pgmtopbm(1)							   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)

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