原因找到了,结帖!
roi is the region-of-interest rectangle (x, y, w, h) to copy from. If not specified, it is equal to the image rectangle which copies the entire image. This argument is not applicable for JPEG images.
这是原版的解释,最后一句This argument is not applicable for JPEG images.
这个是不能用于jpeg图片格式的,原因就是这样
我把图片格式改了以后就可以了