Re: JPEG file internal structure

K
Posted By
kbhat
Sep 3, 2007
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254
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1
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On Aug 31, 10:19 am, wrote:
Hi,

I am looking for some information on the internal structure of a JPEG file. I was playing with a handful of bits in a JPEG file as part of a steganagraphy experiment and I saw something remarkable. I am able to cause a lateral shifting in the file. It is as if someone has torn the right quarter of the picture and pasted it on the extreme left. I was certainly expecting loss of clarity in the picture, but seeing such a remarkable phenomenon has left me completely dumbstruck. Any suggestions what might be causing this?

Thanks,
Bhat

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MS
Marco Schmidt
Sep 4, 2007
fup2 r.p.d; best group is probably comp.compression, though

:

I am looking for some information on the internal structure of a JPEG file. I was playing with a handful of bits in a JPEG file as part of a steganagraphy experiment and I saw something remarkable. I am able to cause a lateral shifting in the file. It is as if someone has torn the right quarter of the picture and pasted it on the extreme left. I was certainly expecting loss of clarity in the picture, but seeing such a remarkable phenomenon has left me completely dumbstruck. Any suggestions what might be causing this?

If part of the bitstream is corrupted, the decoder takes up decoding at the next restart marker or wherever the bitstream makes sense to it again. Decoding resumes at an incorrect position because the decoder does not know how many pixels (MCUs) it skipped over. Hence, everything is shifted.

Regards,
Marco

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