Inkjet printer

1. An inkjet printer comprising:

a first ink head including a plurality of first nozzles arrayed along a sub-scanning direction and ejecting a first ink onto a recording medium; and

a controller, including an m number of data setters including a first data setter to an m-th data setter, where m is a natural number equal to or greater than 2, and an m number of print controllers including a first print controller to an m-th print controller, the controller controlling the first ink head to form ink dots of the first ink on the recording medium; wherein

upon receiving data for the ink dots of the first ink, the first data setter sets a first dot group including some or all of the ink dots of the first ink;

upon receiving the data for the ink dots of the first ink, an n-th data setter sets an n-th dot group including some or all of the ink dots of the first ink, where n is a natural number in a range of from 2 to m;

the first print controller controls the first ink head to form the first dot group on the recording medium;

an n-th print controller controls the first ink head to form the n-th dot group over an (n−1) dot group; and

the first to the m-th data setters set the first to the m-th dot groups so that at least some of ink dots belonging to the first to the m-th dot groups overlap each other.

2. The inkjet printer according to claim 1 , wherein the first to the m-th data setters set the first to the m-th dot groups so that the ink dots belonging to the first to the m-th dot groups include all the ink dots of the first ink.

3. The inkjet printer according to claim 1 , further comprising:

a sub-scanning-direction conveyor that transfers the recording medium in the sub-scanning direction relative to the first ink head; wherein

the first ink head includes the m number of nozzle arrays including a first nozzle array to an m-th nozzle array that are arrayed in that order from upstream to downstream along the sub-scanning direction and include an equal number of the first nozzles;

the first print controller causes the first nozzles of the first nozzle array to eject the first ink onto the recording medium, to form the first dot group on the recording medium; and

the n-th print controller controls the sub-scanning-direction conveyor to transfer the recording medium toward downstream along the sub-scanning-direction, and after the transferring, causes the first nozzles of an n-th nozzle array to eject the first ink to form the n-th dot group over the (n−1) dot group.

4. The inkjet printer according to claim 3 , further comprising:

a second ink head including a plurality of second nozzles arrayed along the sub-scanning direction and ejecting a second ink onto the recording medium; wherein

the first ink head and the second ink head are arranged along a main scanning direction orthogonal to the sub-scanning direction;

the controller further includes an m number of additional data setters including a first additional data setter to an m-th additional data setter, and an m number of additional print controllers including a first additional print controller to an m-th additional print controller, the controller controlling the second ink head and the sub-scanning-direction transfer device to form ink dots of the second ink on the recording medium;

upon receiving data for the ink dots of the second ink, the first additional data setter sets a first additional dot group including some or all of the ink dots of the second ink;

upon receiving the data of the ink dots of the second ink, an n-th additional data setter sets an n-th additional dot group including some or all of the ink dots of the second ink;

the first additional print controller controls the second ink head to form the first additional dot group on the recording medium; and

an n-th additional print controller controls the second ink head to form the n-th additional dot group over an (n−1) additional dot group.

5. The inkjet printer according to claim 4 , wherein the first to the m-th additional data setters set the first to the m-th additional dot groups so that at least some of ink dots belonging to the first to the m-th additional dot groups overlap each other.

6. The inkjet printer according to claim 4 , wherein the first to the m-th additional data setters set the first to the m-th additional dot groups so that the ink dots belonging to the first to the m-th dot additional dot groups include all the ink dots of the second ink.

7. The inkjet printer according to claim 4 , wherein

the second ink head includes an m number of additional nozzle arrays including a first additional nozzle array to an m-th additional nozzle array that are arrayed in that order from upstream to downstream along the sub-scanning direction and include an equal number of the second nozzles;

the first additional nozzle array is aligned with the first nozzle array with respect to the sub-scanning direction;

an n-th additional nozzle array is aligned with the n-th nozzle array with respect to the sub-scanning direction;

the first additional print controller causes the second nozzles of the first additional nozzle array to eject the second ink onto the recording medium, to form the first additional dot group on the recording medium; and

the n-th additional print controller controls the sub-scanning-direction transfer device to transfer the recording medium toward downstream along the sub-scanning-direction, and after the transferring, causes the second nozzles of an n-th additional nozzle array to eject the second ink to form the n-th additional dot group over the (n−1) additional dot group.

8. The inkjet printer according to claim 4 , wherein

the first additional data setter extracts the first additional dot group from all the ink dots of the second ink using a first additional mask having a first additional decimation rate; and

the n-th additional data setter extracts the n-th additional dot group from all the ink dots of the second ink using an n-th additional mask having an n-th additional decimation rate.

9. The inkjet printer according to claim 8 , wherein the controller further includes:

a first additional data input interface that accepts an input of the first additional decimation rate; and

an n-th additional data input interface that accepts an input of the n-th additional decimation rate.

10. The inkjet printer according to claim 1 , wherein

the first data setter extracts the first dot group from all the ink dots of the first ink using a first mask having a first decimation rate; and

the n-th data setter extracts the n-th dot group from all the ink dots of the first ink using an n-th mask having an n-th decimation rate.

11. The inkjet printer according to claim 10 , wherein

the controller further includes:

a first data input interface that accepts an input of the first decimation rate; and

an n-th data input interface that accepts an input of the n-th decimation rate.

12. The inkjet printer according to claim 1 , wherein the second data setter sets a second dot group so that the second dot group includes a smaller number of ink dots than the first dot group.

13. The inkjet printer according to claim 12 , wherein

the number m is a natural number greater than or equal to 3; and

the third data setter sets a third dot group so that the third dot group contains a larger number of ink dots than the second dot group and that the third dot group contains a smaller number of ink dots than the first dot group.

14. The inkjet printer according to claim 1 , further comprising a dryer that dries the ink ejected on the recording medium.