Computing edge length of all half-edges in CGAL::Polyhedron_3

In this post I will show how to compute edge length for a half-edge in CGAL::Polyhedron_3.

ComputeEdgeLength() Functor

ComputeEdgeLength() is a thread-safe functor for computing the edge length of a given half-edge in a CGAL::Polyhedron_3.


namespace SMeshLib   {
namespace Operations {

// The ComputeEdgeLength is a functor (delegate) to compute the length of a halfedge in CGAL::Polyhdeon_3.
// ComputeEdgeLength is thread-safe.
// TPolyhedron is a type of CGAL::Polyhdeon_3.
template<class TPolyhedron>
struct ComputeEdgeLength
	// Redefine types from TPolyhedron for convenience.
	typedef typename TPolyhedron::Point_3  Point3;
	typedef typename TPolyhedron::Halfedge Halfedge;
	// Return type of operator() required by QtConcurrent.
	typedef double result_type;
	// Compute edge length of a given half edge.
	inline double operator () (const Halfedge& h)
		const Point3& p = h.prev()->vertex()->point();
		const Point3& q = h.vertex()->point();
		return CGAL::sqrt(CGAL::squared_distance(p, q));

};	// End namespace Operations.
};	// End namespace SMeshLib.

Using ComputeEdgeLength() Functor

Edge length of a half-edge h can be computed as double length = ComputeEdgeLength(h);.

For most purposes, it is better to compute length of all half-edges once and cache them for later use. It is best to store the results in an associative container which associates the half-edge handle with the edge length. In the following example, I use PropertyMap which is a wrapper for std::set.

#include "ImportOBJ.h"
#include "ComputeEdgeLength.h"
#include "PropertyMap.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;

// Compute the length of all half-edges in a CGAL::Polyhedron_3 and stores it 
// in edgeLengths.
// TPolyhedron is a type of CGAL::Polyhedron_3.
// TEdgeLengths is a property map which associates the edge length to half-edge 
// in the CGAL::Polyhdeon_3.
template<class TPolyhedron, class TEdgeLengths>
void computeEdgeLengths(const TPolyhedron& polyhedron, TEdgeLengths* edgeLengths)
	if(edgeLengths == 0)
	typename TPolyhedron::Halfedge_const_iterator _begin = polyhedron.halfedges_begin();
	typename TPolyhedron::Halfedge_const_iterator _end   = polyhedron.halfedges_end();

	SMeshLib::Operations::ComputeEdgeLength<TPolyhedron> _computeEdgeLength;
	CGAL_For_all(_begin, _end)
		edgeLengths->setValue(&*_begin, _computeEdgeLength(*_begin));

void testComputeEdgeLength()
	CgalPolyhedron _poly;
	SMeshLib::IO::importOBJ("Venus.obj", &_poly);
	typedef SMeshLib::PropertyMap<const CgalPolyhedron::Halfedge*, double> EdgeLengthPM;
	EdgeLengthPM _edgeLengths;
	computeEdgeLengths(_poly, &_edgeLengths);


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