pgmedge - edge-detect a portable graymap
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
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
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.
Copyright (C) 1991 by Jef Poskanzer.