def merge_bbox(bbox1, bbox2):
    """
    Merge two bboxes.

    >>> merge_bbox((-10, 20, 0, 30), (30, -20, 90, 10))
    (-10, -20, 90, 30)

    """
    minx = min(bbox1[0], bbox2[0])
    miny = min(bbox1[1], bbox2[1])
    maxx = max(bbox1[2], bbox2[2])
    maxy = max(bbox1[3], bbox2[3])
    return minx, miny, maxx, maxy
