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I don't think the problem is trivial, but I don't think it's hopeless either. If we can measure a person's pulse and mood with a camera pointed at their face, I'm sure we can come up with a tool that can approximate semantically meaningful diffs of artwork. For image formats like xcf that store parts of the image or the editing history independently, this problem becomes even more tractable.

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It's very tractable if you don't insist on the diff reconstructing the target file byte-for-byte. But this would require pretty invasive changes in the version control system, no?

If you change the top-left pixel of a PNG, for example, between the intra prediction and the DEFLATE compression, the new file can be totally different, and to reconstruct it you either hope the destination is using the exact same libpng with the exact same settings, or you have to find a space-efficient way to write down all the arbitrary encoding decisions the format allows.

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