This program is part of Netpbm(1).
pgmbentleyreads a PTM image as input and performs the Bentley Effect, and writes a PGM image as output.
The Bentley Effect is described in 'Beyond Photography' by Holzmann, chapter 4, photo 4. It's a vertical smearing based on brightness.
Copyright (C) 1990 by Wilson Bent (email@example.com)