CGAL users discussion list

[Marius Kintel <>]

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?

Kind Regards,

 -Marius

Attachment: projection.cpp
Description: Binary data