Patent ID: 6906825

Claim:
An image processor for converting input image data having a first number of tones into halftone-processed image data having a second number of tones which is lower than the first number of tones by halftone processing, and for outputting a dot image corresponding to the halftone-processed image data, said image processor comprising: halftone processing means for halftone-processing the input image data based on thresholds in a threshold array extending over a plurality of dither threshold planes each having a same size unit threshold matrix on which the thresholds are positioned, to produce the halftone-processed image data; and image output means, having a first output position accuracy in a first scan direction and a second output position accuracy which is lower than the first output position accuracy in a second scan direction orthogonal to the first scan direction, for outputting the dot image, which has continuous medium-to-high tone dots in the second scan direction, in accordance with the halftone-processed image data; wherein an order of the thresholds on the unit threshold matrix is defined based on both a reference threshold array indicating the order of the thresholds positioned on the unit threshold matrix on each dither threshold plane and a threshold sequence indicating correlation among the plurality of dither threshold planes with respect to the threshold array; and wherein the medium-to-high tone dots are produced based on the order of the thresholds from medium to high in the unit threshold matrix, the order of the medium-to-high thresholds is of both an aperiodic array and an anisotropic array, and the anisotropic array includes neighboring thresholds having close values in the second scan direction.