def calc_st_pos_for_centering(bg_size, fg_size):
    """
    Calculate start postion for centering.

    Parameters
    ----------
    bg_size : touple(int)
        (width, height) of the background image.

    fg_size : touple(int)
        (width, height) of the foreground image.

    Returns
    -------
    touple (int)
        (st_pos_h, st_pos_v)

    Examples
    --------
    >>> calc_st_pos_for_centering(bg_size=(1920, 1080), fg_size=(640, 480))
    >>> (640, 300)
    """
    bg_width = bg_size[0]
    bg_height = bg_size[1]

    fg_width = fg_size[0]
    fg_height = fg_size[1]

    st_pos_h = bg_width // 2 - fg_width // 2
    st_pos_v = bg_height // 2 - fg_height // 2

    return (st_pos_h, st_pos_v)