CGAL users discussion list

[Andreas Fabri <>]

Hi Juan Pablo,

Juan Pablo Carbajal wrote:
> Andreas,
> Yes, I have read the documentation.
> The problem is - sorry for my newbiness - that I thought that with
> those iterators I could do as follow
>
> typedef CGAL::Alpha_shape_2<Delaunay> Alpha_shape;
>
> Alpha_shape::Alpha_shape_vertices_iterator  it = 
> A.alpha_shape_vertices_begin();
>
> x = (*it).point().x();
> y = (*it).point().y();
>
> where A is an Alpha_shape_2 created from a list of Point_2.
>
> This doesn't compile. The error says that it has no member "point".



You have to write "->point()" and not ".point()".
I admit that this is more our mistake than yours.  The value_type of
the Alpha_shape_vertices_iterator;  is Vertex_handle. I didn't find that
in the manual, but in the code.

  typedef typename std::list< Vertex_handle >::iterator     
Alpha_shape_vertices_iterator;

So it should be changed or documented.

> I was trying to read more on Vertex_handle but I cannot find
> documentation and the header file is a little too much for my C++
> level.

Admittedly the documentation of the Vertex is "hidden".
An Alpha Shape derives from Delaunay_triangulation_2, derives from 
Triangulation_2
http://www.cgal.org/Manual/3.3/doc_html/cgal_manual/Triangulation_2_ref/Class_Triangulation_2.html#Cross_link_anchor_944
where you find

  "The second parameter is the triangulation data structure, it has to be 
instantiated
    by a model of the concept TriangulationDataStructure_2"

When you click on TriangulationDataStructure_2 you find

  "The concept can be seen as a container for the faces and vertices of the 
triangulation.
   The concept TriangulationDataStructure_2 includes two sub-concepts
   TriangulationDataStructure_2::Vertex and 
TriangulationDataStructure_2::Face."

and here we are
http://www.cgal.org/Manual/3.3/doc_html/cgal_manual/TDS_2_ref/Concept_TriangulationDataStructure_2--Vertex.html#Cross_link_anchor_959



best regards,

andreas

> Thanks again,
>
> JPi
>
>
> On Sun, Oct 5, 2008 at 2:07 PM, Andreas Fabri
> <>
>  wrote:
> > Juan Pablo Carbajal wrote:
> > > Dear all,
> > > I am new to CGAL and I am trying to obtain the 2D alpha shape of a set
> > > of points (in the plane, of course).
> > > Everything is working ok but I just cannot figure out how to get the
> > > list of vertices of the boundary of the alpha shape.
> > > I have checked the discussions about alpha shapes boundaries in the
> > > archive of the list but I could not find the solution there.
> > > Is there something like
> > > Alpha_shape_3::get_alpha_shape_vertices
> > >  for Alpha_shape_2?
> > >
> > > Thanks
> > >
> > > JPi
> > Hi Juan Pablo,
> >
> > There is no iterator that allows you to follow the border, if that
> > is what you are looking for.
> >
> > However, the alpha shape class provides iterators for enumerating
> > the vertices and edges
> >
> > http://www.cgal.org/Manual/3.3/doc_html/cgal_manual/Alpha_shapes_2_ref/Class_Alpha_shape_2.html#Cross_link_anchor_995
> >
> > best regards,
> >
> > andreas
> > --
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