Can we convert DVI or PostScript into TeX?

Basically that is the question - if we have a DVI file can we get on the base on it the original TeX file?

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no, that is not possible. –  Herbert Mar 4 '12 at 17:39
It is comparable to the task of decomposing a once baked cake into its ingredients. –  Thorsten Donig Nov 11 '12 at 8:42

The simple answer is no. If you have a dvi and have lost the TeX there are tools such as dvi2tty which will extract the text and some of the paragraph structure, but in general you will have to add the markup back by hand.

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PS files would be tremendously difficult to convert back to TeX.

With DVI files I think it would at least be partially possible. Of course all high-level macros and stuff like that would be lost. Compare the task to recovering, e.g., C++ code from a compiled binary file.

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Somewhat alike to disassembling a compiled executable: Yes, technically it can be done - but you will lose much of the information that was contained in the original TeX source.

You might be able to turn this into text again, but the information that this was originally written as see also chapter \ref{sec:FooBar} is irrevocably lost.

Since this is the case, I don't think anyone has ever made the effort to write such a DVI-to-text converter, since the usability of such text would be severly limited.

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