three.js/src/extras/helpers/WireframeHelper.js

/**
 * @author mrdoob / http://mrdoob.com/
 */

THREE.WireframeHelper = function ( object ) {

	var edge = [ 0, 0 ], hash = {};
	var sortFunction = function ( a, b ) { return a - b };

	var keys = [ 'a', 'b', 'c', 'd' ];
	var geometry = new THREE.Geometry();

	var vertices = object.geometry.vertices;
	var faces = object.geometry.faces;

	for ( var i = 0, l = faces.length; i < l; i ++ ) {

		var face = faces[ i ];
		var length = face instanceof THREE.Face4 ? 4 : 3;

		for ( var j = 0; j < length; j ++ ) {

			edge[ 0 ] = face[ keys[ j ] ];
			edge[ 1 ] = face[ keys[ ( j + 1 ) % length ] ];
			edge.sort( sortFunction );

			var key = edge.toString();

			if ( hash[ key ] === undefined ) {

				geometry.vertices.push( vertices[ edge[ 0 ] ] );
				geometry.vertices.push( vertices[ edge[ 1 ] ] );

				hash[ key ] = true;

			}

		}

	}

	THREE.Line.call( this, geometry, new THREE.LineBasicMaterial( { color: 0xffffff } ), THREE.LinePieces );

	this.matrixAutoUpdate = false;
	this.matrixWorld = object.matrixWorld;

};

THREE.WireframeHelper.prototype = Object.create( THREE.Line.prototype );