This utility takes a raw binary rgb image file as input, and 
outputs a CMYK equivalent image to stdout.
It uses the algorithm documented here:

   http://en.wikipedia.org/w/index.php?title=CMYK_color_model&oldid=147065274#Conversions

Use it as follows:

   > rgb2cmyk rgbfile.raw > cmykfile.raw

Robin Mallinson 18th August 2010