.TH pgmedge 1 "04 February 1990"
.IX pgmedge
.SH NAME
pgmedge - edge-detect a portable graymap

.SH SYNOPSIS
.B pgmedge
.RI [ pgmfile ]

.SH DESCRIPTION
Reads a portable graymap as input.
Outlines the edges, and writes a portable graymap as output.
.IX "edge detection"
Piping the result through
.B pgmtopbm -threshold
and playing with the
threshold value will give a bitmap of the edges.
.IX thresholding
.PP
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
.B pnmdepth
first.

.SH "SEE ALSO"
.BR pgmenhance (1), 
.BR pgmtopbm (1), 
.BR pnmdepth (1),
.BR pgm (5), 
.BR pbm (5)

.SH AUTHOR
Copyright (C) 1991 by Jef Poskanzer.
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