Interesting wiki page. The native format of the image produced by a scanner available via twain is a Dib : https://www.twain.org/docs/TWAIN19.pdf
From there the twain driver from the vendor will typically make available a number of other formats, but as I said it starts with a Dib. So I guess the answer is you don’t get the raw image data from twain. You may find a method for converting the Dib to Raw but that seems rather pointless and seems to run against your ‘digital negative’ idea.
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