CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

On 01/20/2015 11:10 PM, Marius Kintel wrote:
> Hi all,
>
> I’m trying to intersect a Nef polyhedron with a plane in order to get a 2D 
> cross section.
> However, I frequently experience assertion errors when doing this, even 
> though my Nef polyhedrons appear to be simple and valid 2-manifolds:
> CGAL::Assertion_exception: CGAL ERROR: assertion violation!
> Expr: target(s1).vertex()==target(s2).vertex()
> File: 
> /Users/kintel/code/OpenSCAD/libraries/install/include/CGAL/Nef_3/SNC_FM_decorator.h
> Line: 218
>
> As an example, I’ve attached a minimal test program which intersects an 
> object with the XY plane.
> A failing .nef3 file can be found here: 
> http://files.openscad.org/projection.nef3.gz
>
> FYI: The Nef polyhedron was created by importing an STL mesh into a 
> Polyhedron_3 and creating the Nef polyhedron from there.
>
> I had expected this intersection to be numerically stable, but perhaps I’ve 
> slightly misunderstood the limitations of operations on Nef polyhedrons. Any 
> ideas?

There is a header CGAL/Polyhedron_slicer_3.h that is doing what you want 
but on a polyhedron. The code is not totally robust (in particular when 
you have coplanar facets in the plane) for now but the upcoming version 
will be (code is ready but we're in code freeze so it will be in 4.7).

I can share the robust version of the code if needed.

Sebastien.

> Kind Regards,
>
>   -Marius