Copyright (c) 2004 Bart Massey
ppmgrayscreen is a filter I wrote sometime in the early 2000s (if I recall correctly) to remove extraneous grading marks from a student test I had accidentally marked up. Its basic mode of operation is to find pixels that are not a shade of some target color (default black) and change them to white. It's not great, but better than nothing for this job.
Please see the included manual page for invocation details.
This program is licensed under the "3-clause ('new') BSD
License". Please see the file COPYING in this
distribution for license terms.