pgmtopbm(1)

NAME

       pgmtopbm - convert a portable graymap into a portable bitmap

SYNOPSIS

       pgmtopbm   [-floyd|-fs|-threshold   |-hilbert   |-dither8|-d8|-cluster3
       |-c3|-cluster4|-c4 |-cluster8|-c8] [-value val] [-clump size] [pgmfile]

DESCRIPTION

       Reads  a portable graymap as input.  Produces a portable bitmap as out-
       put.

       Note that there is no pbmtopgm converter, because any pgm  program  can
       read pbm files automagically.

OPTIONS

       The  default  quantization  method  is  boustrophedonic Floyd-Steinberg
       error diffusion (-floyd or -fs).  Also available are simple  threshold-
       ing  (-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 flag 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
       flag 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 linearity. Typi-
       cally 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: pnmscale -xysize 2400 3000 image.pgm |  pgm-
       topbm -hil | pnmtops -scale 0.25 > image.ps

       All flags 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

       pbmreduce(1), pgm(5), pbm(5), pnmconvol(1), pnmscale(1), pnmtops(1)

AUTHOR

       Copyright (C) 1989 by Jef Poskanzer.

                                 26 July 1988                      pgmtopbm(1)