I have a scanned PDF material to which I want to add hidden text layer, so I could index the document. I used ghostscript black and white tiff output device (tiffg4) to extract pages as tiff images, and here is example of what they look like:
Processing this image with tesseract, does not give good results.
Changing ghostscript output DPI (600, 300, 150, 96) shows that image at 96 DPI gives best result from tesseract but it's still not satisfactory.
Now I thought to ask for advice which filter would enhance this image for OCR processing.
I could use imagemagick, or numpy/scipy/ndimage