ppmdither
Reduce the number of colors in an image by applying dithering. tldr-pages
On several platforms
Several platforms
Read a PPM image, apply dithering, and save it to a file
ppmdither path/to/image.ppm >path/to/file.ppmIdentifySpecify the desired number of shades for each primary color
ppmdither -red 2 -green 3 -blue 2 path/to/image.ppm >path/to/file.ppmIdentifySpecify the dimensions of the dithering matrix
ppmdither -dim 2 path/to/image.ppm >path/to/file.ppmIdentify