CGAL users discussion list

[Forest Darling <>]

Hello,

I have a "CGAL::Gps_circle_segment_traits_2" Polygon_2  that I generate and manipulate in CGAL. When trying to take the orientation of the polygon (one of the first operations I do), CGAL fails a precondition assertion. This only happens if I use the "CGAL::Simple_cartesian<double>" kernel, it works with the "CGAL::Exact_predicates_exact_constructions_kernel" kernel.

It's a very basic shape, so I was trying to investigate the issue by creating a minimal C++ example to demonstrate the problem. However, I ran into another problem: while I can export the Polygon_2 as binary, I cannot import it!

It seems to be that while General_polygon_2 has the ability to read from an istream:

https://github.com/CGAL/releases/blob/bca0a8035cc3dafaba6f5c0888559c67f372223f/include/CGAL/General_polygon_2.h#L163

The "Traits::X_monotone_curve_2" which resolves to "_X_monotone_circle_segment_2" in the case of "CGAL::Gps_circle_segment_traits_2" only supports exporting to an ostream, not importing from an istream:

https://github.com/CGAL/releases/blob/bca0a8035cc3dafaba6f5c0888559c67f372223f/include/CGAL/Arr_geometry_traits/Circle_segment_2.h#L2477

Is this an oversight? Is it by design? Or am I missing something here? I attached a main.cpp file that will give the compilation error when trying to deserialize the Polygon_2.

I'd appreciate any help!

Thanks,

--Forest

#include <iostream>

// #define USE_EXACT_MATH

#ifdef USE_EXACT_MATH
#include <CGAL/Exact_predicates_exact_constructions_kernel.h>
#else // !defined(USE_EXACT_MATH)
#include <CGAL/Simple_cartesian.h>
#endif // USE_EXACT_MATH

#include <CGAL/Gps_circle_segment_traits_2.h>
#include <CGAL/General_polygon_set_2.h>
#include <CGAL/General_polygon_2.h>

#ifdef USE_EXACT_MATH
typedef CGAL::Exact_predicates_exact_constructions_kernel Kernel;
#else // !defined(USE_EXACT_MATH)
typedef CGAL::Simple_cartesian<double> Kernel;
#endif // USE_EXACT_MATH

typedef Kernel::Point_2                                   Point_2;
typedef Kernel::Circle_2                                  Circle_2;
typedef Kernel::Line_2                                    Line_2;
typedef CGAL::Gps_circle_segment_traits_2<Kernel>         Traits_2;
typedef CGAL::General_polygon_set_2<Traits_2>             Polygon_set_2;
typedef Traits_2::Polygon_2                               Polygon_2;
typedef Traits_2::Polygon_with_holes_2                    Polygon_with_holes_2;
typedef Traits_2::Curve_2                                 Curve_2;
typedef Traits_2::X_monotone_curve_2                      X_monotone_curve_2;

int main(int argc, char **argv)
{
    Polygon_2 poly;
    
    // read Polygon_2 from binary file
    {
        std::ifstream f("bad.bin");
        CGAL::set_binary_mode(f);
        f >> poly;
    }

    std::cout << "poly = " << poly << std::endl;
    const int orient = poly.orientation();
    std::cout << "poly.orientation() = " << orient << std::endl;

    return 0;
}

Attachment: bad.bin
Description: Binary data