CGAL users discussion list

[Benoît Presles <>]

Le 26/08/2010 10:52, Sebastien Loriot (GeometryFactory) a écrit :
> Benoît Presles wrote:
> > > > Hello Everybody,
> > > >
> > > > I think a new bug could be present in the "Alpha_shape_2" class (the
> > > > problem can be reproduced by using the list of points "fin" attached
> > > > in this email).
> > > >
> > > > First, I compute the alpha shape (GENERAL MODE).
> > > > Second, I print the segments which belongs to the alpha shape.
> > > > Third, I print the points which belongs to the alpha shape
> > > > (Alpha_shape_vertices_iterator).
> > > >
> > > > The problem is that I get too many points printed, for example the
> > > > point (60,50) doesn't belong to the alpha shape but is printed.
> > > >
> > > > Output of the program:
> > > > Reading 11 points from file
> > > > Alpha Shape computed
> > > > 7 alpha shape edges
> > > > 7 0 4 4
> > > > 4 4 1 0
> > > > 0 0 1 0
> > > > 1 0 2.5 0
> > > > 2.5 0 4.5 0
> > > > 4.5 0 7 0
> > > > 16 16 20 20
> > > > as_it=4.5 0
> > > > as_it=2.5 0
> > > > as_it=4 4
> > > > as_it=7 0
> > > > as_it=1 0
> > > > as_it=0 0
> > > > as_it=8 12
> > > > as_it=16 16
> > > > as_it=20 20
> > > > as_it=40 40
> > > > as_it=60 50
> > >
> > > It behaves as expected, vertices printed are either REGULAR or SINGULAR
> > > in GENERAL mode and REGULAR in REGULARIZED mode.
> >
> > Maybe I don't understand something but in my point of view, in both
> > cases (GENERAL or REGULARIZED mode), the vertices printed with
> > "Alpha_shape_vertices_iterator" should be the same as the one printed
> > with "Alpha_shape_edges_iterator" and it is not the case here.
> A vertex is singular if it's in the alpha complex but
> no incident simplex is in the alpha complex.
>
> If you only want to have vertices that are on the boundary but not
> singular, you need to use either the REGULARIZED mode (it was designed
> for this purpose I guess) or if you need to use the GENERAL mode, just
> consider regular vertices.

OK but with the alpha value used (FT(10)) the point (60,50) doesn't 
belong to the alpha complex.

Thank you for your help,
Best Regards,
Ben_P

> S.
>
> > Thank you very much for your help,
> > Ben_P
> >
> > > S.