CGAL 5.6.1 - 2D Arrangements
Arrangement_on_surface_2/dual_with_data.cpp
// Checking whether there are three collinear points in a given input set
// using the arrangement of the dual lines.
#include <algorithm>
#include <CGAL/basic.h>
#include <CGAL/Arr_curve_data_traits_2.h>
#include "arr_linear.h"
#include "read_objects.h"
typedef Data_traits::X_monotone_curve_2 Data_x_monotone_curve_2;
typedef CGAL::Arrangement_2<Data_traits> Data_arrangement;
int main(int argc, char* argv[]) {
// Get the name of the input file from the command line, or use the default
// points.dat file if no command-line parameters are given.
const char* filename = (argc > 1) ? argv[1] : "coll_points.dat";
std::vector<Point> points;
read_objects<Point>(filename, std::back_inserter(points));
std::vector<Data_x_monotone_curve_2> dual_lines(points.size());
size_t k{0};
std::transform(points.begin(), points.end(), dual_lines.begin(),
[&](const Point& p) {
Line dual_line(p.x(), -1, -(p.y()));
return Data_x_monotone_curve_2(dual_line, k++);
});
// Construct the dual arrangement by aggregately inserting the lines.
Data_arrangement arr;
insert(arr, dual_lines.begin(), dual_lines.end());
// Look for vertices whose degree is greater than 4.
for (auto vit = arr.vertices_begin(); vit != arr.vertices_end(); ++vit) {
if (vit->degree() > 4) {
// There should be vit->degree()/2 lines intersecting at the current
// vertex. We print their primal points and their indices.
auto circ = vit->incident_halfedges();
for (size_t d = 0; d < vit->degree() / 2; ++d) {
k = circ->curve().data(); // The index of the primal point.
std::cout << "Point no. " << k+1 << ": (" << points[k] << "), ";
++circ;
}
std::cout << "are collinear.\n";
}
}
return 0;
}