Pamditherbw User Manual(0) Pamditherbw User Manual(0)NAMEpamditherbw - dither grayscale image to black and white
SYNOPSISpamditherbw
[-floyd | -fs | -atkinson | -threshold | -hilbert | -dither8 | -d8 |
-cluster3 | -c3 | -cluster4 | -c4 | -cluster8 | -c8]
[-value val]
[-clump size]
[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
distance, 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 doesn't under‐
stand 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
ppmdither.
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 thresh‐
olding is a degenerate case of dithering, so pamditherbw does very sim‐
ple thresholding with its -threshold option. But pamthreshold does
more sophisticated thresholding.
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;
Atkinson
⟨http://www.tinrocket.com/projects/programming/graphics/00158/⟩ ; 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 or Atkinson will almost always give the best looking
results; 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
some claim
⟨http://www.tinrocket.com/projects/programming/graphics/00158/⟩ Atkin‐
son works better.
The -value 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.
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
option 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 lose some grey scale lin‐
earity. 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:
pamscale -xysize 2400 3000 image.pgm | pamditherbw-hilbert | pamtopnm | pnmtops -scale 0.25 > image.ps
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 Halfton‐
ing 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 ALSOpamtopnm(1), pgmtopgm(1), pbmtopgm(1), pamthreshold(1), pbmreduce(1),
pnmconvol(1), pamscale(1), pam(1), pnm(1),
HISTORYpamditherbw 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
linearly) and that it accepts PAM input in addition to PGM and PBM and
produces PAM output.
pamditherbw obsoletes pgmtopbm.
-atkinson was new in Netpbm 10.38 (March 2007).
AUTHOR
Copyright (C) 1989 by Jef Poskanzer.
netpbm documentation 03 March 2007 Pamditherbw User Manual(0)
