def box_mask_string(centre_pix_posns, width):
    """Get a mask string representing box apertures about (x,y) tuples"""
    mask = ''
    if centre_pix_posns is None:
        return mask
    for coords in centre_pix_posns:
        mask += 'box [ [{lx}pix , {ly}pix]  , [{hx}pix, {hy}pix] ]\n'.format(
                          lx=coords[0] - width / 2, ly=coords[1] - width / 2,
                          hx=coords[0] + width / 2, hy=coords[1] + width / 2,)
    return mask