CGAL users discussion list

[Mariette Yvinec <>]

Hi Jonas

First I am getting back to your first question.

If you would like to get an alpha_shape that is a solid withb a manifold 
boundary
you have to use the REGULARIZED version of Alpha_shapes_3
that is the one which is computed by defaut.

If you  don't want to use the REGULARIZED version of the Alpha_shapes
you can still select the facets of type REGULAR
that are facets of the alpha_shapes bounding a 3D component
of the Alpha_shapes.
Then to find the outside normals, check if the facet
is described with the incident cell that is inside
the alpha_shapes or with the one that is outside.
(Every facets has two descriptions)
You can for instance run :
if( A.classify(facet.first) == INTERIOR)
     facet = A.mirror_facet(facet); // which gives the same facet 
viewed from the other adjacent cells

Then  as Monique told you,
t.triangle(facet.first, facet.second) is oriented
so that .....






Jonas Schild wrote:
> 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

--
Mariette Yvinec
Geometrica project team
INRIA  Sophia-Antipolis