Patent ID: 6917446

Claim:
An image processing apparatus for quantizing image data indicative of a density gradation level of a constituent pixel of an image into a discrete value on the basis of quantization levels of a number smaller than the maximum density gradation level and not smaller than two, the apparatus comprising: an N-level/M-level quantization circuit for performing an N-level quantization process with the use of (N−1) N-level quantization thresholds (wherein N is an integer satisfying a relationship of (maximum density gradation level)>N≧2) or performing an M-level quantization process with the use of (M−1) M-level quantization thresholds (wherein M is an integer satisfying a relationship of (maximum density gradation level)>M >N) for quantization of image data of an object pixel on an N-level basis or on an M-level basis; an error diffusion circuit for distributing an error generated through the N-level quantization process or the M-level quantization process performed by the N-level/M-level quantization circuit to peripheral pixels yet to be subjected to the N-level quantization process or the M-level quantization process around the object pixel for modification of image data of the peripheral pixels; an N-level quantization threshold setting circuit for setting the N-level quantization thresholds to be used in the N-level/M-level quantization circuit in a periodically variable manner; and a process setting circuit for causing the N-level/M-level quantization circuit to perform the M-level quantization process for pixels adjacent to a pixel corresponding to at least one of a peak point and a saddle point in the periodic variation of the N-level quantization thresholds and causing the N-level/M-level quantization circuit to perform the N-level quantization process for pixels corresponding to the peak point and the saddle point.