CCITT Compatible Coding of Multilevel Pictures
H. Gharavi, Arun N. Netravali · Bell System Technical Journal · 1983
The Comite Consultatif International Telegraphique et Telephonique (CCITT) has recently recommended a code for two-level (black and white) graphics transmission. A large number of pictures in graphics communication contain areas that cannot be represented adequately by only two shades of gray. We describe techniques by which a composite picture, containing an arbitrary mixture of two- and multilevel areas, can be coded by schemes that are compatible with the CCITT code. First, the composite picture is segmented automatically into two types of areas: one requiring only two levels (text, drawings, etc.) and the other requiring multilevels (for example, photos). A Differential Pulse Code Modulation (DPCM) scheme is then used to code the multilevel areas. Code assignment for the outputs of the DPCM quantizer are based on the local conditional statistics, and the bit stream is processed to change the statistics of the run lengths so that the CCITT run-length code becomes efficient. Results of computer simulations are presented in terms of quality of processed pictures and the required bit rate. Simulations show that our CCITT compatible scheme is as efficient as an incompatible but optimum DPCM coding scheme.