CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

In particular, you are iterating over all the cells.
You need to use the function get_alpha_shape_cells to get only a subset
or add in your loop over cells:

if (as.classify(it)==Alpha_shape_3::EXTERIOR) continue;

You should also have a look at the function find_optimal_alpha that 
might be helpful in your case.

Sebastien.

http://www.cgal.org/Manual/latest/doc_html/cgal_manual/Alpha_shapes_3_ref/Class_Alpha_shape_3.html

On 09/24/2013 09:24 AM, Mariette Yvinec wrote:
> You might
> look at the alpha_spectrum to see if all the alpha-values
> you have been trying are relevant or not.
>
>
> Le 23/09/13 19:40, evantong623 a écrit :
> > I have a 3D point cloud, and need to determine whether there is a big hole or
> > not. So I extract its convex hull and alpha shape, and measure their
> > volumes. If the volume of convex hull is much larger than the one of alpha
> > shape, I say there exist big hole(s) in the point cloud.
> > To compute the volume of convex hull, I first triangulate the convex hull
> > and then sum up the volumes of the tetrahedrons. To compute the volume of
> > alpha shape, I just sum up the volumes of its cells. Here is the code:
> >
> > //generating convex hull
> > Polyhedron_3 poly;
> > CGAL::convex_hull_3(_bundlePoints[ib].begin(), _bundlePoints[ib].end(),
> > poly);
> >
> > //convex hull volume
> > std::vector<Triangulation::Point>  L;
> > for (Polyhedron_3::Vertex_const_iterator  it = poly.vertices_begin(); it !=
> > poly.vertices_end(); it++)
> >         L.push_back(Triangulation::Point(it->point().x(), it->point().y(),
> > it->point().z()));
> > Triangulation T(L.begin(), L.end());
> > float hull_volume = 0;
> > for(Triangulation::Finite_cells_iterator it = T.finite_cells_begin(); it !=
> > T.finite_cells_end(); it++)
> > {
> >         Tetrahedron_3 tetr = T.tetrahedron(it);
> >         hull_volume += tetr.volume();
> > }
> > cout<<"hull volume:"<<  hull_volume<<  endl;
> >
> > //generating alpha shape
> > Alpha_shape_3 as(_bundlePoints[ib].begin(),
> > _bundlePoints[ib].end(),10000,Alpha_shape_3::GENERAL);
> >
> > //alpha shape volume
> > float alpha_volume = 0;
> > for(Alpha_shape_3::Finite_cells_iterator it = as.finite_cells_begin(); it !=
> > as.finite_cells_end(); it++)
> > {
> >         Tetrahedron_3 tetr = as.tetrahedron(it);
> >         alpha_volume += tetr.volume();
> > }
> > cout<<"alpha volume:"<<  alpha_volume<<  endl;
> >
> > I tried different alpha values, but the volumes of convex hull and alpha
> > shape are always almost the same. Is my volume computation not correct? I am
> > very new to CGAL, hope someone could give me a clue for my problem. Thanks,
> >
> >
> >
> > --
> > View this message in 
> > context:http://cgal-discuss.949826.n4.nabble.com/Why-my-volumes-of-convex-hull-and-alpha-shape-are-almost-the-same-tp4658070.html
> > Sent from the cgal-discuss mailing list archive at Nabble.com.
>
> --
> Mariette Yvinec
> Geometrica project team
> INRIA  Sophia-Antipolis