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Comment by Amadou | Hacker News Reader

Anyone got a jpeg un-glitcher? I'm thinking of a tool to fix these sorts of minor corruptions in the jpeg bitstream. The tool would load up the corrupt image, let the user visually select where the glitch starts by clicking the first "weird" looking pixel and then iterate through different values until the user says the picture looks better. Rinse and repeat.

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most pictures don't have hard transitions between block areas, so once the fault area has been identified an algorithm could be made to look for the change that results in the image having the softest transition between neighboring blocks.

we should make your unglitcher part of the standard, that way every jpeg across the whole Internet could be reduced in size by just throwing away certain parts of it. Then your unglitcher could get those parts back. oh, wait....

A paint program!

Its doubtful that the image can be recovered. The information really is gone. I suspect it would be easier to make it simple to have redundant copies of the data, rather than trying to fix the images after they're gone.

If it's only a single bit that's toggled, you can just iterate through all the bits until you find one that fixes the image after you flip it.

For a 1MB JPEG, that'd be over 8 million different images.

If you could point to a region of the image where the corruption begins, you would reduce the number of possible images significantly.

That sounds plausible. Another idea: choose a section of "bad image" and a section of "good image" which should have similar colours and then generate mutations of the corrupted DCT square until you have a close match between the two areas.

Given that we're assuming some kind of correctness metric for the final image, could we not also use this metric to locate the start of the corruption?

For a single byte error that is visibly obvious, you could just iterate through 255 combinations until the user thinks the image looks right.

Yeah maybe you could have the user iterate through a search tree the way an optometrist performs an eye exam, "Which is better, one flip or two?"

But I don't know if there is a way to create a tree of the possibilities.

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