Abstract: An error-diffusion quantization process is conducted on all the pixels of a continuous tone image, pixel by pixel. In the error-diffusion quantization process to an object pixel, a threshold value T is randomly set for the object pixel. Input density data I.sub.x,y of the object pixel is modified by a sum of quantization errors E distributed to the object pixel during error-diffusion quantization processes previously performed for preceding nearby pixels. The modified input density I'.sub.x,y of the object pixel is compared with the threshold value T set for the object pixel, whereby the input density data of the object pixel is converted into a quantized output data Ie. A quantization error "e" between the modified input density I'.sub.x,y and the quantized output data Ie is determined. The quantization error "e" is distributed to subsequent nearby pixels, which have not yet been quantized, with the use of the weight coefficient matrix .alpha..

Abstract: Input chromatic densities IC, IM, and IY are first converted into corrected chromatic densities I'C, I'M, and I'Y based on the input density IK. The input density IK is then added with the corresponding weighted error sum EK, so that a modified density IKmod is obtained. Similarly, the corrected chromatic densities I'C, I'M, and I'Y are added with the weighted error sums EC, EM, and EY, respectively, so that modified densities ICmod, IMmod, and IYmod are obtained. Then, the modified density IKmod is compared with the threshold T, and an output OK is determined. When the obtained output OK is zero, the ICmod, IMmod, and IYmod are compared with the threshold T, and output data OC, OM, and OY are obtained. On the other hand, when the obtained output OK is a dot-forming data, the output data OC, OM, and OY are forcibly set to a non-dot forming data.

Abstract: An apparatus for processing multilevel image data representing a half-tone image, into bilevel image data representing a bilevel image corresponding to the half-tone image, the multilevel image data including sets of multilevel pixel data each set of which represents one of three or more color values as a first color value indicating a color of a corresponding one of pixels of the half-tone image, and is processed into a corresponding one of sets of bilevel pixel data of the bilevel image data so that the corresponding one set of bilevel pixel data represents one of two color values as a second color value indicating a color of a corresponding one of pixels of the bilevel image, the apparatus including a data processing device for processing each set of multilevel pixel data into a corresponding one set of bilevel pixel data, by an error diffusion process wherein an error value occurring in processing each set of multilevel pixel data into a corresponding one set of bilevel pixel data is distributed to one or