pgmtopbm - convert a portable graymap into a portable bitmap
pgmtopbm [-floyd|-fs|-threshold |-hilbert |-dither8|-d8|-cluster3 |-c3|-cluster4|-c4
|-cluster8|-c8] [-value val] [-clump size] [pgmfile]
Reads a portable graymap as input. Produces a portable bitmap as output.
Note that there is no pbmtopgm converter. Any program that uses the Netpbm libraries to
read PGM files, including virtually all programs in the Netpbm package, will read a PBM
file automatically as if it were a PGM file.
If you are using a less intelligent program that expects PGM input, use pnmdepth to con-
vert the PBM file to PGM. As long as the depth is greater than 1, pnmdepth will generate
PGM. This less intelligent program quite probably is also not intelligent enough to deal
with general maxvals, so you should specify a depth of 255.
The default quantization method is boustrophedonic Floyd-Steinberg error diffusion (-floyd
or -fs). Also available are simple thresholding (-threshold); Bayer's ordered dither
(-dither8) with a 16x16 matrix; and three different sizes of 45-degree clustered-dot
dither (-cluster3, -cluster4, -cluster8). A space filling curve halftoning method using
the Hilbert curve is also available. (-hilbert);
Floyd-Steinberg will almost always give the best looking results; however, looking good is
not always what you want. For instance, thresholding can be used in a pipeline with the
pnmconvol tool, for tasks like edge and peak detection. And clustered-dot dithering gives
a newspaper-ish look, a useful special effect.
The -value flag alters the thresholding value for Floyd-Steinberg and simple thresholding.
It should be a real number between 0 and 1. Above 0.5 means darker images; below 0.5
The Hilbert curve method is useful for processing images before display on devices that do
not render individual pixels distinctly (like laser printers). This dithering method can
give better results than the dithering usually done by the laser printers themselves. The
-clump flag alters the number of pixels in a clump. This is usually an integer between 2
and 100 (default 5). Smaller clump sizes smear the image less and are less grainy, but
seem to loose some grey scale linearity. Typically a PGM image will have to be scaled to
fit on a laser printer page (2400 x 3000 pixels for an A4 300 dpi page), and then dithered
to a PBM image before being converted to a postscript file. A printing pipeline might
look something like: pnmscale -xysize 2400 3000 image.pgm | pgmtopbm -hil | pnmtops -scale
0.25 > image.ps
All flags can be abbreviated to their shortest unique prefix.
The only reference you need for this stuff is "Digital Halftoning" by Robert Ulichney, MIT
Press, ISBN 0-262-21009-6.
The Hilbert curve space filling method is taken from "Digital Halftoning with Space Fill-
ing Curves" by Luiz Velho, Computer Graphics Volume 25, Number 4, proceedings of SIGRAPH
'91, page 81. ISBN 0-89791-436-8
pbmreduce(1), pgm(5), pbm(5), pnmconvol(1), pnmscale(1), pnmtops(1)
Copyright (C) 1989 by Jef Poskanzer.
26 July 1988 pgmtopbm(1)