CGAL users discussion list

["Fernando Cacciola" <>]

Hi Qianqian,

> hi Fernando
>
> thank you very much for the quick response. The explanations make sense to
> me.
>
> Now I am trying to hook the Placement_with_fixed_border_vertices policy to
> the edge_collapse() call. I have some difficulty to figure out where to
> insert
> the code.
>
> The original edge_collapse() in
> example/Surface_mesh_simplification/edge_collapse_enriched_polyhedron.cpp
> looks like the following:
>
>  int r = SMS::edge_collapse
>           (surface
>           ,stop
>           ,CGAL::get_cost     (SMS::Edge_length_cost  <Surface>())
>                 .get_placement(SMS::Midpoint_placement<Surface>())
>                 .visitor(&vis)
>           );
>
> I assumed the 3rd parameter sets cost policy, sets the placement policy and
> sets a visitor.

That's correct.

> Can you enlighten me what the 3rd parameter should look like
> if I want to use the Placement_with_fixed_border_vertices policy instead?
OK.

First add this typedef before main():

typedef Placement_with_fixed_border_vertices< SMS::Midpoint_placement<Surface> > 
My_placement ;

then use it like this:

 int r = SMS::edge_collapse
          (surface
          ,stop
          ,CGAL::get_cost     (SMS::Edge_length_cost  <Surface>())
                .get_placement( My_placement() )
                .visitor(&vis)
          );


> in addition, I found in
> "include/CGAL/Surface_mesh_simplification/Detail/Edge_collapse.h"
> a function called is_border( const_vertex_descriptor const& aV), is it
> correct to
> write
>
>  return is_border(v);
>
> inside your is_border_vertex().
Unfortunately that function is a private method of the EdgeCollapse class so you 
cannot call that it from within the policy.
OTOH, you can "borrow" it's implementation, as in the attached file (NOTE: 
That's is off the top of my head, I haven't compiled nor tested it)

HTH

Fernando Cacciola
GeometryFactory 

template<class GetPlacement_>
struct Placement_with_fixed_border_vertices : GetPlacement_
{
 typedef GetPlacement_ GetPlacement ;

 typedef typename GetPlacement::Profile Profile ;

 typedef typename GetPlacement::result_type result_type ;

 typedef typename Profile::const_vertex_descriptor const_vertex_descriptor ;
 typedef typename Profile::const_edge_descriptor   const_edge_descriptor ;
 
 typedef typename Profile::ConstGraphTraits::const_in_edge_iterator const_in_edge_iterator ;
 
 result_type operator()( Profile const& aProfile ) const
 {
    if ( is_border_vertex(aProfile.v0()) || is_border_vertex(aProfile.v1()) )
          return boost::none ;
    else
         return this->GetPlacement::operator()(aProfile);
 }

 bool is_border_vertex( const_vertex_descriptor v ) const
 {
   bool rR = false ;

   const_in_edge_iterator eb, ee ; 
   for ( boost::tie(eb,ee) = in_edges(aV,aProfile.surface()) ; eb != ee ; ++ eb )
   {
     const_edge_descriptor lEdge = *eb ;
     if ( lEdge->is_border() || lEdge->opposite()->is_border() )
     {
       rR = true ;
       break ;
     }
   }  

   return rR ;  
 }
} ;