Boost Geometry: segments intersection not yet implemented? - boost

Question

I am trying a simple test: compute the intersection of 2 segments with Boost Geometry. It does not compile. I also tried with some variations (int points instead of float points, 2D instead of 3D) with no improvement.
Is it really possible that boost doesn't implement segment intersection ? Or what did I do wrong ? Missing some hpp ? Confusion between algorithms "intersects" & "intersection" ?
The code is very basic:
#include <boost/geometry.hpp>
#include <boost/geometry/geometries/point.hpp>
#include <boost/geometry/geometries/segment.hpp>
#include <boost/geometry/algorithms/intersection.hpp>
typedef boost::geometry::model::point<float, 3, boost::geometry::cs::cartesian> testPoint;
typedef boost::geometry::model::segment<testPoint> testSegment;
testSegment s1(
testPoint(-1.f, 0.f, 0.f),
testPoint(1.f, 0.f, 0.f)
);
testSegment s2(
testPoint(0.f, -1.f, 0.f),
testPoint(0.f, 1.f, 0.f)
);
std::vector<testPoint> output;
bool intersectionExists = boost::geometry::intersects(s1, s2, output);
But I got the following errors at compile time by Visual:
- Error C2039 'apply' n'est pas membre de 'boost::geometry::dispatch::disjoint<Geometry1,Geometry2,3,boost::geometry::segment_tag,boost::geometry::segment_tag,false>' CDCadwork C:\Program Files\Boost\boost_1_75_0\boost\geometry\algorithms\detail\disjoint\interface.hpp 54
- Error C2338 This operation is not or not yet implemented. CDCadwork C:\Program Files\Boost\boost_1_75_0\boost\geometry\algorithms\not_implemented.hpp 47

Answer 1

There are indeed two problems:
you're intersecting 3D geometries. That's not implemented
Instead you can do the same operation on a projection.
you're passing an "output" geometry to intersects (which indeed only returns the true/false value as your chosen name intersectionExists suggested). In the presence of a third parameter, it would be used as a Strategy - a concept for which output obviously doesn't satisfy.
Note intersection always returns true: What does boost::geometry::intersection return - although that's not part of the documented interface
Since your geometries are trivially projected onto 2d plane Z=0:
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#include <boost/geometry.hpp>
#include <boost/geometry/geometries/point.hpp>
#include <boost/geometry/geometries/segment.hpp>
#include <iostream>
namespace bg = boost::geometry;
namespace bgm = bg::model;
using Point = bgm::point<float, 2, bg::cs::cartesian>;
using Segment = bgm::segment<Point>;
int main() {
Segment s1{{-1, 0}, {1, 0}};
Segment s2{{0, -1}, {0, 1}};
bool exists = bg::intersects(s1, s2);
std::vector<Point> output;
/*bool alwaysTrue = */ bg::intersection(s1, s2, output);
std::cout << bg::wkt(s1) << "\n";
std::cout << bg::wkt(s2) << "\n";
for (auto& p : output) {
std::cout << bg::wkt(p) << "\n";
}
return exists? 0:1;
}
Prints
LINESTRING(-1 0,1 0)
LINESTRING(0 -1,0 1)
POINT(0 0)