Inkjet printer and printing method

1. An inkjet printer comprising:

a head having a plurality of outlet rows arranged in a predetermined scanning direction, said plurality of outlet rows each having a plurality of outlets arranged in a direction intersecting said scanning direction, said plurality of outlets in each outlet row being provided across the entire width of a printing area on an object with respect to a width direction perpendicular to said scanning direction, wherein when focusing on said head with respect to said width direction, between each pair of adjacent outlets in one outlet row, one outlet from each of the other outlet rows is provided;

a scanning mechanism for moving said object relative to said head in said scanning direction; and

an ejection control part for performing control of ink ejection from said head in parallel with the movement of said object relative to said head, wherein:

said plurality of outlets are each capable of forming dots of a plurality of sizes on said object upon receiving different driving signals from said ejection control part,

under control of said ejection control part, at least one outlet row of said plurality of outlet rows in said head forms dots of only one size out of said plurality of sizes, and the other outlet row(s) forms dots of at least another size, a driving signal for said one size having a different waveform from that of a driving signal for said another size,

the control by said ejection control part is performed based on a halftone image generated by comparing an original image with a threshold matrix,

said threshold matrix has a plurality of positions in a row direction which are associated respectively with a plurality of outlets included in said head, said row direction corresponding to said width direction,

each element in said threshold matrix has threshold values for forming dots of said plurality of sizes, and

values to form no dot are assigned to elements corresponding to said at least one outlet row as threshold values for forming dots of size(s) other than said one size.

2. The inkjet printer according to claim 1 , wherein said other outlet row(s) forms dots of only a size(s) other than said one size.

3. The inkjet printer according to claim 2 , wherein dots of said one size are dots of a largest size out of said plurality of sizes.

4. The inkjet printer according to claim 3 , wherein said plurality of outlets are each provided with a piezoelectric element.

5. The inkjet printer according to claim 2 , wherein said plurality of outlets are each provided with a piezoelectric element.

6. The inkjet printer according to claim 1 , wherein dots of said one size are dots of a largest size out of said plurality of sizes.

7. The inkjet printer according to claim 6 , wherein said plurality of outlets are each provided with a piezoelectric element.

8. The inkjet printer according to claim 1 , wherein said plurality of outlets are each provided with a piezoelectric element.

9. The inkjet printer according to claim 1 , wherein each outlet row in said head is inclined with respect to said width direction.

10. The inkjet printer according to claim 9 , wherein a longitudinal direction of said head is inclined with respect to said width direction.

11. A printing method used in an inkjet printer, comprising the steps of:

a) moving an object relative to a head in a predetermined scanning direction, said head having a plurality of outlet rows arranged in said scanning direction, said plurality of outlet rows each having a plurality of outlets arranged in a direction intersecting said scanning direction, said plurality of outlets in each outlet row being provided across the entire width of an printing area on said object with respect to a width direction perpendicular to said scanning direction, wherein when focusing on said head with respect to said width direction, between each pair of adjacent outlets in one outlet row, one outlet from each of the other outlet rows is provided; and

b) performing control of ink ejection from said head in parallel with said step a), wherein:

said plurality of outlets are each capable of forming dots of a plurality of sizes on said object upon receiving different driving signals,

in said step b), at least one outlet row of said plurality of outlet rows in said head forms dots of only one size out of said plurality of sizes, and the other outlet row(s) forms dots of at least another size, a driving signal for said one size having a different waveform from that of a driving signal for said another size,

the control in said step b) is performed based on a halftone image generated by comparing an original image with a threshold matrix,

said threshold matrix has a plurality of positions in a row direction which are associated respectively with a plurality of outlets included in said head, said row direction corresponding to said width direction,

each element in said threshold matrix has threshold values for forming dots of said plurality of sizes, and

values to form no dot are assigned to elements corresponding to said at least one outlet row as threshold values for forming dots of size(s) other than said one size.

12. The printing method according to claim 11 , wherein said other outlet row(s) forms dots of only a size(s) other than said one size.

13. The printing method according to claim 12 , wherein dots of said one size are dots of a largest size out of said plurality of sizes.

14. The printing method according to claim 11 , wherein dots of said one size are dots of a largest size out of said plurality of sizes.

15. The printing method according to claim 11 , wherein each outlet row in said head is inclined with respect to said width direction.

16. The printing method according to claim 15 , wherein a longitudinal direction of said head is inclined with respect to said width direction.