def get_new_corners(bb1,bb2):
    """returns tuple of corners describing box bounding two bounding boxes"""
    (ux1,uy1),(lx1,ly1) = bb1
    (ux2,uy2),(lx2,ly2) = bb2
    ul_corner = min(ux1,ux2),min(uy1,uy2)
    lr_corner = max(lx1,lx2),max(ly1,ly2)
    return ul_corner,lr_corner