CGAL Nef_polyhedron_3 Union issue

[JamesYangLim]

Issue Details

Hi, I am trying to union 53 blocks from OFF files. However, the result is not closed and seems incorrect. Has anyone seen this kind of behaviour from Nef_polyhedron_3 union? Is this a bug?

These blocks are just random generated.

Data Files

Zip containing the cube OFF input files and OFF output file, 14kB

Output Visualisation

The 53 Blocks look like this:

After the union operation:

Top view:

Source Code

#include "pch.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 <CGAL/Exact_predicates_exact_constructions_kernel.h>

#include <iostream>
#include <fstream>
#include <vector>

typedef CGAL::Exact_predicates_exact_constructions_kernel Kernel;
typedef Kernel::Point_3 Point_3;
typedef CGAL::Nef_polyhedron_3<Kernel> Nef_polyhedron_3;
typedef CGAL::Polyhedron_3<Kernel> Polyhedron_3;
typedef CGAL::Surface_mesh<Point_3> Mesh;

TEST(NefTest, unionTest)
{
	std::vector<Polyhedron_3> ps;

	for (int i = 0; i < 53; ++i)
	{
		Polyhedron_3 tempP;
		std::stringstream ss;
		ss << "C:\\Temp\\NefBoolean\\Mesh_" << i << ".off";
		std::ifstream input(ss.str().c_str());
		input >> tempP;
		input.close();
		ps.push_back(tempP);
	}

	std::vector<Nef_polyhedron_3> ns;
	for (auto p : ps)
	{
		ns.emplace_back(Nef_polyhedron_3(p));
	}

	Nef_polyhedron_3 unionedN(ns[0]);
	for (int i = 1; i < ns.size(); ++i)
	{
		unionedN += ns[i];
	}
	
	Mesh m;
	CGAL::convert_nef_polyhedron_to_polygon_mesh(unionedN, m);

	std::ofstream out;
	out.open("c:\\Temp\\OutputMesh.off");
	out << m;
	out.close();
}

Environment

• Operating system (Windows 64 bits):
• Release mode
• CGAL version: 4.11
• Boost version: 1_66_0

[sloriot]

The operation with Nef is not an issue, the error you have comes from the conversion from Nef to a polygon mesh. In your output you have some non-manifold edges (edges incident to more that 2 faces) and this kind of data is not representable with a polygon mesh.

[JamesYangLim]

@sloriot Thanks Sebastian, this is very helpful. May I suggest that it throws an exception just like Polyhedron_3 says that the surface is not simple.

[sloriot]

You can use the function nef.is_simple() for that purpose.

[ankofl]

The operation with Nef is not an issue, the error you have comes from the conversion from Nef to a polygon mesh. In your output you have some non-manifold edges (edges incident to more that 2 faces) and this kind of data is not representable with a polygon mesh.

@sloriot In other words, if two grids have touching edges, they cannot be combined?