CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

On 03/22/2013 04:39 AM, wenchao wrote:
> Dear CGAL developers,
>
> I encountered a problem when using incremental builder to build a
> polyhedron. I used the add_vertex() and add_vertex_to_facet() to construct a
> polyhedron from a triangulation. It looks something like:
>
> //////////////////////////////////////////////////////////////////////////////////
> builder.begin_surface(points.size(), points.size()/3,
> CGAL::Polyhedron_incremental_builder_3<HDS>::ABSOLUTE_INDEXING);
>
> for (unsigned int i = 0; i<  points.size(); i++)
> {
>          Vertex_handle vh = builder.add_vertex(points[i].p);
>          vh->group_id = points[i].group_id;
>          vh->mp = points[i].prj_pnt;
> }
>
> some_function();
>
> builder.end_surface();
>
> //////////////////////////////////////////////////////////////////////////////
>
> void some_function(unsigned int i, unsigned int j, unsigned int k)
> {
>
> // some orientation code check here.
>
>
> b.begin_facet();
> b.add_vertex_to_facet(k);
> b.add_vertex_to_facet(j);
> b.add_vertex_to_facet(i);
> b.end_facet();
> }

A mistake could be that the orientation of the facets are not consistent.

Looking here:
http://www.cgal.org/Manual/latest/doc_html/cgal_manual/Triangulation_3/Chapter_main.html#Section_39.1

you can see how the orientation should be.

if i is the index opposite to the face you want to display,
the following code should orient the facets of the polyhedron
counterclockwise when seen from outside.

int indices[3]={(i+1)%4,(i+2)%4,(i+3)%4};
if (i%2==0) swap(indices[1],indices[2]);
b.begin_facet();
b.add_vertex_to_facet(indices[0]);
b.add_vertex_to_facet(indices[1]);
b.add_vertex_to_facet(indices[2]);
b.end_facet();

> ///////////////////////////////////////////////////////////////////////////////
>
> However, after I built this polyhedron P, the validity check
> P.is_valid(false, 1) always fails.
>
> More surprisingly, I run the example in
> examples/Polyhedron/polyhedron_prog_incr_builder.cpp. I added the same check
> in this example. But it still failed.
What check are you talking about?

Sebastien.

> Can anyone help me find the reason for this trouble?
>
> Thanks a lot!
>
> Best,
> Wenchao
>
>
>
>
>
> --
> View this message in context: 
> http://cgal-discuss.949826.n4.nabble.com/Troubles-in-converting-triangulation-to-polyhedron-tp4657006.html
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