code: function normal( va, vb, vc ) {
  
  		var cb = new THREE.Vector3();
  		var ab = new THREE.Vector3();
  
  		cb.sub( vc, vb );
  		ab.sub( va, vb );
  		cb.crossSelf( ab );
  
  		cb.normalize();
  
  		return cb;
  
  	}
docs: Face normal