pgmtopbm(1)pgmtopbm(1)NAMEpgmtopbm - convert a portable graymap into a portable
bitmap
SYNOPSISpgmtopbm [-floyd|-fs|-threshold |-hilbert
|-dither8|-d8|-cluster3 |-c3|-cluster4|-c4 |-cluster8|-c8]
[-value val] [-clump size] [pgmfile]
DESCRIPTION
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, includ
ing 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 convert the PBM file to PGM.
As long as the depth is greater than 1, pnmdepth will gen
erate PGM. This less intelligent program quite probably
is also not intelligent enough to deal with general max
vals, so you should specify a depth of 255.
OPTIONS
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, -clus
ter4, -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 news
paper-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 means lighter.
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 pre
fix.
REFERENCES
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 "Dig
ital Halftoning with Space Filling Curves" by Luiz Velho,
Computer Graphics Volume 25, Number 4, proceedings of
SIGRAPH '91, page 81. ISBN 0-89791-436-8
SEE ALSOpbmreduce(1), pgm(5), pbm(5), pnmconvol(1), pnmscale(1),
pnmtops(1)AUTHOR
Copyright (C) 1989 by Jef Poskanzer.
26 July 1988 pgmtopbm(1)
