Image forming apparatus

1. An image forming apparatus, comprising:

a print engine configured to eject an ink droplet from a nozzle and thereby print an image to be printed on a print sheet; and

a control unit configured to control the print engine;

wherein when performing on-sheet preliminary ink ejection, the control unit (a) determines a nozzle that performs the on-sheet preliminary ink ejection, (b) randomly selects on the print sheet an ejection position candidate of an ink droplet ejected from the determined nozzle, (c) determines whether the selected ejection position candidate is included by a mask area that includes a postprocessing object, (d1) if the selected ejection position candidate is not included by the mask area, sets as the selected ejection position candidate an ejection position of an ink droplet ejected from the determined nozzle, and (d2) if the selected ejection position candidate is included by the mask area, does not set as the selected ejection position candidate an ejection position of an ink droplet ejected from the determined nozzle and sets an ejection position of an ink droplet ejected from the determined nozzle outside of the mask area.

2. The image forming apparatus according to claim 1 , wherein the postprocessing object is either a trim mark or a color patch.

3. The image forming apparatus according to claim 1 , wherein if the selected ejection position candidate is included by the mask area, the control unit makes ejection timing of the ink droplet ejected from the determined nozzle earlier and thereby sets the ejection position of the ink droplet ejected from the determined nozzle outside of the mask area.

4. The image forming apparatus according to claim 1 , wherein if the selected ejection position candidate is included by the mask area, the control unit delays ejection timing of the ink droplet ejected from the determined nozzle and thereby sets the ejection position of the ink droplet ejected from the determined nozzle outside of the mask area.

5. The image forming apparatus according to claim 1 , wherein if the selected ejection position candidate is included by the mask area, the control unit performs ejection of the ink droplet ejected from the determined nozzle on a subsequent print sheet to the print sheet on which the mask area is set.

6. The image forming apparatus according to claim 1 , wherein

the control unit selects a control type from a predetermined three control types in accordance with user setting, and if the selected ejection position candidate is included by the mask area, performs the on-sheet preliminary ink ejection of the determined nozzle in accordance with the selected control type;

a first control type among the three control types makes ejection timing of the ink droplet ejected from the determined nozzle earlier and thereby sets the ejection position of the ink droplet ejected from the determined nozzle outside of the mask area;

a second control type among the three control types delays ejection timing of the ink droplet ejected from the determined nozzle and thereby sets the ejection position of the ink droplet ejected from the determined nozzle outside of the mask area; and

a third control type among the three control types performs ejection of the ink droplet ejected from the determined nozzle on a subsequent print sheet to the print sheet on which the mask area is set.

7. The image forming apparatus according to claim 1 , wherein

the mask area includes (a) the postprocessing object and (b) a margin set with a predetermined width on an outer edge of the postprocessing object; and

the width of the margin is enabled to be set by a user.