Pamditherbw User Manual(1) General Commands Manual Pamditherbw User Manual(1)
NAME
pamditherbw - dither grayscale image to black and white
SYNOPSIS
pamditherbw
[-floyd | -fs | -atkinson | -threshold | -hilbert | -dither8 | -d8 |
-cluster3 | -c3 | -cluster4 | -c4 | -cluster8 | -c8]
[-value val]
[-clump size]
[-randomseed=integer]
[pamfile]
All options can be abbreviated to their shortest unique prefix.
DESCRIPTION
This program is part of Netpbm(1).
pamditherbw dithers a grayscale image. Dithering means turning each
shade of gray into a pattern of black and white pixels that, from a dis-
tance, look the same as the gray.
The input should be a PGM image or a PAM image of tuple type GRAYSCALE.
However, pamditherbw doesn't check, so if you feed it e.g. a PPM image,
it will produce arbitrary results (actually, it just takes the first
channel of whatever you give it and treats it as if it represented gray
levels).
The output is a PAM with tuple type BLACKANDWHITE. You can turn this
into a PBM (if you need to use it with an older program that doesn't un-
derstand PAM) with pamtopnm.
To do the opposite of dithering, you can usually just scale the image
down and then back up again with pamscale, possibly smoothing or blur-
ring the result with pnmsmooth or pnmconvol. Or use the special case
program pbmtopgm.
To dither a color image (to reduce the number of pixel colors), use pp-
mdither.
Another way to convert a grayscale image to a black and white image is
thresholding. Thresholding is simply replacing each grayscale pixel
with a black or white pixel depending on whether its brightness is above
or below a threshold. That threshold might vary. Simple thresholding
is a degenerate case of dithering, so pamditherbw does very simple
thresholding with its -threshold option. But pamthreshold does more so-
phisticated thresholding.
If all you want is to change a PGM image with maxval 1 to a PBM image,
pamtopnm will do that.
OPTIONS
In addition to the options common to all programs based on libnetpbm
(most notably -quiet, see ]8;;index.html#commonoptions\ Common Options]8;;\ ), pamditherbw recognizes the
following command line options:
Quantization Method
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; ]8;;http://www.tinrocket.com/projects/programming/graphics/00158/\ Atkinson]8;;\ ; and three different
sizes of 45-degree clustered-dot dither (-cluster3, -cluster4, -clus-
ter8).
A space filling curve halftoning method using the Hilbert curve is also
available (-hilbert).
Floyd-Steinberg or Atkinson will almost always give the best looking re-
sults; however, looking good is not always what you want. For instance,
you can use thresholding in a pipeline with the pnmconvol, for tasks
such as edge and peak detection. And clustered-dot dithering gives a
newspaper-ish look, a useful special effect.
Floyd-Steinberg is by far the more traditional, but ]8;;http://www.tinrocket.com/projects/programming/graphics/00158/\ some claim]8;;\ Atkin-
son works better.
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 op-
tion alters the number of pixels in a clump. 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 con-
verted to a postscript file. A printing pipeline might look something
like:
pamscale -xysize 2400 3000 image.pgm | pamditherbw -hilbert | \
pamtopnm | pnmtops -scale 0.25 > image.ps
Other Options
-value This option alters the thresholding value for Floyd-Steinberg,
Atkinson, and simple thresholding. It should be a real number
between 0 and 1. Above 0.5 means darker images; below 0.5 means
lighter.
-clump This option alters the number of pixels in a clump when the
Hilbert curve method is used. 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 lose some grey scale linearity.
-randomseed=integer
The Floyd-Steiberg and Atkinson methods use random numbers to
diffuse the error. This is the seed for the random number gener-
ator. The other methods do not employ random numbers and ignore
this option.
Use this to ensure you get the same image on separate invoca-
tions.
By default, pamditherbw uses a seed derived from the time of day
and process ID, which gives you fairly uncorrelated results in
multiple invocations.
This option was new in Netpbm 10.45 (December 2008).
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 "Digital 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 ALSO
pamtopnm(1), pgmtopgm(1), pbmtopgm(1), pamthreshold(1), pbmreduce(1),
pnmconvol(1), pamscale(1), pam(1), pnm(1),
HISTORY
pamditherbw was new in Netpbm 10.23 (July 2004), but is essentially the
same program as pgmtopbm that has existed practically since the begin-
ning. pamditherbw differs from its predecessor in that it properly adds
brightnesses (using gamma transformations; pgmtopbm just adds them lin-
early) and that it accepts PAM input in addition to PGM and PBM and pro-
duces PAM output.
pamditherbw obsoletes pgmtopbm.
-atkinson was new in Netpbm 10.38 (March 2007).
AUTHOR
Copyright (C) 1989 by Jef Poskanzer.
DOCUMENT SOURCE
This manual page was generated by the Netpbm tool 'makeman' from HTML
source. The master documentation is at
http://netpbm.sourceforge.net/doc/pamditherbw.html
netpbm documentation 10 May 2010 Pamditherbw User Manual(1)
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