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. 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 pro-
    gram 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 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.