CGAL users discussion list

[Philipp Moeller <(005b7a4cd6%hidden_head%005b7a4cd6)philipp.moeller(005b7a4cd6%hidden_at%005b7a4cd6)geometryfactory.com(005b7a4cd6%hidden_end%005b7a4cd6)>]

Julian Hodgson 
<(005b7a4cd6%hidden_head%005b7a4cd6)julian.of.london(005b7a4cd6%hidden_at%005b7a4cd6)gmail.com(005b7a4cd6%hidden_end%005b7a4cd6)>
 writes:

> I'm trying to build up an array of data per vertex on a Polyhedron e.g.
>
>         vector<FT> vArray( nVertices, 0.0f);
>         Halfedge_handle he_itr = polyhedron.halfedges_begin();
>
> for ( Polyhedron::Facet_iterator fitr = polyhedron.facets_begin(); fitr !=
> polyhedron.facets_end(); fitr++)
> {
> Halfedge_facet_circulator cir = fitr->facet_begin();
>
> do
> {
>    // Get vertex index (as per subdivision tutorial)
>    int i = (int) std::distance( he_itr, Halfedge_handle( cir.operator->()));
>
>    vArray[ i] = // some Data
>
> }
> while ( ++cir != fitr->facet_begin());
> }
>
> Here I'm calculating the vertex index as the distance of the half edge
> iterator from the start half edge.
>
> Is this correct, and why?
>
> Julian

This will be rather inefficient as it requires traversal of the internal
list structure for every index calculation.

You have basically two options to store extra data in a Polyhedron:
  - store indices in every vertex and use those for look-up in your
    data-structure
  - store the data directly in the vertex

I'd prefer the second approach. For this you need to write a new class
that fulfills the PolyhedronItems_3 [1] concept. The example on this
page shows how to do it for a Face. Doing it for a Vertex is analog.

To store indices inside a vertex, do exactly the same.

Footnotes: 
[1]  
http://www.cgal.org/Manual/latest/doc_html/cgal_manual/Polyhedron_ref/Class_Polyhedron_items_3.html#Cross_link_anchor_1116

HTH,
Philipp Moeller
GeometryFactory