PROBLEM TO BE SOLVED: To improve image stability, to prevent the time for image processing from being prolonged, to prevent a memory capacitance from being increased, and to prevent an image defect from becoming remarkable.SOLUTION: An image forming apparatus comprises a storage part for storing a first dither pattern group, a second dither pattern group and a third dither pattern group. The first dither pattern group includes a plurality of dither patterns formed from a line screen which corresponds to each of gradation values within a first range and in which the line width increases as the gradation value increases. The second dither pattern group includes a plurality of dither patterns formed from a void dot screen which corresponds to each of gradation values within a second range, the gradation values within the second range being higher than the gradation values within the first range, and in which dots increase and a void part surrounded by the dots decreases as the gradation value increases. The third dither pattern group includes a plurality of dither patterns for transition from the line screen to the void dot screen, the plurality of dither patterns completing the transition to the void dot screen before achieving a first reference gradation value at which an image defect occurs in image data to be inputted.SELECTED DRAWING: Figure 1
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