CGAL Voronoi Diagram for Segments - cgal

Question

I have used a CGAL Segment_Delaunay_graph to create a voronoi diagram for a collection of line segments. I would like to extract the edges of the Segment_Delaunay_graph which correspond to the voronoi cells. There are methods draw_dual() and draw_skeleton() for this purpose, but both include some additional edges that I don't want to retain (and draw_skeleton removes some edges that I do want to retain).
Here is an image to show the issue:
example voronoi diagram
The black lines are inputs. They are typically groups of several segments connected end-to-end to form longer lines, although each segment is input to the Segment_Delaunay_graph individually like this:
s 1677850.1951146198 466276.4198628192 1784307.2726912862 466276.4198628192
s 1784307.2726912862 466276.4198628192 1784307.2726912862 567677.3831007502
s 1784307.2726912862 567677.3831007502 1677850.1951146198 567677.3831007502
The red and blue lines are output by draw_dual(). I want to retain the red lines which represent voronoi cell boundaries around the connected input lines, but I do not want to keep the blue lines. Is is possible to filter out the unwanted edges based on information stored in the Segment_Delaunay_graph? If so, how?
Some sample code:
#include <iostream>
#include <fstream>
#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
#include <CGAL/Segment_Delaunay_graph_filtered_traits_2.h>
#include <CGAL/Segment_Delaunay_graph_2.h>
typedef CGAL::Exact_predicates_inexact_constructions_kernel K;
typedef CGAL::Segment_Delaunay_graph_filtered_traits_2<K> Gt;
typedef CGAL::Segment_Delaunay_graph_2<Gt> SDG2;
using namespace std;
int main()
{
//read line segments from input file
string inFile = "example-a.in";
ifstream ifs(inFile);
SDG2 sdg;
SDG2::Site_2 site;
std::vector<SDG2::Site_2> sites;
while (ifs >> site) {
sites.push_back(site);
}
ifs.close();
//add line segments to diagram
sdg.insert(sites.begin(), sites.end(), CGAL::Tag_true());
//save voronoi edges to a file
string outFile = "example-a.out";
ofstream ofs(outFile);
sdg.draw_dual(ofs);
ofs.close();
}

Answer 1

I observe the red lines you want to keep. Are they the edges that far away to the sites that are not the junction sites? You can set a distance threshold d, if the distance between the edge to its closest site is smaller than d and the site is not a junction, then remove this edge.