Graded facsimile image signal coding system

Abstract

A graded facsimile image signal coding system is disclosed which is provided with means for distributing a multi-graded facsimile picture signal, represented by 2 n gradation levels, into n bit planes corresponding to n digits; initial encoding means for detecting, from picture elements forming each bit plane, picture elements at intervals of ΔX picture elements on every ΔY-th line and encoding the detected picture elements; and additional coding means for .[.further .]. coding each one of .[.said encoded.]. .Iadd.picture elements except said detected .Iaddend.picture elements with reference to encoded four picture elements positioned around said one picture element to be coded.

What we claim is: 
     
       1. A graded facsimile image signal coding system, comprising means for distributing a multi-graded facsimile picture signal, represented by 2 n  gradation levels, into n bit planes corresponding to n digits; initial encoding means for detecting, from picture elements forming each bit plane, picture elements at intervals of ΔX picture elements on every ΔY-th line and encoding the detected picture elements; and additional encoding means for .[.further.]. encoding each one of said .[.encoded.]. .Iadd.picture elements except said detected .Iaddend.picture elements with reference to encoded four picture elements positioned around said one picture element to be encoded. 
     
     
       2. A graded facsimile image signal coding system according to claim 1, in which said additional encoding means comprises mode-1 encoding means for encoding, by referring to four of the encoded picture elements, a picture element surrounded by the four reference picture elements and lying centrally thereof; and mode-2 encoding means for encoding, by referring to four picture elements encoded by the initial encoding means and the mode-1 encoding means, a picture element surrounded by the four picture elements lying above and below it and on its left and right; wherein the initial encoding and mode-1 and mode-2 encoding are effected for each of the n bit planes, using 2 n  (where n is an integer) as an initial value of each of the ΔX and ΔY and thereafter the mode-1 and mode-2 encoding operations are repeated with the values of the ΔX and ΔY reduced by half. 
     
     
       3. A graded facsimile image signal coding system according to claim 2, in which said mode-1 encoding means and said mode-2 encoding means are formed so as to successively perform for each ones of a plurality of categories of said encoded picture elements, said categories being determined in accordance with the number of black picture elements in said four reference picture elements.