Encoding facsimile images for packet-switched networks

An image compression algorithm is presented that is suitable for transmitting a high-resolution (400 dot/in) black-and-white (two-tone) facsimile image over a packet network. The algorithm decomposes the image into nondroppable (essential) and droppable (supplementary) bits. The nondroppable bits describe the image at low resolution (200 dot/in) and can be coded with any of the standard CCITT Group 3 or Group 4 techniques. Thus, the algorithm is compatible with the large installed base of Group 3 facsimile machines. The droppable (supplementary) bits are approximately 45% of the total number of bits and can be dropped by the network to relieve congestion. The supplementary bits are encoded with a predictive Huffman method. Resynchronization after missing some of the supplementary information presents no serious difficulties. The algorithm is information preserving, assuming that all droppable bits are available, and its compression efficiency is equal to or better than that of direct two-dimensional Group 3 or 4 CCITT encoding of the high-resolution (400 dot/in) image. Frame memory is not needed