quantize the colors in a portable pixmap down to a specified number
see also :
pnmquant - ppmquantall - pnmdepth - ppmdither
[-floyd|-fs] ncolors [ppmfile]
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.
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is a newer, more general program that is backward compatible
with ppmquant. ppmquant may be faster,
Reads a PPM
image as input. Chooses ncolors colors to best
represent the image, maps the existing colors to the new
ones, and writes a PPM image as output.
quantization method is Heckbert’s "median
you can skip the color-choosing step by specifying your own
set of colors with the -mapfile option. The
mapfile is just a 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
0 0 0
255 0 0
0 255 0
0 0 255
255 255 0
255 0 255
0 255 255
255 255 255
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.
-floyd/-fs option enables a Floyd-Steinberg
error diffusion step. 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.
means not to use the Floyd-Steinberg error diffusion. This
is the default.
"Color Image Quantization for Frame Buffer Display" by Paul
Heckbert, SIGGRAPH ’82 Proceedings, page 297.
ppmquantall , pnmdepth ,
ppmdither , ppm
1989, 1991 by Jef Poskanzer.