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)