Patent ID: 7034964

Claim:
A method for gradation reproduction of a continuous tone image based on an error diffusion method in which, when the gradation of input image data on a continuous tone image is converted into multivalue data based on at least three values using at least two sets of threshold values to reproduce a gradation, an error between a data item before multivaluing and the corresponding multivalued data item is spread, as a corrected error and under spreading conditions determined by weight coefficients, to a plurality of subsequent data items successively input to obtain corrected data, and the corrected data is multivalued, the gradation reproduction method comprising the step of determining the interval between the threshold values so that an amplitude of the corrected data according to rising and falling variations in the corrected error spikes outward beyond the threshold values at least at a halftone gradation between a darkest gradation and a lightest gradation, whereby generation of a transient region where the amplitude of the corrected data becomes excessively small at the halftone gradation of the corrected data is prevented.