Wavefront OBJ reader for building CGAL::Polyhedron_3

CGAL provides high quality generic half-edge data structure for representing polyhedral surfaces as well as many algorithms for mesh processing. However, CGAL doesn’t have any in-build support for building a polyhedron from Wavefront OBJ or PLY file. The following code is a basic OBJ file loader which reads vertex coordinates and faces (can be polygons) from OBJ file. Note that it doesn’t read vertex normals, face normals, or texture coordinates. Code is well commented and should be fairly obvious.

importOBJ() Function

#include "CGAL/Polyhedron_incremental_builder_3.h"
#include "CGAL/Modifier_base.h"
#include "CGAL/exceptions.h"
#include <string>
#include <fstream>
#include <exception>

namespace SMeshLib {
namespace IO       {

// The BuildCgalPolyhedronFromObj class builds a CGAL::Polyhedron_3 from Wavefront OBJ file.
// This is very simple reader and only reads vertex coordinates and vertex index for faces.
// Faces can be polygons and doesn't have to be triangles.
template<class HDS>
class BuildCgalPolyhedronFromObj : public CGAL::Modifier_base<HDS>
	BuildCgalPolyhedronFromObj(const std::string& fileName) : mFileName(fileName) {}
	void operator() (HDS& hds)
		typedef typename HDS::Vertex   Vertex;
		typedef typename Vertex::Point Point;
		// Open obj file for reading.
		std::ifstream _file(mFileName.c_str());
		// Count the number of vertices and facets.
		// This is used to reserve memory in HDS.
		std::string _line;
		int _numVertices = 0;
		int _numFacets   = 0;
			std::getline(_file, _line);
			if(_line.size() > 1)
				if(_line[0]=='v' && _line[1]==' ') {++_numVertices;}
				if(_line[0]=='f' && _line[1]==' ') {++_numFacets;}
		// Rewind file to beginning for reading data.

		// Postcondition: hds is a valid polyhedral surface.
		CGAL::Polyhedron_incremental_builder_3<HDS> B(hds, true);
		// Load the data from OBJ file to HDS.
		B.begin_surface(_numVertices, _numFacets, int((_numVertices + _numFacets - 2)*2.1));
			std::string _token;
				_token = ""; // Reset token.
				_file >> _token;
				// if token is v then its a vertex.
					double x, y, z;
					_file >> x >> y >> z;
					B.add_vertex(Point(x, y, z));
				// There are 4 type of facets.
				// a     only vertex index.
				// a/b   vertex and texture index.
				// a/b/c vertex, texture and normal index.
				// a//c  vertex and normal index.
				else if(_token=="f")
					// Read the remaining line for the facet.
					std::string _line;
					std::getline(_file, _line);
					// Split the line into facet's vertices.
					// The length of _vertices is equal to the number of vertices for this face.
					std::istringstream _stream(_line);
					std::vector<std::string> _vertices;
					// For each vertex read only the first number, which is the vertex index.
					for(size_t i=0 ; i<_vertices.size() ; ++i)
						std::string::size_type _pos = _vertices[i].find('/', 0);
						std::string _indexStr = _vertices[i].substr(0, _pos);
						B.add_vertex_to_facet(stoi(_indexStr)-1); // -1 is because OBJ file uses 1 based index.
	std::string mFileName;

// Import a OBJ file given by fileName to polyhedron.
// TPoly is a type of CGAL::Polyhdeon_3.
template<class TPoly>
void importOBJ(const std::string& fileName, TPoly* polyhedron)
			// Build Polyhedron_3 from the OBJ file.
			BuildCgalPolyhedronFromObj<TPoly::HalfedgeDS> _buildPolyhedron(fileName);
			// Calls is_valid at the end. Throws an exception in debug mode if polyhedron is not
			// manifold.
			// CGAL::Assert_exception is thrown in the debug mode when 
			// CGAL::Polyhedron_incremental_builder_3 is destroyed in BuildCgalPolyhedronFromObj.
			// However, in the release mode assertions is disabled and hence no exception is thrown.
			// Thus for uniform error reporting, if the polyhedron is not valid then throw a dummy 
			// exception in release mode.
				throw CGAL::Assertion_exception("", "", "", 0, "");
		catch(const CGAL::Assertion_exception&)
			std::string _msg = "SMeshLib::importOBJ: Error loading " + fileName;
			throw std::exception(_msg.c_str());

};	// End namespace IO.
};	// End namespace SMeshLib.

Using importOBJ()
#include "ImportOBJ.h"
#include "CGAL/Simple_cartesian.h"
#include "CGAL/Polyhedron_items_3.h"
#include "CGAL/HalfedgeDS_list.h"
#include "CGAL/Polyhedron_3.h"

typedef CGAL::Simple_cartesian<double> Kernel;
typedef CGAL::Polyhedron_3<Kernel, 
						   CGAL::HalfedgeDS_list> CgalPolyhedron;

void testImportOBJ()
	CgalPolyhedron _poly;
	SMeshLib::IO::importOBJ("Venus.obj", &_poly);


  • Minliang Reply

    Thank you very much!

  • Andrei Serebro Reply

    Thank you very much for sharing this useful stuff!
    There is a small typo in the code above, though. It should be


    rather than


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