public void bounce(Direction d) {
		if (d == null) return;
		switch (d) {
		case UP:    v_y = Math.abs(v_y); break;  
		case DOWN:  v_y = -Math.abs(v_y); break;
		case LEFT:  v_x = Math.abs(v_x); break;
		case RIGHT: v_x = -Math.abs(v_x); break;
		}
	}