// detect device orientation
    override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {

        if UIDevice.current.orientation.isLandscape {
            self.view.backgroundColor = UIColor(patternImage: UIImage(named: "back-landscape")!)

        } else {
            self.view.backgroundColor = UIColor(patternImage: UIImage(named: "back-potrait")!)
        }
    }