CGAL users discussion list

[Efi Fogel <>]

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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