CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

On 04/26/2012 10:05 AM, luchia wrote:
> Hallo,
>
> I'm a new CGAL user and I'm facing a problem that actually I do not know how
> to solve.
>
> I'm writing the procedure Lower_hull that takes in input a Convex_hull_d
> CH_d, an empty
> vector vfacet of type std::vector<Facet>  with Facet of type
> Convex_hull_d::Facet_handle             
> and returns in vfacet the vector of all the facet of CH_d that belong to the
> lower hull.
>
> Here is the code:
>
> *void Lower_hull(Convex_hull_d&  CH_d, VFacet vfacet) {
>         Hyperplane hp;
>         Vector_d normal;
>         for (Facet_iterator i = CH_d.facets_begin(); i != CH_d.facets_end(); 
> ++i) {
>                 hp = CH_d.hyperplane_supporting(i); // Hyperplane of the i-th 
> facet
>                 normal = hp.orthogonal_vector(); // Normal to hp
>                 if(normal[(normal.dimension())-1]<0)
>                 {
>                         vfacet.push_back(i);
>                         for (Convex_hull_d::Point_const_iterator j = 
> i->points_begin(); j!=
> i->points_end(); ++j){}
>                 }
>         }
> }*
>
> Questions:
>
> 1. The idea to use the Hyperplane passing throuth a facet to extract its
> orthogonal vector
> does it make sense?
you can extract 3 points for the facet and compute the normal vector
directly, but using the hyperplane is fine too (you have an extra 
construction).

> 2. Why the statement CH_d.hyperplane_supporting(i) is it correct? The method
> hyperplane_supporting takes in input a facet_handle, while "i" its an
> iterator over facet. Thus ... shouldn't we pass (*i) insteat of i?
Iterators value_type are Facet, not Facet_handle.
There is a convertion from iterator to Facet_handle.

> 3. The cicle:
>
> for (Convex_hull_d::Point_const_iterator j = i->points_begin(); j!=
> i->points_end(); ++j){}
>
> was designed in order to iterate over the points of a given facet i. But it
> does not work ... I obtained
> a type error message.
Look at here:
http://www.cgal.org/Manual/latest/doc_html/cgal_manual/Convex_hull_d_ref/Class_Convex_hull_d.html

Vertex_handle   C.vertex_of_facet ( Facet_handle f, int i)
        returns the vertex corresponding to the i-th vertex of f.

for (int k=0;k<dim-1; ++k)
  CH_d.vertex_of_facet(i,k)->point();

Sebastien.

> Thaks your patience and for your help,
>
> Luca
>
>
>
>
>
>
> --
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