CGAL users discussion list

[Ben Supnik <>]

Hi Y'all,

I am using get_intersection_points_2 to try to find all places where 
bezier curves intersect each other (excluding end-points) - I am using 
the standard bezier curve traits from the arrangement package (e.g. from 
the demo code) with CORE for my numeric type.

The surprising result is that if I pass in 4 curves (each with only two 
control points, so they are really line segments) I get an assertion 
failure at Bezier_bounding_rational_trait:247... there are 5 
intersection pairs, not just 1.

My "curves" are really quite boring...just a bow-tie shape.

Are there any known gotchas here?  I've posted a heavily cut down sample 
reproducer here:

http://dev.x-plane.com/download/test_bez.cpp

"interesting" part of the code is at the end of the email.  The output 
gives me:

CGAL error: assertion violation!
Expr: intersection_pairs.size() == 1
File: 
../../libs/local/include/CGAL/Arr_traits_2/Bezier_bounding_rational_traits.h
Line: 247
Explanation:
terminate called after throwing an instance of 'CGAL::Assertion_exception'
  what():  CGAL ERROR: assertion violation!
Expr: intersection_pairs.size() == 1
File: 
../../libs/local/include/CGAL/Arr_traits_2/Bezier_bounding_rational_traits.h
Line: 247
Abort trap
mac-pro:XESTools bsupnik$ scp test_bez.cpp 
:/shared/download/test_bez.cpp

Anyway, if I am doing something really, really idiotic, please let me know!!

cheers
ben


        vector<Rat_point_2>       c;
        vector<Bezier_curve_2> curves;
        vector<Bezier_point_2>    p;
        
        c.resize(2);
        c[0] = Rat_point_2(-71.026688,42.395582);
        c[1] = Rat_point_2(-71.011037,42.396439);
        curves.push_back(Bezier_curve_2(c.begin(),c.end()));

        c[0] = Rat_point_2(-71.011037,42.396439);
        c[1] = Rat_point_2(-71.010892,42.390653);
        curves.push_back(Bezier_curve_2(c.begin(),c.end()));

        c[0] = Rat_point_2(-71.010892,42.390653);
        c[1] = Rat_point_2(-71.022341,42.400083);
        curves.push_back(Bezier_curve_2(c.begin(),c.end()));

        c[0] = Rat_point_2(-71.022341,42.400083);
        c[1] = Rat_point_2(-71.026688,42.395582);
        curves.push_back(Bezier_curve_2(c.begin(),c.end()));

        Bezier_traits_2 traits;
        CGAL::get_intersection_points(curves.begin(),curves.end(),
                back_inserter(p), false, traits);


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