CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

Use one of the following examples [1] to build the triangulation of your 
points and set their indices. Then build the alpha shape from the 
triangulation.

then you can do:

typedef CGAL::Delaunay_triangulation_3<.....> DT3;

....

for (std::list<Facet>::iterator it = facets.begin(); it != facets.end(); 
 ++it)
{
  it->first->vertex( DT3::vertex_triple_index(it->second,0) )->info();
  it->first->vertex( DT3::vertex_triple_index(it->second,1) )->info();
  it->first->vertex( DT3::vertex_triple_index(it->second,2) )->info();
}

Sebastien.

[1] 
http://www.cgal.org/Manual/latest/doc_html/cgal_manual/Triangulation_3/Chapter_main.html#Subsection_39.5.3

On 08/01/2013 08:44 PM, rcasero wrote:
> Dear all,
>
> I'm computing alpha shapes on a set of points
>
>    // read points from function
>    std::vector<Point>  x =
>      some_function_to_read_points();
>
>    // compute alpha shape
>    Alpha_shape_3 as(x.begin(), x.end());
>
> based on the example from the "3D Alpha shapes" documentation:
>
> http://www.cgal.org/Manual/latest/doc_html/cgal_manual/Alpha_shapes_3/Chapter_main.html#Subsection_43.5.1
>
> What I would like to do now is extract the triangulation of a given alpha
> shape in index form, e.g.
>
> tri = [3 2 8
>          2 1 3
>         ...]
>
> where the indices are the indices of the points in the x vector above.
>
> This is similar to a previous question I got help from Sebastien Loriot to
>
> http://cgal-discuss.949826.n4.nabble.com/Getting-index-of-facet-closest-to-a-point-tp4657635.html
>
> but I don't manage to find the right pointers in this case so that I can do
> something like
>
>     std::distance(x.begin(), some_iterator_here);
>
> to get the index.
>
> I can get a list of the facets
>
>    // get alpha-shape surface
>    std::list<Facet>        facets;
>    as.get_alpha_shape_facets(std::back_inserter(facets),
>                              Alpha_shape_3::REGULAR);
>
> and I can iterate said facets
>
>    for (std::list<Facet>::iterator it = facets.begin(); it != facets.end();
> ++it) {
>
>    }
>
>  From the documentation and an old question replied to by sloriot
>
> http://stackoverflow.com/questions/7938311/cgal-help-getting-triangles-coordinates-from-delaunay-triangulation
>
> I know I can the coordinates of the facet vertices as
>
>    for (std::list<Facet>::iterator it = facets.begin(); it != facets.end();
> ++it) {
>
>      it->first->vertex((it->second+1)%4)->point();
>      it->first->vertex((it->second+2)%4)->point();
>      it->first->vertex((it->second+3)%4)->point();
>
>    }
>
> but the it->...->point(); above are not pointing to elements in x, neither
> it->first->vertex((it->second+1)%4).
>
> Many thanks for any help or, ahem, pointers.
>
> Best regards,
>
> Ramon.
>
>
>
> --
> View this message in context: 
> http://cgal-discuss.949826.n4.nabble.com/Extracting-triangulation-from-alpha-shape-tp4657870.html
> Sent from the cgal-discuss mailing list archive at Nabble.com.