CGAL users discussion list

[Stephane Tayeb <>]

Ianic wrote:
> I've edited the File_medit.h file to add a "Neighbours" section, like so:
>
>   os << "Neighbours" << std::endl
>      << c3t3.number_of_cells() << std::endl;
>   for( Cell_iterator cit = c3t3.cells_begin() ;
>        cit != c3t3.cells_end() ;
>        ++cit )
>   {
>     const Cell_handle& c = cit;
>     for ( int i=0 ; i<4 ; ++i ) {
>       const Cell_handle& neighbor = c->neighbor(i);
>       if ( c3t3.is_in_complex(neighbor) )
>         os << C[neighbor] << " ";
>       else
>         os << -1 << " ";
>     }
>     os << std::endl;
>   }
>
> Here C is std::map<Cell_handle, int> to keep track of cell numbers. The
> point of doubt that I have is whether my detection of boundary elements is
> correct. What exactly does c3t3.is_in_complex() check for? My naive
> understanding right now is that if a neighbour is outside a complex, it
> means that the element I look from (const Cell_handle& c) lies on a boundary
> and therefore some of its neighbours do not belong to a mesh. Am I right
> here?

From the documentation: 
(http://www.cgal.org/Manual/last/doc_html/cgal_manual/Mesh_3_ref/Concept_MeshComplex_3InTriangulation_3.html#Cross_link_anchor_1476)
bool c3t3.is_in_complex(Cell_handle c) "Returns true iff the cell c 
belongs to the 3D complex.".

It means that it checks whether a tetrahedron belongs to the mesh (= the 
discretization of your input domain) or not.

In your case, if c->neighbor(i) does not belongs to the mesh (c does), 
there is a boundary facet between c and c->neighbor(i). So I think you 
are right :).

Best,
Stéphane.

--
Stephane Tayeb
Software engineer - INRIA Sophia Antipolis
Geometrica Project-Team