CGAL users discussion list

[Monique Teillaud <>]

Hi Pratyush,

thanks for the mail. What you suggest sounds good below. Actually, outputting the convex hull as a polyhedron_3 would have made my life much simple. But i think there is a problem. I have additional information stored on the vertices of the triangulation. I guess when I output the convex hull as a polyhedron, i lose this additional information on the vertices?

I would “guess” the opposite, but in general I don’t trust guesses… I trust the manual better, and/or results of experiments. 

I guess i could circumvent this problem if I can also inherit the polyhedron from the vertex_base_with_if_3 class ( the name of the class is not accurate probably, i am spewing it out of my mind). Do you think it is possible to inherit poly from vertex with info? I am sorry, it has been many years since i touched CGAL, and i am on a bit of shaky ground here.

Alternatively, if this is not possible,

to be checked

I guess I can work with the original triangulation by restricting myself to the simplifies on the convex hull only. I think, for this i need to devise a function that checks if a particular simplex is on the convex hull or not. I write below my idea of how to check if a particular simplex is on the hull.

— for a vertex it is trivial, because by design they are al on the surface of the sphere (or do i need additional checks?)

you can check if it is incident to the infinite vertex

— for an edge, i plan to circulate over all the incident facets, and if one of the facet is an infinite facet, the edge must be on the convex hull (?)

yes (provided that this edge is not itself infinite, of course)

— i am not sure how to check this for facets. For a particular facet, i could again check for all the edges that constitute it, and look if they are on the convex hull. if all the edges of the facet are on the hull, the facet must be on the hull?

Similar. If the facet is not infinite, and if one of its incident cells is infinite, then the facet is on the convex hull. 

--

Monique Teillaud 
http://www.loria.fr/~teillaud/ 
INRIA Nancy - Grand Est, LORIA 
Institut National de Recherche en Informatique et Automatique 

On 5 Apr 2016, at 15:39, Monique Teillaud <> wrote:

Hi Pratyush,

I suggest that you have a closer look at the manual: at http://doc.cgal.org/latest/Triangulation_3/classCGAL_1_1Triangulation__3.html you will find functions is_infinite(vertex/ cell/ facet/ edge). 

If you are using convex hulls, IIRC one possibility is to get a Polyhedron as output, then I guess that you have iterators/circulators to traverse the surface. However, the manual is more reliable than my memory… 

Best,

--

Monique Teillaud 
http://www.loria.fr/~teillaud/ 
INRIA Nancy - Grand Est, LORIA 
Institut National de Recherche en Informatique et Automatique