CGAL users discussion list

[Andreas Fabri <>]

Hi Joel,

You must make a copy of the iterator, increment it and then work on the copy 
in the body of the loop.
This is because the halfedges are in an in-place list, your join operation 
removes them and the iterator
no longer points on something reasonable

for ( Halfedge_iterator j = polyhedron.halfedges_begin(); j != 
polyhedron.halfedges_end(); ){
  Halfedge_iterator i = j;
  ++j;
 // and now your code again
  .
  .
}


I think you have even to increment j twice, as the twin halfedges are stored 
consecutively.
Alternatively you should use an Edge_iterator.


andreas



 wrote:
> 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