Return a center crop of an image

def center_crop(im, min_sz=None):
    """ Return a center crop of an image """
    r,c,*_ = im.shape
    if min_sz is None: min_sz = min(r,c)
    start_r = math.ceil((r-min_sz)/2)
    start_c = math.ceil((c-min_sz)/2)
    return crop(im, start_r, start_c, min_sz)