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.
This program is part of Netpbm.
pnmquant reads a PNM image as input. It chooses ncolors colors to best represent the image, maps the existing colors to the new ones, and writes a PNM image as output.
This program is simply a combination of pnmcolormap and pnmremap, where the colors of the input are remapped using a color map which is generated from the colors in that same input. The options have the same meaning as in those programs. See their documentation to understand pnmquant.
You may actually get fewer than ncolors colors in the output because the method pnmcolormap uses to choose the best set of colors for the image is not the same as the method pnmremap uses to determine the best color from the set to represent an individual color. For example, pnmcolormap may include salmon in the color map as the best representative of a pink pixel in the input and include coral in the color map as the best representative of an actual coral pixel in the input. But pnmremap is free to use any color in the color map to represent that pink pixel and would find coral is a closer match for pink than salmon and therefore use coral for pink. pnmremap might not use salmon for any pixel.
This waste of a slot in the color map is a consequence of the approximate method pnmcolormap uses in order to compute the color map with a practical amount of computation.
It is much faster to call pnmcolormap and pnmremap directly than to run pnmquant. You save the overhead of the Perl interpreter and creating two extra processes. pnmquant is just a convenience.
Here is an example of the relationship between the programs:
This:
$ pnmquant 256 myimage.pnm >/tmp/colormap.pnm >myimage256.pnm
does essentially this:
$ pnmcolormap 256 myimage.pnm >/tmp/colormap.pnm
$ pnmremap -mapfile=/tmp/colormap.pnm myimage.pnm >myimage256.pnm
In addition to the options common to all programs based on libnetpbm (most notably -quiet, see Common Options), pnmquant recognizes the following command line options:
These options control the selection of the palette. They are options to pnmcolormap.
These options control which color from the palette the program uses to replace a pixel of a certain color from the input. They are options to pnmremap.
pnmquant did not exist before Netpbm 9.21 (January 2001). Before that, ppmquant did the same thing, but only on PPM images. ppmquant continues to exist, but is only a front end (for name compatibility) to pnmquant.
-version did not exist before Netpbm 10.75 (June 2016).
-norandom did not exist before Netpbm 10.82 (March 2018).