Ubuntu Manpage: pgmtopbm - convert a portable graymap into a portable bitmap

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

AUTHOR