work:

for ( Halfedge_iterator i = polyhedron.halfedges_begin(); i != 
polyhedron.halfedges_end(); ++i) 
{
    if( i->is_border_edge() )
        continue;

    Polyhedron::Facet_handle f1 = i->facet();
    Polyhedron::Facet_handle f2 = i->opposite()->facet();
    
    Halfedge_handle he1 = f1->halfedge();
    
    Point_3 p0 = he1->vertex()->point();
    Point_3 p1 = he1->next()->vertex()->point();
    Point_3 p2 = he1->next()->next()->vertex()->point();
    
    Kernel::Vector_3 n_i = CGAL::cross_product(p0-p2, p1-p2);
    n_i = n_i / sqrt(n_i.squared_length());

    Halfedge_handle he2 = f2->halfedge();
    p0 = he2->vertex()->point();
    p1 = he2->next()->vertex()->point();
    p2 = he2->next()->next()->vertex()->point();

    Kernel::Vector_3 n_j = CGAL::cross_product(p0-p2, p1-p2);
    n_j = n_j / sqrt(n_j.squared_length());

    if(n_i == n_j)
    {
        if( circulator_size(i->opposite()->vertex_begin()) >= 3 && 
circulator_size(i->vertex_begin()) >= 3 )
            polyhedron.join_facet(i);
    }
}

There seem to be 2 problems: 
- after joining to faces the iterator becomes invalid
- some faces cannot be joined, but why and how can I change that?

Can anyone help me please?

Have a nice day,
Joel
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