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floyd(9) [plan9 man page]

FLOYD(9.1)																FLOYD(9.1)

NAME
floyd, halftone, hysteresis - create 1-bit images by dithering SYNOPSIS
fb/floyd [ input ] fb/halftone screen [ input ] fb/hysteresis low high [ input ] DESCRIPTION
Floyd reads a grey-scale input file (default standard input), and reduces it to one bit per pixel using Floyd-Steinberg error-diffusion dither, as improved by Ulichney. The resulting TYPE=bitmap picture file is written to standard output. Ulichney's algorithm involves randomly varying the Floyd-Steinberg diffusion coefficients. As the random number generator is seeded from the clock, floyd may produce different output if rerun on the same input. Halftone reduces grey-scale images to one bit per pixel using ordered dither. The screen argument is the name of a file containing a dither matrix. Halftone searches for screens in /lib/fb/screens. Hysteresis creates one-bit-per-pixel images by thresholding with hysteresis. Any value in the input image less than low is mapped to zero. Any input value less than high is mapped to zero if any of its eight neighbors is less than low. If low and high are equal, this is just an ordinary thresholding operation. Hysteresis makes a useful edge-detection operator if used on a high-pass filtered image. FILES
/lib/fb/screens/* SOURCE
/sys/src/fb/floyd.c /sys/src/fb/halftone.c /sys/src/fb/hysteresis.c SEE ALSO
picfile(9.6) FLOYD(9.1)

Check Out this Related Man Page

pgmtopbm(1)                                                   General Commands Manual                                                  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 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 gen- eral 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 (-clus- ter3, -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, 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. 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)
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