Efficient fax transmission of halftone image sequences

We propose an algorithm in which a sequence of digital halftone images is efficiently transmitted using encoding compatible with conventional facsimile devices. To enhance the CCITT standardized coding schemes for Group 3 and Group 4 facsimile apparatus, pre-encoding is done on each image in the sequence: The image is either pre-encoded as a combination of bit representation of block means and an "error" image, according to the ToneFac algorithm, or it is pre-encoded as an "interimage" in which interframe redundancy is converted into spatial redundancy, and the least "busy' ' of the above images is encoded and transmitted. The approach is referred to as the ToneSec algorithm.