A full color process image that can be printed with cyan, magenta, yellow, and black inks is produced as a duotone image using only two colors of ink. Color data defining the colors used in the full color image serve as input to a process that maps the colors into a duotone gamut defined by the two colors...http://www.google.com.hk/patents/US5982924?utm_source=gb-gplus-share專利 US5982924 - Method and system for reproducing color images as duotones
