def unique_image(pair_list) -> set:
    """Return unique image path in pair_list.txt"""
    with open(pair_list, 'r') as fd:
        pairs = fd.readlines()
    unique = set()
    for pair in pairs:
        id1, id2, _ = pair.split()
        unique.add(id1)
        unique.add(id2)
    return unique