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Old 28th Jul 2021, 8:32 am   #15
GMB's Avatar
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
Posts: 2,664
Default Re: Scanning of Manuals & Circuit Diagrams?

1 bit monochrome is mostly out of the question unless you want the scan either to loose lots of detail or you're willing to do pixel and palet editing by hand
It depends on your starting material. If you have a nice clear document then if you scan it for monochrome at quite high resolution (minimum 300 dpi) then the result is usually just fine and requires no editing unless your are a perfectionist, and you have lost no useful detail. I do a lot of document reprocessing and find that this usually works just fine.

The point here is that until quite recently there was no way to print pages with proportional shades. If you get the microscope out you will find that on the paper are scattered bits of solid black ink. But even quite old printing was surprisingly high resolution, so if you exceed its resolution in scanning then the result in pure monochrome is as good as the original. What you are doing by using greyscale is blurring the black patterned areas to obtain the grey intent. For photos this is a way to reduce the dpi and increase the bits per sample and the result can look better in a PDF than the original document (because PDFs tend to be viewed on small screens).

The trouble comes with old photocopies of documents that were not flat. Or photos of them, and with electronic photos you have added JPEG fringes which defeat thresholding and ideally need to be blurred but this looses detail. Then the advice to use the smallest number of greyscale bits that works is the way to go. The point here is that the human eye and brain are superbly good at compensating for the errors software will struggle to do the same.
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