After writing the post on converting PNMs to DjVu, I’ve run 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 converting it afterward 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 was 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 could find a better solution.
Continue reading Scanning Documents Written in Blue Ink – biscan