After writing the post on converting PNMs to DjVu I’ve ran into some trouble scanning documents written in blue ink. The problem: XSane didn’t allow me to set the threshold for converting the scanned image to line-art (B&W). So, I tried scanning the document in grayscale and in color and convert it afterwards to bitonal using imagemagick. This ended up with two results. When I used the -monochrome command line switch, the conversion looked good, but it used halftones (dithering), when I tried to convert it to DjVu it resulted in a document size twice as large as normal B&W would. The other thing that I tried is using the -threshold switch. The DjVu compressed document size was much better now, but the document was awful looking, either it was too dark, or some of the text disappeared. After giving it some thought I knew I can find a better solution.
Continue reading Scanning Documents Written in Blue Ink – biscan