Patent US 4,229,768

In the transmission of binary image signals obtained by scanning of an original document, a high signal compression ratio without deterioration in image quality is achieved by subjecting the binary image signals to a selective thinning process in which the signals of a run-length within a determined range are converted into signals of a determined length and by subsequently coding thus selectively thinned signals. A further improved signal compression is also established without deterioration of image quality by conducting, prior to the selective thinning process, preliminary processes such as elimination of isolated dots in the image or smoothing in the principal and auxiliary scanning directions. Furthermore signal compression ratios are increased by preparing state signals representing the correlation between the first image signals present on a first scan line and the second image signals present on a second scan line to encode and transmit the first image signals and the state signals, and still to allow regeneration of the second image signals at the reception end. Transmission of signals either with coding of a high compression or with a coding of a higher popularity can also be achieved through simple switching of the coding system. Furthermore there is also provided effective thinning on graphic information which could not be easily handled with the conventional thinning processes.