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  2. How to print the faces of a Voronoi diagram?

How to print the faces of a Voronoi diagram?

由 只谈情不闲聊 提交于 2019-12-05 16:13:34

I needed to print the faces of the Voronoi diagram as well-known text polygons. This entailed iterating over the edges of the faces and giving finite representations to the infinite points. I did so as follows.

//Generate WKT polygons from Voronoi cells using CGAL
//Compile with: g++ main.cpp -Wall -lCGAL -lgmp
//Author: Richard Barnes (rbarnes.org)
#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
#include <CGAL/Regular_triangulation_filtered_traits_2.h>
#include <CGAL/Regular_triangulation_adaptation_traits_2.h>
#include <CGAL/Regular_triangulation_adaptation_policies_2.h>
#include <CGAL/Regular_triangulation_2.h>
#include <CGAL/Voronoi_diagram_2.h>
#include <CGAL/intersections.h>
#include <CGAL/bounding_box.h>
#include <CGAL/Polygon_2.h>
#include <iostream>
#include <cstdint>

typedef CGAL::Exact_predicates_inexact_constructions_kernel K;
typedef CGAL::Regular_triangulation_filtered_traits_2<K>  Traits;

typedef CGAL::Regular_triangulation_2<Traits> RT2;
typedef CGAL::Regular_triangulation_adaptation_traits_2<RT2>         AT;
typedef CGAL::Regular_triangulation_degeneracy_removal_policy_2<RT2> DRP;
typedef CGAL::Voronoi_diagram_2<RT2, AT, DRP> VD;

int main(int argc, char **argv){
  std::vector<RT2::Weighted_point> wpoints;

  std::cout.precision(4);
  std::cout.setf(std::ios::fixed);

  //Generated random points
  for(int i=0;i<100;i++)
    //Weight of 0 gives a Voronoi diagram. Non-zero weight gives a power diagram
    wpoints.push_back(RT2::Weighted_point(K::Point_2(rand()%100,rand()%100), 0)); 

  //Create a Regular Triangulation from the points
  RT2 rt(wpoints.begin(), wpoints.end());
  rt.is_valid();

  //Wrap the triangulation with a Voronoi diagram adaptor. This is necessary to
  //get the Voronoi faces.
  VD vd(rt);

  //CGAL often returns objects that are either segments or rays. This converts
  //these objects into segments. If the object would have resolved into a ray,
  //that ray is intersected with the bounding box defined above and returned as
  //a segment.
  const auto ConvertToSeg = [&](const CGAL::Object seg_obj, bool outgoing) -> K::Segment_2 {
    //One of these will succeed and one will have a NULL pointer
    const K::Segment_2 *dseg = CGAL::object_cast<K::Segment_2>(&seg_obj);
    const K::Ray_2     *dray = CGAL::object_cast<K::Ray_2>(&seg_obj);
    if (dseg) { //Okay, we have a segment
      return *dseg;
    } else {    //Must be a ray
      const auto &source = dray->source();
      const auto dsx     = source.x();
      const auto dsy     = source.y();
      const auto &dir    = dray->direction();
      const auto tpoint  = K::Point_2(dsx+1000*dir.dx(),dsy+1000*dir.dy());
      if(outgoing)
        return K::Segment_2(
          dray->source(),
          tpoint
        );
      else
        return K::Segment_2(
          tpoint,
          dray->source()
        );
    }
  };

  std::cout<<"\"id\",\"geom\"\n";

  int fnum = 0;
  //Loop over the faces of the Voronoi diagram in some arbitrary order
  for(VD::Face_iterator fit = vd.faces_begin(); fit!=vd.faces_end();++fit,fnum++){
    CGAL::Polygon_2<K> pgon;

    //Edge circulators traverse endlessly around a face. Make a note of the
    //starting point so we know when to quit.
    VD::Face::Ccb_halfedge_circulator ec_start = fit->ccb();

    //Find a bounded edge to start on
    for(;ec_start->is_unbounded();ec_start++){}

    //Current location of the edge circulator
    VD::Face::Ccb_halfedge_circulator ec = ec_start;

    do {
      //A half edge circulator representing a ray doesn't carry direction
      //information. To get it, we take the dual of the dual of the half-edge.
      //The dual of a half-edge circulator is the edge of a Delaunay triangle.
      //The dual of the edge of Delaunay triangle is either a segment or a ray.
      // const CGAL::Object seg_dual = rt.dual(ec->dual());
      const CGAL::Object seg_dual = vd.dual().dual(ec->dual());

      //Convert the segment/ray into a segment
      const auto this_seg = ConvertToSeg(seg_dual, ec->has_target());

      pgon.push_back(this_seg.source());

      //If the segment has no target, it's a ray. This means that the next
      //segment will also be a ray. We need to connect those two rays with a
      //segment. The following accomplishes this.
      if(!ec->has_target()){
        const CGAL::Object nseg_dual = vd.dual().dual(ec->next()->dual());
        const auto next_seg = ConvertToSeg(nseg_dual, ec->next()->has_target());
        pgon.push_back(next_seg.target());
      }
    } while ( ++ec != ec_start ); //Loop until we get back to the beginning

    std::cout<<fnum<<", "
    "\"POLYGON ((";
    for(auto v=pgon.vertices_begin();v!=pgon.vertices_end();v++)
      std::cout<<v->x()<<" "<<v->y()<<", ";
    std::cout<<pgon.vertices_begin()->x()<<" "<<pgon.vertices_begin()->y()<<"))\"\n";
  }

  return 0;
}

Panagiotis Mike gave me the answer:

// standard includes
#include <iostream>
#include <string>
#include <fstream>
#include <cassert>
// includes for defining the Voronoi diagram adaptor
#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
#include <CGAL/Voronoi_diagram_2.h>
#include <CGAL/Segment_Delaunay_graph_2.h>
#include <CGAL/Segment_Delaunay_graph_adaptation_traits_2.h>
#include <CGAL/Segment_Delaunay_graph_adaptation_policies_2.h>
#include <CGAL/Segment_2.h>
#include <CGAL/Segment_Delaunay_graph_traits_2.h>

typedef CGAL::Exact_predicates_inexact_constructions_kernel K;
typedef CGAL::Segment_Delaunay_graph_traits_2<K> Gt;
typedef CGAL::Segment_Delaunay_graph_2<Gt> DT;
typedef CGAL::Segment_Delaunay_graph_adaptation_traits_2<DT> AT;
typedef CGAL::Segment_Delaunay_graph_degeneracy_removal_policy_2<DT> AP;
typedef CGAL::Voronoi_diagram_2<DT, AT, AP> VD;
typedef AT::Site_2                    Site_2;
typedef VD::Face_handle               Face_handle;

int main()
{
    std::ifstream ifs("data.cin");
    assert( ifs );
    VD vd;
    Site_2 t;
    while ( ifs >> t ) { vd.insert(t); }
    ifs.close();
    assert( vd.is_valid() );
    std::cout << vd.number_of_faces() << std::endl;
    VD::Face_iterator it = vd.faces_begin(),       
    beyond = vd.faces_end();
    for (int f=0; it != beyond; ++f, ++it) {
        std::cout << "Face" << f << ": \n";
        VD::Ccb_halfedge_circulator hec = it->ccb();
        do {
            VD::Halfedge_handle heh = static_cast<VD::Halfedge_handle>(hec);
                if (heh->has_target())
                    std::cout << heh->target()->point() << "\n";
                else
                    std::cout << "point at infinity\n";

        } while (++hec != it->ccb());
        std::cout << std::endl;
    }  
    return 0;
}
标签
c++
boost
computational-geometry
cgal
voronoi
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