错误发生于opencv颜色空间转换时：
im_lab = cv2.cvtColor(im_bgr, cv2.COLOR_BGR2LAB)
根据上面第二个链接里的回答，数据不能是浮点型，只能是整型。但根据我的实验，float64会报如题的错误，但只要改成float32就可以运行。
im_bgr = im_bgr.astype(np.float64) # throws an error in the title
im_bgr = im_bgr.astype(np.float32) # runs without error
参考：
https://github.com/llSourcell/Object_Detection_demo_LIVE/issues/6
https://stackoverflow.com/questions/52739143/opencv-error-215assertion-failed-vscncontainsscn-vdcncontainsdcn