A halftone cell is composed of a plurality of pixel groupings predefined to correspond to optimal tone level gradations. Selected output pixel groupings vary in number, and in shape of subpixels used from other pixel groupings, in order that successive input levels correspond to successive halftone cells...http://www.google.de/patents/US6643032?utm_source=gb-gplus-sharePatent US6643032 - Marking engine and method to optimize tone levels in a digital output system
