def build_empty_bbox_mask(bboxes):
    """
    Generate a mask, 0 means empty bbox, 1 means non-empty bbox.
    :param bboxes: list[list] bboxes list
    :return: flag matrix.
    """
    flag = [1 for _ in range(len(bboxes))]
    for i, bbox in enumerate(bboxes):
        # empty bbox coord in label files
        if bbox == [0,0,0,0]:
            flag[i] = 0
    return flag