CGAL users discussion list

[Marius Kintel <>]

Hi all,

As the author of OpenSCAD, I get a lot of user testing of various CGAL 
functions. I experience a lot of CGAL assertions, mostly due to malformed 
data.
This one looks suspiciously like a CGAL bug though:

CGAL error: assertion violation!
Expression : orientation(v1,v2,vz) != COPLANAR
File       : 
/Users/kintel/code/OpenSCAD/libraries/install/include/CGAL/convex_hull_3.h
Line       : 350

Looking at my minimal example below, it appears that CGAL is using two 
different methods for determining whether three points are collinear; a 
collinearity test and a coplanarity test involving a fourth point. The issue 
seems to be that the second test is more strict than the first and this 
second test is being used as an assert.
If you look at the vertices in my code, it’s clear that it’s possible to 
select three non-collinear vertices if a more strict test was used, this 
avoiding the assertion error.

Any idea how to fix or avoid this?

Cheers,

 -Marius

#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
#include <CGAL/convex_hull_3.h>

typedef CGAL::Epick K;

int main()
{
  std::vector<K::Point_3> points;
  points.push_back(K::Point_3(-71.199011502334841, 14.664467308583257, 
32.003440443428268));
  points.push_back(K::Point_3(-71.199011502334841, 26.092309142341527, 
28.509598571991564));
  points.push_back(K::Point_3(-71.199011502334841, 37.520150976099799, 
25.01575670055486));
  points.push_back(K::Point_3(-71.199011502334841, 0, 0));

  CGAL::Polyhedron_3<K> r;
  CGAL::convex_hull_3(points.begin(), points.end(), r);

  return 0;
}