I am using Clipper and want to determine if two (multi-)polygons intersect.
My expectation was that the library would have a nice, abstract way of asking this question, but it does not seem to.
I thought that the Area()
method might be useful, but it only works on Path
and the Execute()
method returns Paths
.
I've built the following M(almost)WE demonstrating the issue:
#include <iostream>
#include "clipper.hpp"
using namespace ClipperLib;
Paths MakeBox(int xmin, int xmax, int ymin, int ymax){
Paths temp(1);
temp[0] << IntPoint(xmin,ymin) << IntPoint(xmax,ymin) << IntPoint(xmax,ymax) << IntPoint(xmin,ymax);
return temp;
}
bool Intersects(const Paths &subj, const Paths &clip){
ClipperLib::Clipper c;
c.AddPaths(subj, ClipperLib::ptSubject, true);
c.AddPaths(clip, ClipperLib::ptClip, true);
ClipperLib::Paths solution;
c.Execute(ClipperLib::ctIntersection, solution, ClipperLib::pftNonZero, ClipperLib::pftNonZero);
return Area(solution);
}
int main(){
Paths subj = MakeBox(0,10,0,10);
Paths clip1 = MakeBox(1,2,1,2);
Paths clip2 = MakeBox(15,20,15,20);
Intersects(subj,clip1);
Intersects(subj,clip2);
}
It seems as though the simplest way of doing this is to count the number of paths in the Paths
object returned by the Execute()
method. Paths
is a simple vector, so, if it has size()==0
, there is not intersection.
#include <iostream>
#include "clipper.hpp"
using namespace ClipperLib;
Paths MakeBox(int xmin, int xmax, int ymin, int ymax){
Paths temp(1);
temp[0] << IntPoint(xmin,ymin) << IntPoint(xmax,ymin) << IntPoint(xmax,ymax) << IntPoint(xmin,ymax);
return temp;
}
bool Intersects(const Paths &subj, const Paths &clip){
ClipperLib::Clipper c;
c.AddPaths(subj, ClipperLib::ptSubject, true);
c.AddPaths(clip, ClipperLib::ptClip, true);
ClipperLib::Paths solution;
c.Execute(ClipperLib::ctIntersection, solution, ClipperLib::pftNonZero, ClipperLib::pftNonZero);
return solution.size()!=0;
}
int main(){
Paths subj = MakeBox(0,10,0,10);
Paths clip1 = MakeBox(1,2,1,2);
Paths clip2 = MakeBox(15,20,15,20);
Intersects(subj,clip1);
Intersects(subj,clip2);
}