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[sdobashi <>]

Hi everyone, 

I created a rectangular tube shaped Nef_polyhedron_3 object by subtracting
'box2' from 'box1' (fig1, 2). 
The resultant nef polyhedron looks like what I expected (fig2). 

However, when I converted this to Polyhedron_3 using 
"CGAL::convert_nef_polyhedron_to_polygon_mesh" or "convert_to_polygon"
method, 
The top and bottom of the rectangular tube were capped (fig3). 

I attached the relevant code to the end of this message. 
The output "diff12.off" has a rectilinear hole inside a box but the hole was
capped on the surface. 
I want to preserve the shape like fig3 
even after converting the Nef polyhedron to an ordinary Polyhedron. 

Is there a way to prevent the covering on the surface 
when converting Nef polygon to an ordinary polygon? 

Thanks, 
S Dobashi 

Fig1
<http://cgal-discuss.949826.n4.nabble.com/file/t375861/fig1_box1_and_box2.png>
 

Fig2
<http://cgal-discuss.949826.n4.nabble.com/file/t375861/fig2_box1_minus_box2_as_Nef.png>
 

Fig3
<http://cgal-discuss.949826.n4.nabble.com/file/t375861/fig3_box1_minus_box2_as_Polyhedron.png>
 

#include <CGAL/Exact_predicates_exact_constructions_kernel.h>
#include <CGAL/Polyhedron_3.h>
#include <CGAL/Surface_mesh.h>
#include <CGAL/Nef_polyhedron_3.h>
#include <CGAL/boost/graph/convert_nef_polyhedron_to_polygon_mesh.h>

#include <sstream>
#include <fstream>

typedef CGAL::Exact_predicates_exact_constructions_kernel Exact_kernel;
typedef CGAL::Polyhedron_3<Exact_kernel> Polyhedron;
typedef CGAL::Surface_mesh<Exact_kernel::Point_3> Surface_mesh;
typedef CGAL::Nef_polyhedron_3<Exact_kernel> Nef_polyhedron;

void set_box1(Polyhedron& P)
{
    std::stringstream ss;
    ss << "OFF\n"
        << "8 6 0\n"
        << "0 0 0\n"
        << "1 0 0\n"
        << "1 1 0\n"
        << "0 1 0\n"
        << "0 0 1\n"
        << "1 0 1\n"
        << "1 1 1\n"
        << "0 1 1\n"
        << "4 0 3 2 1\n"
        << "4 0 1 5 4\n"
        << "4 1 2 6 5\n"
        << "4 2 3 7 6\n"
        << "4 3 0 4 7\n"
        << "4 4 5 6 7\n";
    ss >> P;
}

void set_box2(Polyhedron& P)
{
    std::stringstream ss;
    ss << "OFF\n"
        << "8 6 0\n"
        << "0.25 0.25 -0.25\n"
        << "0.75 0.25 -0.25\n"
        << "0.75 0.75 -0.25\n"
        << "0.25 0.75 -0.25\n"
        << "0.25 0.25 1.25\n"
        << "0.75 0.25 1.25\n"
        << "0.75 0.75 1.25\n"
        << "0.25 0.75 1.25\n"
        << "4 0 3 2 1\n"
        << "4 0 1 5 4\n"
        << "4 1 2 6 5\n"
        << "4 2 3 7 6\n"
        << "4 3 0 4 7\n"
        << "4 4 5 6 7\n";
    ss >> P;
}

int main()
{
    Polyhedron box1, box2;
    set_box1(box1);
    set_box2(box2);

    std::ofstream out;
    out.open("box1.off");
    out << box1;
    out.close();

    out.open("box2.off");
    out << box2;
    out.close();

    Nef_polyhedron nef1(box1);
    Nef_polyhedron nef2(box2);
    Nef_polyhedron nef_diff12 = nef1 - nef2;

    Surface_mesh output;
    CGAL::convert_nef_polyhedron_to_polygon_mesh(nef_diff12, output);
    
    out.open("diff12.off");
    out << output;
    out.close();
}




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