man > pamditherbw(1)

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 | -clus‐
       ter4 | -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 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  (actu‐
       ally,  it  just takes the first channel of whatever you give it and treats it as if it repre‐
       sented 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 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 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.  Thresh‐
       olding 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 sophisticated 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 (-clus‐
       ter3, -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,  look‐
       ing  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;;\  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



   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 im‐
              ages; 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 generator.  The other methods do not  employ  random
              numbers and ignore this option.

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

              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),  pam‐
       scale(1), pam(1), pnm(1),


HISTORY
       pamditherbw  was new in Netpbm 10.23 (July 2004), but is essentially the same program as pgm‐
       topbm that has existed practically since the beginning.  pamditherbw differs from its  prede‐
       cessor 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 out‐
       put.

       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)