def on_touch_move(self, touch):
        floor = self.player1.height / 2
        ceiling = self.height - floor
        if touch.x < self.width / 3:
            if self.player1.center_y >= floor and self.player1.center_y <= ceiling:
                self.player1.center_y += touch.dy
            elif self.player1.center_y > ceiling:
                self.player1.center_y = ceiling
            else:
                self.player1.center_y = floor
        if touch.x > self.width - self.width / 3:
            if self.player2.center_y >= floor and self.player2.center_y <= ceiling:
                self.player2.center_y += touch.dy
            elif self.player2.center_y > ceiling:
                self.player2.center_y = ceiling
            else:
                self.player2.center_y = floor