Sometimes you photo some book in the library. The effect usually looks like that:
The light is never equal on all picture - some parts are brighter, some parts are darker. This is bad if you plan to OCR it or convert to djvu. enhance_book_photo is a simple command line tool which equalizes such photo so it looks like that:
java -jar enhance_book_photo.jar picture.png
This tool is licenced under GPLv2 or later, or any other OSI approved or FSI approved licence - whichever fits your needs.