CGAL users discussion list

[Andreas Fabri <>]

Thomas Zangl - Home wrote:
>  
> Am Wed, 15 Aug 2007 14:25:50 -0400, schrieb "Satyajit Sarangi" <>:
>
> Hi!
>
> > 1) How do i extract an edge from a triangle i have in 3D delaunay. (Not from 
> > the cell ) since i extract a triangle from each cell. So how do i extract 
> > each edge from the triangle.
>
> You mean, you have a facet iterator and you process each facet of the
> triangulation?
> You can easily get the edges of the given facet:
>
> // fSl = facet iterator 
> Cell_handle cell = fSl->first;
> // this is the index of the facet's opposite vertex
> int vertexIdx = fSl->second;
>
> // get the three points forming a facet
> Point p1=cell->vertex((vertexIdx+1) % 4)->point();
> Point p2=cell->vertex((vertexIdx+2) % 4)->point();
> Point p3=cell->vertex((vertexIdx+3) % 4)->point();

Note that there is also a static helper function in all
3D triangulations, coming from the base class Triangulation_utils_3


static int vertex_triple_index(const int i, const int j)
  {
    // indexes of the  jth vertex  of the facet of a cell
    // opposite to vertx i


What is nice about it, is that the indices are then
in counterclockwise order, which you cannot guarantee
with the mod 4

andreas


> // construct the edges
> Segment sideA(p1,p2);
> Segment sideB(p2,p3);
> Segment sideC(p3,p1);
>
> Now you have the edges defined by the respective vertices. 
>
> > 2) How do i get all the other triangles incident to this edge.
>
> You can use       t.incident_facets ( Cell_handle c, int i, int j).
>
> Given the possibility above, you have an cell handle (Variable "cell")
> and you have the vertexIdx (integer). The vertexIdx is the vertex
> opposite of the facet in the cell. E.g. you have a cell formed by four
> vertices ABCD and you look at the facet formed by ABC then is D the
> opposite vertice of the facet ABC. 
>
> In fact you need the vertexIdx to define to parameters i and j. 
> e.g. if you want sideA of the example above, you will write:
>
> Facet_circulator circ = t.incident_facets(cell, vertexIdx+1, vertexIdx+2
> ); 
>
> for sideB you will write:
> Facet_circulator circ = t.incident_facets(cell, vertexIdx+2, vertexIdx+3
> ); 
>
> and so on. Please keep in mind that I defined sideA as the side
> connecting the vertices A-B, etc. This might be different from you
> understanding. I hope you got the conecept :-) 
>
> Best regards,