CGAL users discussion list

[Jonas Schild <>]

Hi all,

I have the following problem:

I'm constructing a 3d alpha shape from a set of points using CGAL. After 
creation of the alpha shape, I'm iterating over all facets to extract 
the triangles. But:
The orientation of the normal (either by using cross product or using 
CGAL's ortho_vector(point, point, point) of which results are the same) 
is partially wrong. I've read that the orientation of the triangle is 
based on orientation of the cell (i.e. pointing into the cell). However, 
I'm not able to retrieve the normals pointing outside the alpha shape. 
Part of the problem is, that for certain alpha values, the constructed 
shape is no solid object. Is it possible to iterate only over alphas 
where the shape is solid, not necessarily convex?

Furthermore, I don't understand how ccw(int) on the triangulation 
structure is supposed to work regarding finding the correct orientation 
thus the normals are pointing outside the volume.

Hopefully someone could give me a hint or two ;)

Thanks in advance
Jonas