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Thank you. It worked! However I have a problem I don't seem to be  
able to work around. Using  
Exact_predicates_exact_constructions_kernel slows don't the code so  
much that it becomes almost unusable. Is there a way around this? I  
don't really care for accuracy. What I need is the code to not crash/ 
fail. I would prefer if I could use double for the intersection test  
but that it didn't give assertion failures in the degenerate cases.  
E.g. it would be ok if the code just chose not to treat those cases.

On  20 Jan 2007, at 10:30 AM, Efi Fogel wrote:

> Sounds like a classic case of  limited precision problem.
> Try to use an exact number type. For example:
>
> #include <CGAL/Exact_predicates_exact_constructions_kernel.h>
>
> typedef CGAL::Exact_predicates_exact_constructions_kernel  Kernel;
>
> Erik Engheim wrote:
>
> > 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;
> > }
>
>
>
> --
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