def interpret_signal(signal):
    """signal interpretation
    returning numbers:
        1: to the next image
        -1: previous image
        2: exit
        None: remain and do nothing
    returning sequence:
        add_page, model_inference, quit
    """
    if signal == 1:
        return 1, False
    elif signal == 2:
        return None, True
    elif signal == -1:
        return -1, False
    else:
        return 0, False
