CGAL users discussion list

[Andreas Fabri <>]

Hello,

You might call, or have a look at the implementation of
http://doc.cgal.org/4.7/Polygon_mesh_processing/index.html#PMPNormalComp

best,

andreas

On 06/02/2016 18:44, Petros Kataras wrote:
> Hello,
>
> I am trying to render 3d meshes produced by CGAL through OpenGL and I am
> hitting a wall regarding the calculation of properly oriented normals.
>
> I have reduced my test case to the absolute minimal of creating and
> rendering a simple tetrahedron generated from
> Polyhedron_3::make_tetrahedron.
>
> After the creation I am iterating over the vertices of the tetrahedron
> and calculate the per vertex-normal based on the following functions
> that I found :
>
> // Compute facet normal
> template <class Facet, class Kernel>
> typename Kernel::Vector_3 compute_facet_normal(const Facet& f)
> {
>    typedef typename Kernel::Point_3 Point;
>    typedef typename Kernel::Vector_3 Vector;
>    typedef typename Facet::Halfedge_around_facet_const_circulator
> HF_circulator;
>    Vector normal = CGAL::NULL_VECTOR;
>    HF_circulator he = f.facet_begin();
>    HF_circulator end = he;
>    CGAL_For_all(he,end)
>    {
>      const Point& prev = he->prev()->vertex()->point();
>      const Point& curr = he->vertex()->point();
>      const Point& next = he->next()->vertex()->point();
>
>      Vector n = CGAL::cross_product(next-curr,prev-curr);
>      normal = normal + n;
>    }
>    return normal / std::sqrt(normal * normal);
> }
>
> // Compute vertex normal
> template <class Vertex, class Kernel>
> typename Kernel::Vector_3 compute_vertex_normal(const Vertex& v)
> {
>
>    typedef typename Kernel::Vector_3 Vector;
>    typedef typename Vertex::Halfedge_around_vertex_const_circulator
> HV_circulator;
>    typedef typename Vertex::Facet Facet;
>    Vector normal = CGAL::NULL_VECTOR;
>    HV_circulator he = v.vertex_begin();
>    HV_circulator end = he;
>
>    CGAL_For_all(he,end)
>    {
>      if(!he->is_border())
>      {
>        Vector n = compute_facet_normal<Facet,Kernel>(*he->facet());
>        normal = normal + (n / std::sqrt(n*n));
>      }
>    }
>    return normal / std::sqrt(normal * normal);
> }
>
> I would expect this to be enough to give me properly oriented normals
> based on the fact that CGAL maintains CCW order but obviously I am
> missing something since the result is not correct. The normals of the
> front facet especially look like the winding order is reversed for the
> specific triangle. The attached image shows the result that I get.
>
> Is there a way to ensure CCW order on the vertices of facets so that
> normals are pointing outwards or am I totally missing something?
>
> Thanks in advance for any hints,
>
> Petros

--
Andreas Fabri, PhD
Chief Officer, GeometryFactory
Editor, The CGAL Project

phone: +33.492.954.912    skype: andreas.fabri