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You can use this map as input to pnmremap on the same input image to quantize the colors in that image, I.e. produce a similar image with fewer colors. pnmquant does both the pnmcolormap and pnmremap steps for you.
A PNM colormap is a PNM image of any dimensions that contains at least one pixel of each color in the set of colors it represents.
The quantization method is Heckbert's "median cut". See the section QUANTIZATION METHOD.
If the input image is a PPM, the output image is a PPM. If the input image is a PBM or PGM, the output colormap is a PGM. Note that a colormap of a PBM image is not very interesting.
The colormap generally has the same maxval as the input image, but pnmcolormap may reduce it if there are too many colors in the input, as part of its quantization algorithm.
If you want to create a colormap without basing it on the colors in an input image, see ppmcolors.
The single parameter, which is required, is the number of colors you want in the output colormap. pnmcolormap may produce a color map with slightly fewer colors than that. You may specify all to get a colormap of every color in the input image (no quantization).
A quantization method is a way to choose which colors, being fewer in number than in the input, you want in the output. pnmcolormap uses Heckbert's "median cut" quantization method.
This method involves separating all the colors into "boxes," each holding colors that represent about the same number of pixels. You start with one box and split boxes in two until the number of boxes is the same as the number of colors you want in the output, and choose one color to represent each box.
When you split a box, you do it so that all the colors in one sub-box are "greater" than all the colors in the other. "Greater," for a particular box, means it is brighter in the color component (red, green, blue) which has the largest spread in that box. pnmcolormap gives you two ways to define "largest spread.": 1) largest spread of brightness; 2) largest spread of contribution to the luminosity of the color. E.g. red is weighted much more than blue. Select among these with the -spreadbrightness and -spreadluminosity options. The default is -spreadbrightness.
pnmcut provides three ways of choosing a color to represent a box: 1) the center color - the color halfway between the greatest and least colors in the box, using the above definition of "greater"; 2) the mean of the colors (each component averaged separately by brightness) in the box; 3) the mean weighted by the number of pixels of a color in the image.
Note that in all three methods, there may be colors in the output which do not appear in the input at all.
Select among these with the -center, -meancolor, and -meanpixel options. The default is -center.