pamditherbw



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




NAME

       pamditherbw - dither grayscale image to black and white



SYNOPSIS

       pamditherbw

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

       [-value val]

       [-clump size]

       [pamfile]



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.



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; 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 will almost always give the best looking results;  how-
       ever,  looking good is not always what you want.  For instance, thresh-
       olding can be used in a pipeline with the  pnmconvol  tool,  for  tasks
       like  edge  and  peak  detection.   And clustered-dot dithering gives a
       newspaper-ish look, a useful special effect.

       The -value option alters the thresholding value for Floyd-Steinberg 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 loose 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

       All options can be abbreviated to their shortest unique prefix.



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), 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 begin-
       ning.  pamditherbw differs from its predecessor  in  that  it  properly
       deals  with  gamma adjustment and that it accepts PAM input in addition
       to PGM and PBM and produces PAM output.

       pamditherbw obsoletes pgmtopbm.



AUTHOR

       Copyright (C) 1989 by Jef Poskanzer.



netpbm documentation             20 June 2004       Pamditherbw User Manual(0)

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