pngquant

About pngquant

From Debian's package description

pngquant is a command-line conversion utility to quantize and dither truecolor PNG images, especially those with a full alpha channel, down to 8-bit (or smaller) RGBA-palette PNGs. Such images are usually two to four times smaller than the full 32-bit versions, and partial transparency is preserved quite nicely. This makes pngquant especially useful both for Web sites and for PlayStation 2 development, where one of the texture formats is RGBA-palette-based (though not PNG-compressed). This is the same technique used for many of the images on the Miscellaneous Transparent PNGs page (http://www.libpng.org/pub/png/pngs-img.html), and the results are often indistinguishable from the original, truecolor PNG images.

Optimizers (like pngcrush and optipng) optimize the compression, usually losslessly, while pngquant quantizes colors down to 256 (or fewer) distinct RGBA combinations, which is lossy.

From Debian's description of the pngquant package, under that package's license.