def bbox_union(bbox_a, bbox_b):
    """ realize union between to given bbox"""
    x_min_a, y_min_a, x_max_a, y_max_a = bbox_a
    x_min_b, y_min_b, x_max_b, y_max_b = bbox_b
    return min(x_min_a, x_min_b), min(y_min_a, y_min_b), max(x_max_a, x_max_b
        ), max(y_max_a, y_max_b)
