Extracting a region from a Jpeg

Hi

Does anyone know if it would be possible in principle to extract a
rectangular region from a Jpeg file without decoding the pixels - or put
another way could a jpeg file be cropped without first decoding it? I'd be
prepared to accept that the width and height of the rectangle had to be
aligned multiples of 8.

My sketchy understanding of Jpeg suggests that the entropy encoding scheme
would prevent this. If that is the case, would it at least be possible to
crop a jpeg without inverting the DCT?

Thanks
Jeff


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