This program is part of Netpbm.
pgmbentley reads a PGM 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.
There are no command line options defined specifically for pgmbentley, but it recognizes the options common to all programs based on libnetpbm (See Common Options.)