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 output.

       Note that there is no pbmtopgm converter.  Any program that uses the Netpbm  libraries  to
       read  PGM  files,  including virtually all programs in the Netpbm package, will read a PBM
       file automatically as if it were a PGM file.

       If you are using a less intelligent program  that  expects  PGM  input,  use  pnmdepth  to
       convert  the  PBM  file  to  PGM.   As  long as the depth is greater than 1, pnmdepth will
       generate PGM.  This less intelligent program quite probably is also not intelligent enough
       to deal with general maxvals, so you should specify a depth of 255.

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; however, looking good is
       not always what you want.  For instance, thresholding 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. 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: pnmscale -xysize 2400 3000 image.pgm | pgmtopbm -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  Halftoning  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)