edge-detect a portable graymap
see also :
pgmenhance - pgmtopbm - pnmdepth
add an example, a script, a trick and tips
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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.
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
pgmtopbm , pnmdepth , pgm,
1991 by Jef Poskanzer.