CGAL users discussion list

[Erik Engheim <>]

I am using the Polygon_2 class. But I don't seem to get intersection  
tests with "do_intersect" from "Boolean_set_operations_2.h" to work  
properly. Most of the time it works but it doesn't seem to handle  
degenerate cases. In my case when the corner of a polygon intersects  
a line segment on another polygon.

I get the following assertion failure:
CGAL error: assertion violation!
Expr: *slIter == curve
File: /Users/Shared/CGAL-3.2.1/include/CGAL/Basic_sweep_line_2.h
Line: 591

Sometimes I have gotten this failure:
CGAL error: precondition violation!
Expr: Segment_assertions::_assert_is_point_on (p, cv,  
Has_exact_division()) && compare_xy(cv.left(), p) == SMALLER &&  
compare_xy(cv.right(), p) == LARGER
File: /Users/Shared/CGAL-3.2.1/include/CGAL/Arr_segment_traits_2.h
Line: 730

The program below demonstrates the problem. When run I get the first  
assertion failure. Any suggestions to what is causing this? Am I  
using the library wrong or is this a bug in CGAL?

Cheers,
Erik

#include <CGAL/Cartesian.h>
#include <CGAL/basic.h>
#include <CGAL/Point_2.h>

#include <CGAL/Boolean_set_operations_2.h>

#include <iostream>

using namespace std;
using namespace CGAL;

typedef Cartesian<double> K;
typedef Point_2<K> Point2;
typedef Polygon_2<K> Polygon2;

int main()
{
  Point2 small[] = {
    Point2(3.0721924234843      , -1.0673205240039),
    Point2(5.067320524004, -0.9278075765157),
    Point2(4.9278075765157, 1.0673205240039),
    Point2(2.9326794759961, 0.9278075765157)};

  Point2 large[] = {Point2(-3,-3),
                    Point2(3,-3),
                    Point2(3,3),
                    Point2(-3,3)};

  Polygon2 smallPoly(small, small+4);
  Polygon2 largePoly(large, large+4);

  cout << "do intersect? " << do_intersect(smallPoly, largePoly) <<  
endl;
  return 0;
}