CGAL users discussion list

[Gard Spreemann <>]

Hello list,

I'm seeing a behavioral difference between constructing a
Nef_polyhedron_3 from a Polyhedron_3, and reading the former directly
from an OFF file using one of the supplied IO methods. I'm sure this
boils down to a misunderstanding of mine, and I'm very grateful if
anyone has any insight on the matter.

Suppose we have two OFF files:

a.off:

 OFF
 5 5 10
 -0.193282 0.100467 0.339231
 -0.183855 0.102082 0.334972
 -0.183473 0.103303 0.332011
 -0.181318 0.104688 0.348585
 0.000000 0.000000 0.000000
 3 4 0 1
 3 4 1 2
 3 4 2 3
 3 4 3 0
 4 3 2 1 0

b.off:

 OFF
 8 6 12
  1.0  0.0 1
  0.0  1.0 1
 -1.0  0.0 1
  0.0 -1.0 1
  1.0  0.0 0.0
  0.0  1.0 0.0
 -1.0  0.0 0.0
  0.0 -1.0 0.0
 4 0 1 2 3
 4 7 4 0 3
 4 4 5 1 0
 4 5 6 2 1
 4 3 2 6 7

Define for brevity's sake the following common.hpp:

 #pragma once
 #include <iostream>
 #include <fstream>
 #include <cassert>
 #include <CGAL/Exact_predicates_exact_constructions_kernel.h>
 #include <CGAL/Polyhedron_3.h>
 #include <CGAL/Nef_polyhedron_3.h>
 #include <CGAL/IO/Polyhedron_OFF_iostream.h>
 #include <CGAL/OFF_to_nef_3.h>
 using Exact_kernel = CGAL::Exact_predicates_exact_constructions_kernel;
 using Polyhedron = CGAL::Polyhedron_3<Exact_kernel>;
 using Surface_mesh = CGAL::Surface_mesh<Exact_kernel::Point_3>;
 using Nef_polyhedron = CGAL::Nef_polyhedron_3<Exact_kernel>;

The following code builds a `Nef_polyhedron_3` by reading an OFF file
directly:

 #include "common.hpp"
 int main(int argc, char ** argv)
 {
   Nef_polyhedron nef_poly;
   std::ifstream is("test.off", std::ios::in);
   CGAL::OFF_to_nef_3(is, nef_poly);
   is.close();
   return 0;
 }

That code does what I would expect with both of the above OFF
files.

The following code, however, builds a Polyhedron_3 by reading an OFF
file, and then converts that Polyhedron_3 into a Nef_polyhedron_3. This
code does what I'd expect on b.off:

 #include "common.hpp" 
 int main(int argc, char ** argv)
 {
   Polyhedron poly;
   std::ifstream is("test.off", std::ios::in);
   bool success = CGAL::IO::read_OFF(is, poly);
   is.close();
   assert(success);
   assert(poly.is_valid());
   assert(poly.is_closed()); 
   Nef_polyhedron nef_poly(poly); // <--- X (see below).
   return 0;
 }

However, then the latter code is used with a.off, it crashes at the
point marked X due to a failed assertion:

 terminate called after throwing an instance of 'CGAL::Assertion_exception'
   what():  CGAL ERROR: assertion violation!
 Expr: ss_circle.has_on(sp)
 File: /usr/include/CGAL/Nef_3/polygon_mesh_to_nef_3.h
 Line: 253


Can anyone help illuminate why I'm confused about these two snippets of
code not producing the same Nef_polyhedron_3?


 Best,
 Gard

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