what is boost::geometry::correct doing in this case? - c++

Question

The following code generates the output I expect:
MULTILINESTRING((5 5,4 4),(2 2,1 1))
However, if I remove the call to boost::geometry::correct() it returns the incorrect result:
MULTILINESTRING((5 5,1 1))
Code below:
#include <boost/geometry.hpp>
#include <boost/geometry/geometries/polygon.hpp>
#include <boost/geometry/geometries/linestring.hpp>
#include <boost/geometry/multi/geometries/multi_linestring.hpp>
#include <iostream>
namespace bg = boost::geometry;
namespace bgm = boost::geometry::model;
using point = bgm::point<double, 2, bg::cs::cartesian>;
using polygon = bgm::polygon<point>;
using polyline = bgm::linestring<point>;
using polylines = bgm::multi_linestring<polyline>;
int main()
{
polygon poly = {
{ {1,5}, {5,5}, {5,1}, {1,1} },
{ {2,4}, {2,2}, {4,2}, {4,4} }
};
polyline line = { {6,6},{0,0} };
bg::correct(poly);
polylines result;
bg::intersection(poly, line, result);
std::cout << bg::wkt(result) << "\n";
return 0;
}
The geometry defined in the above looks like the following. The red line segment and blue polygon with a hole should intersect to the green line segments.
I defined the vertices of the hole in counter-clockwise order as it is typical in computational geometry for holes to have reverse orientation of non-holes (It's also typical for non-holes to have counter-clockwise orientation but boost::geometry seems to default to clockwise). If I flip the orientation of the hole it does not fix the problem (although I do get a different wrong result). I am not sure what else correct could be doing.

Answer 1

boost::correct() is closing both the inner and outer polygons.
That is, the following returns the expected output:
namespace bg = boost::geometry;
namespace bgm = boost::geometry::model;
using point = bgm::point<double, 2, bg::cs::cartesian>;
using polygon = bgm::polygon<point>;
using polyline = bgm::linestring<point>;
using polylines = bgm::multi_linestring<polyline>;
int main()
{
polygon poly = {
{ {1,5}, {5,5}, {5,1}, {1, 1}, {1,5}},
{ {2,4}, {2,2}, {4,2}, {4,4}, {2,4}}
};
polyline line = { {6,6},{0,0} };
polylines result;
bg::intersection(poly, line, result);
std::cout << bg::wkt(result) << "\n";
return 0;
}