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 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 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.
       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 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
        Common Options ⟨index.html#commonoptions⟩ ), 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;
        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/⟩     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
              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 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),
       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 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
       produces 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