CCITT Compatible Coding of Multilevel Pictures

Simultaneous developments (algorithmic as well as systems) have taken place for many years in coding and transmission of two-level (black and white) document facsimile, and multilevel (many shades of gray) pictures. 1,2 T h e former type of pictures require very high spatial resolution to preserve the sharpness and have been coded by one* Bell Laboratories. °Copyright 1983, American Telephone & Telegraph Company. Photo reproduction for noncommercial use is permitted without payment of royalty provided that each reproduction is done without alteration and that the Journal reference and copyright notice are included on the first page. The title and abstract, but no other portions, of this paper may be copied or distributed royalty free by computer-based and other information-service systems without further permission. Permission to reproduce or republish any other portion of this paper must be obtained from the Editor. 2765 dimensional run-length coding and two-dimensional edge difference coding [Comite Consultatif International Telegraphique et Telephonique (CCITT) one- and two-dimensional codes].3 On the other hand, multilevel pictures contain gradual luminance transitions, and therefore require lower spatial resolution. They have been coded by Differential Pulse Code Modulation (DPCM) and transform methods. Most pictures used in business facsimile systems and audiographics conferencing contain a mixture of two-level and multilevel segments or subpictures. Coding such pictures using two-level techniques would not be adequate from the point of view of the picture quality, and using multilevel techniques would generate enormous data rates.