Method and apparatus for binary coding of image data including adding

Abstract

Error data .SIGMA.e.sub.P accumulated for a target pixel of binary coding or pixel of interest P is read from an error memory (20) and inputted into an adder (22). The adder (22) adds the error data to multi-valued image data regarding the pixel of interest P, so as to correct the multi-valued image data. A comparator (24) compares the corrected multi-valued image data outputted from the adder (22) with a predetermined reference value and outputs binary image data based on a result of the comparison. A bit converter (28) converts the one-bit binary image data to 8-bit data. A subtracter (26) subtracts the binary-coded image data from the image data prior to binary coding to obtain an error `e`. An error distribution circuit (30) distributes the error `e` into pixels A, C, D, and E, where the pixels A, C, and D are respectively located on the immediate right of, immediately below, and on the lower left of the pixel of interest P and the pixel E is located on the immediate left of the pixel D. The error memory (20) adds distributed portions of the error e.sub.A, e.sub.C, e.sub.D, and e.sub.E to error data .SIGMA.e.sub.A, .SIGMA.e.sub.C, .SIGMA.e.sub.D, and .SIGMA.e.sub.E, so as to update the values of the error data .SIGMA.e.sub.A, .SIGMA.e.sub.C, .SIGMA.e.sub.D, and .SIGMA.e.sub.E. This method of distributing the error `e` into the above peripheral pixels enables multi-valued image data for the pixel of interest P having a relatively low tone value or a relatively high tone value to be binary coded without damaging the quality of picture.