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



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]

       [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 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 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)