code: function visible( face, vertex ) {
  
  		var va = vertices[ face[ 0 ] ];
  		var vb = vertices[ face[ 1 ] ];
  		var vc = vertices[ face[ 2 ] ];
  
  		var n = normal( va, vb, vc );
  
  		// distance from face to origin
  		var dist = n.dot( va );
  
  		return n.dot( vertex ) >= dist; 
  
  	}
docs: Whether the face is visible from the vertex