Image forming apparatus capable of improving image quality of image formed on sheet, and determination method

A determination method executed in an image forming apparatus including a first ejection portion which includes a plurality of first nozzles arranged along a width direction orthogonal to a conveying direction of a sheet, and causes ink to be ejected from each of the first nozzles, and a second ejection portion which includes a plurality of second nozzles arranged along the width direction, some of the plurality of second nozzles being arranged so as to overlap with some of the plurality of first nozzles in the width direction, and causes the ink to be ejected from each of the second nozzles, the determination method comprising:

• a detection step of detecting, for each line data for one line along a main scanning direction corresponding to the width direction, that is included in image data used for driving the first ejection portion and the second ejection portion, edges included in a correspondence area corresponding to an overlapping portion between the plurality of first nozzles and the plurality of second nozzles in the line data;
• and
• a determination step of randomly determining, when the edges are detected in the detection step, a division position for dividing the line data into first line data used for driving the first ejection portion and second line data used for driving the second ejection portion in, out of two density areas sectioned by the edge on an upstream side or a downstream side of a specific direction along the main scanning direction out of the edges included in the correspondence area, the density area having a relatively high density, and randomly determining, when the edges are not detected in the detection step, the division position in the correspondence area.