EFFICIENT TRANSMISSION OF HALF-TONES VIA FACSIMILE

The conventional method for sending halftone images via facsimile mach ines is inefficient The previously proposed ToneFac algorithm improves the transmission of halftone images. ToneFac represents a halftone im age by mean gray values of the disjoint blocks and an error image, whi ch records the difference between the desired halftone and the halfton e generated using the mean gray values. To improve on ToneFac, we prop ose additional processing techniques: searching for the error-minimizi ng gray value for each block; quantization and coding of block values; bit switching, which transforms the error image into a more compressi ble image; optimal block sizing; and spurious dot filtering, which rem oves perceptually insignificant dots. The new algorithm is compared to other methods, including adaptive arithmetic coding, and is shown to provide improvement in bit rate. A theoretical consideration of the co mpression ratio from the ToneFac algorithm is also given. (C) 1996 SPI E and IS&T.