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

       pamditherbw - dither grayscale image to black and white


       [-floyd	|  -fs	|  -atkinson | -threshold | -hilbert | -dither8 | -d8 | -cluster3 | -c3 |
       -cluster4 | -c4 | -cluster8 | -c8]

       [-value val]

       [-clump size]



       All options can be abbreviated to their shortest unique prefix.

       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-

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

       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

       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.

	      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).

       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

       pamtopnm(1)  , pgmtopgm(1) , pbmtopgm(1) , pamthreshold(1) , pbmreduce(1) , pnmconvol(1) ,
       pamscale(1) , pam(1) , pnm(1) ,

       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).

       Copyright (C) 1989 by Jef Poskanzer.

netpbm documentation			   10 May 2010		       Pamditherbw User Manual(0)
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