Exports a THREE.js scene mesh to STL, making it suitable for 3d printing

LICENSE

The MIT License

Copyright © 2010-2016 three.js authors

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
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copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.

STLExporter.js

/**
 * Based on https://github.com/mrdoob/three.js/blob/a72347515fa34e892f7a9bfa66a34fdc0df55954/examples/js/exporters/STLExporter.js
 * Tested on r68 and r70
 * @author kjlubick / https://github.com/kjlubick
 * @author kovacsv / http://kovacsv.hu/
 * @author mrdoob / http://mrdoob.com/
 */
THREE.STLExporter = function () {};

THREE.STLExporter.prototype = {

	constructor: THREE.STLExporter,

	parse: ( function () {

		var vector = new THREE.Vector3();
		var normalMatrixWorld = new THREE.Matrix3();

		return function ( scene ) {

			var output = '';

			output += 'solid exported\n';

			scene.traverse( function ( object ) {

				if ( object instanceof THREE.Mesh ) {

					var geometry = object.geometry;
					var matrixWorld = object.matrixWorld;
					var mesh = object;

					if ( geometry instanceof THREE.Geometry ) {

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

						normalMatrixWorld.getNormalMatrix( matrixWorld );

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

							vector.copy( face.normal ).applyMatrix3( normalMatrixWorld ).normalize();

							output += '\tfacet normal ' + vector.x + ' ' + vector.y + ' ' + vector.z + '\n';
							output += '\t\touter loop\n';

							var indices = [ face.a, face.b, face.c ];

							for ( var j = 0; j < 3; j ++ ) {
								var vertexIndex = indices[ j ];
								if (mesh.geometry.skinIndices.length == 0) {
									vector.copy( vertices[ vertexIndex ] ).applyMatrix4( matrixWorld );
									output += '\t\t\tvertex ' + vector.x + ' ' + vector.y + ' ' + vector.z + '\n';
								} else {
									vector.copy( vertices[ vertexIndex ] ); //.applyMatrix4( matrixWorld );
									
									// see https://github.com/mrdoob/three.js/issues/3187
									boneIndices = [];
									boneIndices[0] = mesh.geometry.skinIndices[vertexIndex].x;
									boneIndices[1] = mesh.geometry.skinIndices[vertexIndex].y;
									boneIndices[2] = mesh.geometry.skinIndices[vertexIndex].z;
									boneIndices[3] = mesh.geometry.skinIndices[vertexIndex].w;
									
									weights = [];
									weights[0] = mesh.geometry.skinWeights[vertexIndex].x;
									weights[1] = mesh.geometry.skinWeights[vertexIndex].y;
									weights[2] = mesh.geometry.skinWeights[vertexIndex].z;
									weights[3] = mesh.geometry.skinWeights[vertexIndex].w;
									
									inverses = [];
									inverses[0] = mesh.skeleton.boneInverses[ boneIndices[0] ];
									inverses[1] = mesh.skeleton.boneInverses[ boneIndices[1] ];
									inverses[2] = mesh.skeleton.boneInverses[ boneIndices[2] ];
									inverses[3] = mesh.skeleton.boneInverses[ boneIndices[3] ];

									skinMatrices = [];
									skinMatrices[0] = mesh.skeleton.bones[ boneIndices[0] ].matrixWorld;
									skinMatrices[1] = mesh.skeleton.bones[ boneIndices[1] ].matrixWorld;
									skinMatrices[2] = mesh.skeleton.bones[ boneIndices[2] ].matrixWorld;
									skinMatrices[3] = mesh.skeleton.bones[ boneIndices[3] ].matrixWorld;
									
									var finalVector = new THREE.Vector4();
									for(var k = 0; k<4; k++) {
										var tempVector = new THREE.Vector4(vector.x, vector.y, vector.z);
										tempVector.multiplyScalar(weights[k]);
										//the inverse takes the vector into local bone space
										tempVector.applyMatrix4(inverses[k])
										//which is then transformed to the appropriate world space
										.applyMatrix4(skinMatrices[k]);
										finalVector.add(tempVector);
									}
									output += '\t\t\tvertex ' + finalVector.x + ' ' + finalVector.y + ' ' + finalVector.z + '\n';
								}
							}
							output += '\t\tendloop\n';
							output += '\tendfacet\n';
						}
					}
				}

			} );

			output += 'endsolid exported\n';

			return output;
		};
	}() )
};

// Use FileSaver.js 'saveAs' function to save the string
function saveSTL( scene, name ){  
  var exporter = new THREE.STLExporter();
  var stlString = exporter.parse( scene );
  
  var blob = new Blob([stlString], {type: 'text/plain'});
  
  saveAs(blob, name + '.stl');
}

Nice gist, would be a fine addition to @joostn work on OpenJSCAD https://github.com/joostn/OpenJsCad/blob/gh-pages/src/formats.js