CGAL users discussion list

["Laurent Rineau (CGAL/GeometryFactory)" <>]

Le Friday, April 14, 2017 5:05:36 AM CEST mikef a ?crit :
> I am using CGAL  "make_surface_mesh" to create a mesh out of an ellipsoid
> function, then using OpenGL to draw it by plotting triangle by triangle. I
> have no clue what is going on...
> 
> *Problem:* the plotted surface is weird, some triangle has color, some not,
> some are like holes.
> 
> *My codes for mesh generation:*
>         Surface_3 surface(earth_function,             // pointer to
> function Sphere_3(CGAL::ORIGIN, 7000*7000));
>         // bounding sphere
>         // Note that "2." above is the *squared* radius of the bounding
> sphere!
>         // defining meshing criteria
>         CGAL::Surface_mesh_default_criteria_3 criteria(30.,  // angular
> bound
>                 500,  // radius bound
>                 50); // distance bound
>                          // meshing surface
>         CGAL::make_surface_mesh(c2t3, surface, criteria ,
> CGAL::Manifold_tag());
> 
> *My codes for mesh plotting: *
>         glNewList(EARTH_S, GL_COMPILE);
> 
>         Point_3 pt0, pt1, pt2, pt3;
>         for (auto it = c2t3.facets_begin(); it != c2t3.facets_end(); it++)
> {
> 
>                 pt0 = it->first->vertex((it->second) & 3)->point();
>                 pt1 = it->first->vertex((it->second + 1) & 3)->point();
>                 pt2 = it->first->vertex((it->second + 2) & 3)->point();
>                 pt3 = it->first->vertex((it->second + 3) & 3)->point();
> 
>                 auto vv = pt1 - pt0;
>                 auto n = CGAL::normal(pt1, pt2, pt3);
>                 if (CGAL::angle(n, vv) == CGAL::ACUTE) { n = -n; }
>                 //auto n = ((it->second % 2 ) == 1)?CGAL::normal(pt1, pt2,
> pt3): CGAL::normal(pt1, pt3, pt2);

First: to get the oriented list of three points of a facets, you should use 
the function tr.vertex_triple_index(i, j), a function in a base class of the 
triangulation.

http://doc.cgal.org/latest/TDS_3/
classCGAL_1_1Triangulation__utils__3.html#a71288c8c2833f954a73090336ed60054

Second: the facets of a c2t3 are actually not oriented, and that means that 
their orientation is random. That probably explains your rendering issues. 
You should use the OpenGL lightening model "two sides", and disable the back-
culling.

-- 
Laurent Rineau, PhD
R&D Engineer at GeometryFactory           http://www.geometryfactory.com/
Release Manager of the CGAL Project       http://www.cgal.org/