Using boost geometry to find intersection of line segments - c++

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))