osx man page for pgmedge

pgmedge(1)                                                    General Commands Manual                                                   pgmedge(1)

NAME
       pgmedge - edge-detect a portable graymap

SYNOPSIS
       pgmedge [pgmfile]

DESCRIPTION
       Reads  a  portable  graymap  as  input.   Outlines  the edges, and writes a portable graymap as output.  Piping the result through pgmtopbm
       -threshold and playing with the threshold value will give a bitmap of the edges.

       The edge detection technique used is to take the Pythagorean sum of two Sobel gradient operators at 90 degrees to  each  other.   For  more
       details see "Digital Image Processing" by Gonzalez and Wintz, chapter 7.

       The  maxval  of the output is the same as the maxval of the input The effect is better with larger maxvals, so you may want to increase the
       maxval of the input by running it through pnmdepth first.

SEE ALSO
       pgmenhance(1), pgmtopbm(1), pnmdepth(1), pgm(5), pbm(5)

AUTHOR
       Copyright (C) 1991 by Jef Poskanzer.

                                                                 04 February 1990                                                       pgmedge(1)