This is a small program dedicated to reducing the number of colors in an image. It operates with files in PNG or binary PPM format (Portable PixMap), and features automatic palette generation (using a median-cut algorithm) and dithering implemented with the Floyd-Steinberg method.

$ brew install tessarin/core/dither

First, install libpng. Running make will then compile the program and generate the documentation (requires Perl). To install, move the executable and manual files to appropriate directories in your system.

$ make
$ mv dither ~/bin
$ mv dither.1 ~/man/man1

Individual targets can also be specified to only compile the program or to generate the manual page:

$ make dither
$ make doc

$ dither [-p name.size] [-dv] input output

Detailed information about the program options are included in the manual.

By default, dither will use a 3-bit RGB palette and dithering when processing an image. Other possible palettes include:

• Grayscale of any given size
• Automatic, generated with the given size using a median-cut quantization algorithm
• Custom palette

Dithering on the final image can be disabled and the program can also be used just to generate a palette.

$ dither -p auto.32 bird-original.png bird-auto.32.png

$ dither -p bw flower-original.png flower-bw.png

Generates a perfect checkerboard pattern.

$ dither -p bw gray-original.png gray-dithered.png