CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

 wrote:
> Thank you so much, really! you helped me a lot, and I think I'm very close
> now :).
>
> I have one more question if you don't mind. I noticed that if I change the
> order of the vertices added to a facet the program will give an error, and
> I couldn't understand this. For example, in the second facet, if you add
> the vertices in the order 1-2-3 rather than 1-3-2 (like you did it) the
> program will crash. Could you please tell me why this happens? I mean I'm
> going to create random polyhedra with 20 surfaces or more, how will I
> judge the right order to take?

You have to look at a reference given in the manual page indicated
(for example DCEL in Mark de Berg, Marc van Kreveld, Mark Overmars, and 
Otfried Schwarzkopf. Computational Geometry: Algorithms and 
Applications. Springer-Verlag, Berlin, 1997. )

The idea is that the way an halfedge is oriented indicates on which side 
of the edge the facet described is. If you look at the picture on the 
manual page, you will see that vertices of a facet must be given 
counter-clockwise.

> Thank you,
> Regards,
> Samer Afach
[snip]