Using boost geometry to find intersection of line segments

Question

I am trying to use the boost geometry method intersects with my own point class, successfully registered with the boost geometry library.

The boost documentation ( https://www.boost.org/doc/libs/1_73_0/libs/geometry/doc/html/geometry/reference/algorithms/intersection/intersection_3.html ) says that I can use a vector of points as the output parameter. So I wrote this:

#include <iostream>
#include <deque>
#include <boost/geometry.hpp>
#include <boost/geometry/geometries/register/point.hpp>

namespace bg = boost::geometry;
using namespace std;

class cxy
{
public:
    double x;
    double y;
    cxy( double X, double Y )
        : x( X )
        , y( Y )
    {

    }
    /// boost geometry insists on a default constructor
    cxy()
        : cxy(0,0)
    {

    }
};

BOOST_GEOMETRY_REGISTER_POINT_2D( cxy, double, bg::cs::cartesian, x, y )
typedef bg::model::segment<cxy> segment_t;

int main()
{
    cxy a(0,0);
    cxy b(1,1);
    cxy c(1,0);
    cxy d(0,1) ;

    std::vector<cxy> out;

    //////////////// this compiles

    bg::intersection(segment_t{a, b}, segment_t{c, d}, out);

    //////////////// this does not!!!

    segment_t ab( a, b );
    segment_t cd( c, d );
    bg::intersects( ab, cd, out );

    return 0;
}

To be clear: my problem was that I was confused between intersection and intersects

The following code compiles and produces the expected results:

#include <iostream>
#include <deque>
#include <boost/geometry.hpp>
#include <boost/geometry/geometries/register/point.hpp>

namespace bg = boost::geometry;
using namespace std;

class cxy
{
public:
    double x;
    double y;
    cxy( double X, double Y )
        : x( X )
        , y( Y )
    {

    }
    /// boost geometry insists on a default constructor
    cxy()
        : cxy(0,0)
    {

    }
};

BOOST_GEOMETRY_REGISTER_POINT_2D( cxy, double, bg::cs::cartesian, x, y )
typedef bg::model::segment<cxy> segment_t;

int main()
{
    cxy a(0,0);
    cxy b(1,1);
    cxy c(1,0);
    cxy d(0,1) ;

    segment_t ab( a, b );
    segment_t cd( c, d );
    std::vector<cxy> out;
    if( ! bg::intersection( ab, cd, out ) ) {
       std::cout << "no intersection\n";
       return 1;
    }
    std::cout << "intersection at " << out[0].x <<" " << out[0].y << "\n";

    return 0;
}

Answer 1

You're asking "whether a intersects b".

However, your third argument suggests you wanted instead to ask for "the intersection":

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#include <boost/geometry.hpp>
#include <boost/geometry/geometries/register/point.hpp>
#include <boost/geometry/geometries/segment.hpp>
#include <iostream>

namespace bg = boost::geometry;
using namespace std;

struct cxy {
    double x = 0;
    double y = 0;
    cxy(double X, double Y) : x(X), y(Y) {}
    cxy() = default;
};

BOOST_GEOMETRY_REGISTER_POINT_2D(cxy, double, bg::cs::cartesian, x, y)
using segment_t = bg::model::segment<cxy>;
using points_t = bg::model::multi_point<cxy>;

int main() {
    cxy a{0, 0}, b{1, 1}, c{1, 0}, d{0, 1};
    std::vector<cxy> out;
    bg::intersection(segment_t{a, b}, segment_t{c, d}, out);

    // for output, use a multipoint model or register your vector as one
    points_t pts;
    bg::intersection(segment_t{a, b}, segment_t{c, d}, pts);
    std::cout << bg::wkt(pts) << "\n";
}

Prints

MULTIPOINT((0.5 0.5))

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