An Extension of the CCITT Facsimile Codes for Dithered Pictures

The International Telegraph and Telephone Consultative Committee (CCITT) has recently recommended two redundancy reduction codes for the digital transmission of two-level facsimile over the general switched telephone networks. 1 In the future, most facsimile apparatus using redundancy reduction techniques will use these codes. The first code is a one-dimensional run length code called the modified Huffman code (MHC). The second code is a two-dimensional extension of the modified Huffman code, called the modified READ code (MRC). Dithering is an image processing technique which creates a twolevel picture that gives the illusion of a multilevel picture by appropriately controlling the spatial density of black and white picture elements. 2 " 6 It is useful in systems that use inherently bilevel displays (e.g., plasma panels). Dithering allows a picture to be transmitted as 391 a two-level picture, thus greatly reducing the number of bits to be transmitted. The high frequency components of a dithered picture prevent the use of CCITT codes as an efficient redundancy reduction scheme. 5 Judice, 5 Netravali et al.,' and Johnsen 8 have proposed several coding techniques for dithered pictures. Judice 5 has proposed a bit interleaving scheme which regroups pels with the same or similar threshold levels, thus allowing the use of run-length coding. We are interested in coding schemes for dithered pictures that would require only a slight modification to the standardized one-dimensional and two-dimensional CCITT codes.