Patent ID: 7710606

Claim:
An image processing method for processing image data to be output to an image forming apparatus that is capable of making a two-way recording to form an image on a recording medium by recording in a forward path and a return path of a scan by an ink-jet recording head, comprising: a halftone process that is based on an inclined line-group keytone and maintains keytone continuity, wherein the halftone process includes a dither process in which the inclined line-group keytone appears at a stage where the recording in the forward path is made, wherein the dither process uses a dither mask that is formed by first patterns and second patterns, the first patterns have a plurality of different threshold values by combinations of dots recognizable as inclined line-group tone patterns, and the second patterns interpolate between the first patterns to obtain linear gradation values, and wherein first threshold values of the dither mask at dot positions recorded during the recording in the forward oath are small relative to second threshold values of the dither mask at dot positions recorded during the recording in the return oath in a section between the first and second threshold values, the first threshold values emphasize the inclined line-group keytone pattern by a combination of specific dots, and the second threshold values are higher than the first threshold values and emphasize the inclined line-group keytone pattern by a combination of specific dots.