Image forming apparatus and method

1. An image forming apparatus, comprising:

a first group of large nozzles which eject large droplets of liquid for a color;

a second group of small nozzles which eject small droplets of the liquid for the color, the small droplets having volume smaller than the large droplets;

a dot data creation device which creates dot data according to input image data;

a dot data correction device which corrects the dot data if there is an abnormal nozzle in one of the first and second groups, in such a manner that a corrective nozzle is selected from the other of the first and second groups, and droplet ejection performed by the corrective nozzle substitutes for droplet ejection that is originally to be performed by the abnormal nozzle; and

a driving device which drives the large and small nozzles to eject the large and small droplets according to the corrected dot data, wherein:

the large nozzles and the small nozzles are arranged in a first nozzle row and a second nozzle row, respectively, along a first direction;

the driving device drives the large and small nozzles to eject the large and small droplets while moving the first and second nozzle rows relatively to a recording medium in a second direction perpendicular to the first direction;

the dot data correction device selects the corrective nozzle located in a vicinity of the abnormal nozzle with regard to the first direction;

the image forming apparatus further comprises a nozzle row position calculation device which calculates an amount of relative displacement between the first nozzle row and the second nozzle row in the first direction; and

the dot data correction device corrects the dot data while taking the calculated amount of the relative displacement into consideration.

2. The image forming apparatus as defined in claim 1 , wherein:

the large nozzles and the small nozzles are arranged in a first nozzle row and a second nozzle row, respectively, along a first direction;

the driving device drives the large and small nozzles to eject the large and small droplets while moving the first and second nozzle rows relatively to a recording medium in a second direction perpendicular to the first direction; and

the dot data correction device selects the corrective nozzle located at a position same with the abnormal nozzle with regard to the first direction.

3. The image forming apparatus as defined in claim 1 , further comprising an abnormal nozzle finding device which finds the abnormal nozzle.

4. An image forming method for an image forming apparatus having a first group of large nozzles and a second group of small nozzles, the large nozzles and the small nozzles being arranged in a first nozzle row and a second nozzle row, respectively, along a first direction, the large nozzles ejecting large droplets of liquid for a color, the small nozzles ejecting small droplets of the liquid for the color, the small droplets having volume smaller than the large droplets, the method comprising the steps of:

creating dot data according to input image data;

finding an abnormal nozzle in the large and small nozzles;

calculating an amount of relative displacement between the first nozzle row and the second nozzle row in the first direction;

correcting the dot data while taking the calculated amount of the relative displacement into consideration if the abnormal nozzle is found in one of the first and second groups, in such a manner that a corrective nozzle located in a vicinity of the abnormal nozzle with regard to the first direction is selected from the other of the first and second groups, and droplet ejection performed by the corrective nozzle substitutes for droplet ejection that is originally to be performed by the abnormal nozzle; and

driving the large and small nozzles to eject the large and small droplets according to the corrected dot data while moving the first and second nozzle rows relatively to a recording medium in a second direction perpendicular to the first direction.