CGAL users discussion list

[Markus Eich <>]

Thank you for the input. My question is now how can I access the 
neighbors of an edge using CGAL?

Is there an overridden function like as.segment(*it).get_neighbor() ? 
The link

http://www.cgal.org/Manual/latest/doc_html/cgal_manual/TDS_2_ref/Concept_TriangulationDSFaceBase_2.html#Cross_link_anchor_1316 


doesn't tell me anything (sorry i am a CGA newbie). If I have the 
segment A from the Alpha_shape_edges_iterator, how can I get the 
adjactend Vertices. How can I extend my little example to get the two 
adjacend edges?

////////////////////
for(Alpha_shape_edges_iterator it =  as.alpha_shape_edges_begin();it != 
as.alpha_shape_edges_end(); ++it) {
        segments.push_back(as.segment(*it));



}

////////////////////

Cheers,

Markus



On 06.06.2011 08:55, Sebastien Loriot (GeometryFactory) wrote:
> Markus Eich wrote:
> > Dear all,
> >
> > I am using CGAL in order the get one or more alpha polygons from a 
> > shape. IN a first approach I assume there are no holes. I generate 
> > the segements as follows:
> >
> >
> >     Alpha_shape_2 as(lp.begin(),lp.end());
> >
> >     // find optimal alpha value
> >
> >     Alpha_iterator opt = as.find_optimal_alpha(1);
> >     as.set_alpha((*opt)*2.0f);
> >     as.set_mode(Alpha_shape_2::REGULARIZED);
> >
> >     std::vector<K::Segment_2> segments;
> >     std::vector<K::Segment_2> sorted_segments;
> >
> >     for(Alpha_shape_edges_iterator it =  
> > as.alpha_shape_edges_begin();it != as.alpha_shape_edges_end(); ++it)
> >         segments.push_back(as.segment(*it));
> >
> >     std::cout << segments.size() << " alpha shape edges" << std::endl;
> >
> >
> > This works fine an I get all the segments. My problem is that I need 
> > a single polygon (in my example I only have one concave shape). If I 
> > iterate over  the std::vector<K::Segment_2> segments vector, the 
> > segments are like E,A,V,D,C,F , instead of A,B,C,D,E,F. In need the 
> > segments in an ordered way i.e. that the  .target() of A is the 
> > .source() of B, the .target() of B the source of C, and so on.
> >
> > How can I achieve this using CGAL? Is there a way to sort the 
> > segments directly?
> There is nothing to do it directly in CGAL.
>
> This is a simple graph problem: each edge has 2 vertices and two
> adjacent edges share one identical vertex. In particular each such
> vertex is adjacent to two other vertices. Starting from one vertex
> and going from adjacent vertex to adjacent vertex you can reconstruct
> the polygon.
>
> You can use for example a
> std::map<Vertex_handle,std::vector<Vertex_handle> >
> to get the adjacency between vertices.
>
> For the definition of an edge as std::pair<Face_handle,int>, see:
> http://www.cgal.org/Manual/latest/doc_html/cgal_manual/TDS_2_ref/Concept_TriangulationDSFaceBase_2.html#Cross_link_anchor_1316 
>
>
>
>
>
> S.
>
>
>
>
> > Thank you and cheers,
> >
> > Markus


--
Dipl. Inf. Markus Eich
Researcher

DFKI Bremen
Robotics Innovation Center
Mary-Somerville-Str. 9
28359 Bremen, Germany

Phone: +49 (0)421 17845-4105
Fax:   +49 (0)421 17845-6613
E-Mail: 


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