CGAL users discussion list

[Andreas Fabri <>]

Andreas Fabri wrote:
> 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 have to correct myself. It is documented, as it says
Alpha_shape_2<Dt>::Alpha_shape_vertices_iterator
        A bidirectional and non-mutable iterator that allow to traverse the 
vertices
        which belongs to the -shape for the current .
        Precondition:   Its value_type is Dt::Vertex_handle

It is nevertheless surprising.

andreas

> > 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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