[Boost-users] [polygon] Incorrect output from get_trapezoids(..., HORIZONTAL)

Subject: [Boost-users] [polygon] Incorrect output from get_trapezoids(..., HORIZONTAL)
From: Patrick J. LoPresti (lopresti_at_[hidden])
Date: 2014-10-07 15:29:37

I apologize for sending this to the mailing list, but when I tried to
file a bug report via Trac, it said "Trac thinks your submission might
be Spam. To prove otherwise please provide a response to the
following"... And then there was no "following".

Please let me know if there is a better forum to report this sort of issue.

I am using Boost 1.56. The following program uses boost::polygon to
slice up a figure (with a "hole" cut out of it) into horizontal
trapezoids. The output I get is:

Output:
polygon  (4 3) (4 0) (0 0) (0 3) (4 3)

...which is clearly wrong. I am pretty sure my code is correct,
because if I replace bg::HORIZONTAL with bg::VERTICAL, I get the
correct output of a bunch of vertical trapezoids.

For my application, I really want horizontal traps. I suppose I can
just swap x and y on input and output... But this seemed worth
reporting.

Thanks.

 - Pat

#include <boost/polygon/polygon.hpp>

#include <iostream>

namespace bp = boost::polygon;

typedef bp::polygon_set_data<int> PolygonSet;
typedef bp::polygon_data<PolygonSet::coordinate_type> Polygon;
typedef bp::polygon_traits<Polygon>::point_type Point;

std::ostream &
operator<<(std::ostream &os, const Point &pt)
{
    return os << "(" << bp::x(pt) << " " << bp::y(pt) << ")";
}

std::ostream &
operator<<(std::ostream &os, const Polygon &poly)
{
    os << "polygon ";
    for (Polygon::iterator_type p = poly.begin() ; p != poly.end() ; ++p)
        os << " " << *p;
    return os;
}

std::ostream &
operator<<(std::ostream &os, const PolygonSet &ps)
{
    std::vector<Polygon> temp;
    ps.get(temp);
    os << "polygonset ";
    for (std::vector<Polygon>::const_iterator p = temp.begin()
             ; p != temp.end() ; ++p)
        os << " " << *p;
    return os;
}

int
main(int argc, char *argv[])
{
    const Point points[] = {
        Point(2,0),
        Point(4,0),
        Point(4,3),
        Point(0,3),
        Point(0,0),
        Point(2,0),
        Point(2,1),
        Point(1,1),
        Point(1,2),
        Point(3,2),
        Point(3,1),
        Point(2,1)
    };
    Polygon poly;
    bp::set_points(poly, &points[0],
                   &points[sizeof(points)/sizeof(points[0])]);
    std::cout << "Input: " << poly << "\n";

    PolygonSet ps;
    ps.insert(poly);
    std::cout << "ps = " << ps << "\n";

    std::vector<Polygon> traps;
    // Note: bp::VERTICAL works fine
    bp::get_trapezoids(traps, ps, bp::HORIZONTAL);

    std::cout << "Output:\n";
    for (std::vector<Polygon>::const_iterator p = traps.begin()
             ; p != traps.end() ; ++p)
        std::cout << *p << "\n";
}