pamditherbw(1) - phpMan

Pamditherbw User Manual(0) Pamditherbw User Manual(0)

NAME
pamditherbw - dither grayscale image to black and white

SYNOPSIS
pamditherbw

[-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 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 understand 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 blurring the result with pnmsmooth or pnm-
convol. 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 depend-
ing 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 sophisticated thresh-
olding.

If all you want is to change a PGM image with maxval 1 to a PBM image, pamtopnm will do
that.

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 dif-
ferent 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/> Atkinson
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 option 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 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

-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. 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
This is the seed for the random number generator that generates the pixels.

Use this to ensure you get the same image on separate invocations.

By default, pgmnoise 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 Fill-
ing 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), pam-
scale(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 beginning. 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 pro-
duces PAM output.

pamditherbw obsoletes pgmtopbm.

-atkinson was new in Netpbm 10.38 (March 2007).

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(0)

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