Improve contrast and brightness enhancements for grayscale documents

[Axinalis]

I'm very impressed with this application and it's really easy to use and scan documents.

But I noticed that the resulting scan has low contrast and if it is to be printed and scanned again it would become barely readable. So I wanted to leave a request to increase the contrast of output image (or add an option to do so)

For context adding random document I wanted to scan, FairScan output and desired output

Original photo

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FairScan output

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Desired output

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[pynicolas]

Thanks a lot for reporting this with a precise example, that's very helpful!
Your images clearly show that the automatic brightness and contrast enhancements could be improved for grayscale documents.
I recently received similar feedback through other channels and improving the automatic processing is my main focus.

[pynicolas]

As mentioned in another issue (#112), some grey parts of documents (for example pencil annotations on sheet music) tend to become unreadable after being processed by FairScan.

Looking at several examples, I suspect there is a common pattern: when a document contains very dark areas (bold text, music symbols, etc.), the automatic contrast and brightness normalization tends to push the rest of the page too far toward white. This improves the appearance of the background, but it can also wash out lighter grey details.

Improving this behavior for grayscale documents is something I want to investigate.

[pynicolas]

Fixed by #138.