Gray scale pictures via Group 3 facsimile

The CCITT has developed a recommendation for Group 3 facsimile that is based on the transmission of binary picture elements (pels) having a brightness value that is either black or white. By introducing a minor modification into the basically bilevel facsimile system, dither coding delivers an output image that conveys gray-scale information. An analysis is made of three dither coding techniques: random dither, clumped dither and distributed dither. The comparison of random and clumped dither was also studied by simulation. The random dither simulation runs having the intermediate random number range (RNR=192) exhibited the best combination of maximum image quality and minimum number of transmitted bits, relative to the other random dither runs. The clumped dither images are superior to the best random dither image when both image quality and transmitted bits are considered. For clumped dither, the average compression ratio for the three images is only 1.39, which is significantly poorer than for the same images without dither coding. This compression ratio corresponds to a transmission time of 4.5 min over the public switched telephone network. Although this time is significantly longer than the 1 min typical for nondithered pictures, this performance may be most satisfactory for many requirements and applications.<>