CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

On 07/12/2014 05:14 PM, donben wrote:
> Hello and thank you for the impressive work on CGAL.
>
> I'm beginning with the library and trying to use the Surface_mesher example.
> What I want to do is to get positions and indices to feed VBOs that I can
> display with glDrawArray.
>
> Thanks to this post
> (http://cgal-discuss.949826.n4.nabble.com/Need-to-write-facets-created-from-Implicit-surface-meshing-td951670.html)
> I'm able to get the vertices of the Complex_2_in_triangulation_3 and display
> them (nice skull !).
>
> But my indices array seems uncorrect and the resulting displayed triangles
> are disordered.
> Is there a simple example code to build VBOs adn draw them ?
>
> And how to access normals in the result if they are performed ?
>
> Thank you and sorry for my bad english.
>
> Benoît

If you're using something like the following:

const Vertex_handle vh1( c.vertex( (index + 1)%4 ) );
const Vertex_handle vh2( c.vertex( (index + 2)%4 ) );
const Vertex_handle vh3( c.vertex( (index + 3)%4 ) );
_face_indices.push_back( vh_to_index_map[vh1] );
_face_indices.push_back( vh_to_index_map[vh2] );
_face_indices.push_back( vh_to_index_map[vh3] );

your facets will be inconsistently oriented.
You need something like the following:

Vertex_handle vh1 = c.vertex( c2t2.vertex_triple_index(index,1)%4 );
Vertex_handle vh2 = c.vertex( c2t2.vertex_triple_index(index,2)%4 );
Vertex_handle vh3 = c.vertex( c2t2.vertex_triple_index(index,3)%4 );

No normal vector is computed in the c2t3.
Here is some pseudo code for one way to do it.

std::size_t nb_vertices=XXX;
std::vector< K::Vector_3 > normals(CGAL::NULL_VECTOR, nb_vertices);
std::vector< int > nb_incident_facets(0, nb_vertices);

foreach facet f:
  K::Vector_3 n = normal of f; //using CGAL::cross_product
  Vertex_handle vh1, vh2, vh3;
  //assuming index_of(vh) is a function returning the id of vh
  normals[index_of(vh1)] = normals[index_of(vh1)] + n;
  ++nb_incident_facets[index_of(vh1)];
  //same for vh2, vh3

foreach vertex_handle vh:
  normals[index_of(vh)] = normals[index_of(vh)] /
  nb_incident_facets[index_of(vh1)];

// normals now contains non-normalized normal vector per vertex

HTH

Sebastien.

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