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How does jpeg handle errors? From a naive understanding of...

<< Back to: JPEG image compression FAQ, part 1/2

Question by Cushy
Submitted on 5/24/2004
Related FAQ: JPEG image compression FAQ, part 1/2
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How does jpeg handle errors?  From a naive understanding of huffman compression it seems that one or more omitted or incorrect bits would make the file undecodable.  Experiments have shown this is not the case even with files that lack restart markers (0xffd0-0xffd7).  Shifted and off-color portions of image usually result, but the stream appears to still be readable. How?  (I'm writing a jpeg repair utility)

Answer by Iain
Submitted on 9/1/2004
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Hi, was wondering if you made any progress with your error fixing utility?
I have some images off a bad CD that are corrupted by minor errors. Most are viewable but get mixed up at some point. I suspect that it is just a single bit error or maybe a bit missing.
It is easy to tell which 8x8 block the error is in since that is where the image starts going wrong, so I figured that it might be possible to fix them with a bit of guidance. Had a look at them in WinHex and all the headers look okay, but I don't know enough about JPEG encoding to find where I might need to stuff an extra bit. Have been unable to find any utlities out there which do this.
Can send you a sample image if you need. Would be glad to hear any ideas you might have.

Iain Phillips (iainph at hotmail)


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<< Back to: JPEG image compression FAQ, part 1/2

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