.TH ppmquant 1 "12 January 1991"
.IX ppmquant
.SH NAME
ppmquant - quantize the colors in a portable pixmap down to a specified number
.SH SYNOPSIS
.B ppmquant
.RB [ -floyd | -fs ]
.I ncolors
.RI [ ppmfile ]
.br
.B ppmquant
.RB [ -floyd | -fs ]
.RB [ -nofloyd | -nofs ]
.B -mapfile
.I mapfile
.RI [ ppmfile ]
.fi

All options can be abbreviated to their shortest unique prefix.  You
may use two hyphens instead of one to designate an option.  You may
use either white space or equals signs between an option name and its
value.

.SH DESCRIPTION
.B pnmquant 
is a newer, more general program that is backward compatible with 
.BR ppmquant .
.B ppmquant
may be faster, though.

Reads a PPM image as input.  Chooses
.I ncolors
colors to best represent the image, maps the existing colors
to the new ones, and writes a PPM image as output.
.IX "colormap reduction"
.PP
The quantization method is Heckbert's "median cut".
.IX "median cut"
.PP
Alternately, you can skip the color-choosing step by
specifying your own set of colors with the
.B -mapfile
option.  The
.I mapfile
is just a
.I ppm
file; it can be any shape, all that matters is the colors in it.
For instance, to quantize down to the 8-color IBM TTL color set, you
might use:
.nf
    P3
    8 1
    255
      0   0   0
    255   0   0
      0 255   0
      0   0 255
    255 255   0
    255   0 255
      0 255 255
    255 255 255
.fi
If you want to quantize one image to use the colors in another one,
just use the second one as the mapfile.  You don't have to reduce it
down to only one pixel of each color, just use it as is.

If you use a mapfile, the output image has the same maxval as the
mapfile.  Otherwise, the output maxval is the same as the input
maxval, or less in some cases where the quantization process reduces
the necessary resolution.

.PP
The
.BR -floyd / -fs
option enables a Floyd-Steinberg error diffusion step.
.IX Floyd-Steinberg
.IX "error diffusion"
Floyd-Steinberg gives vastly better results on images where the unmodified
quantization has banding or other artifacts, especially when going to a
small number of colors such as the above IBM set.
However, it does take substantially more CPU time, so the default is off.

.BR -nofloyd / -nofs
means not to use the Floyd-Steinberg error diffusion.  This is the default.

.SH REFERENCES
"Color Image Quantization for Frame Buffer Display" by Paul Heckbert,
SIGGRAPH '82 Proceedings, page 297.

.SH "SEE ALSO"
.BR pnmquant (1),
.BR ppmquantall (1), 
.BR pnmdepth (1), 
.BR ppmdither (1), 
.BR ppm (5)

.SH AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer.
.\" Permission to use, copy, modify, and distribute this software and its
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