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[Eivind LM <>]

Greetings,
I'm completely new to CGAL, and am trying to model a 2D terrain surface in 3D 
using Triangulation_2 and Triangulation_euclidean_traits_xy_3. My application 
requires me to add points to an existing triangulation.

During insertion of points, I get an assertion error which to me indicates 
that the triangulation has become invalid at some point.

But as far as I have understood the documentation I should be able to insert 
vertices at any point in a triangulation, and CGAL should keep the 
triangulation valid. Is this correct? Please find below example code to 
reproduce my problem.

Best regards,
Eivind Lyche Melvaer

Sample code to reproduce the assertion failure, using CGAL version 3.3.3:
#include <CGAL/basic.h>
#include <CGAL/Triangulation_euclidean_traits_xy_3.h>
#include <CGAL/Triangulation_2.h>
#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>


typedef CGAL::Exact_predicates_inexact_constructions_kernel Kernel;
typedef Kernel::Point_3                                     Point_3;
typedef CGAL::Triangulation_euclidean_traits_xy_3<Kernel>   
TriangulationTraits;
typedef CGAL::Triangulation_2<TriangulationTraits>          Triangulation;


int main(int argc, char** argv)
{
  Triangulation t;

  //Add two triangles, to represent a coarse triangulation of the unit square
  t.insert(Point_3(0, 0, 0));
  t.insert(Point_3(1, 0, 0));
  t.insert(Point_3(0, 1, 0));
  t.insert(Point_3(1, 1, 0));

  int numi = 10;
  int numj = 10;

  //Then a 10x10 grid of vertices within the unit square are inserted.
  for(int i = 0; i < numi; i++) {
    for( int j = 0; j < numj; j++) {
      std::cout << i << " " << j << std::endl;
      double x = i*1.0/(numi-1);
      double y = j*1.0/(numj-1);
      double z = 0;
      t.insert(Point_3(x, y, z));
    }
  }
}

The assertion failure:
CGAL error: precondition violation!
Expr: oriented_side(f,p) == ON_POSITIVE_SIDE
File: /mn/sarpanitu/ansatte-u21/eivindlm/env/include/CGAL/Triangulation_2.h
Line: 920
terminate called after throwing an instance of 'std::logic_error'
  what():  basic_string::_S_construct NULL not valid
Aborted