I take a lot of photos of whiteboards, blackboards, and so on for teaching purposes (examples online through May 2010). I'm interested in cleaning them up for archival purposes, preferably using Linux. Commercial products ClearBoard and PhotoNote are priced a little aggressively for my purposes, plus my students would like to have this capability too.

Does anyone know of any good, open source software for

• Converting photographs to images with just a few colors?
• Eliminating perspective distortion?
• Removing unwanted junk from around the edges of an image?

or anything like that? I'm imagining that I start out with a picture of my whiteboard using red and black markers, and I end up with a three-color image using just white, red, and black. Or I photograph a laser-printed document and end up with a clean black-and-white image. I have tried standard tools that reduce the number of colors in an image, and they do a terrible job—probably because they are trying to reproduce the uneven illumination of the original image.

Command-line Linux tools would be ideal.