Inkjet printing apparatus

1. An inkjet printing apparatus that performs printing process of printing an image by ejecting an ink droplet from a nozzle on a recording medium being transported, the printing apparatus comprising:

a transport mechanism that transports said recording medium in a transport direction;

a first line head group including a first line head in number of N that ejects first ink;

a second line head group including second line heads in number of M that ejects second ink; and

a controller that controls the operations of said transport mechanism, said first line head, and said second line heads, wherein

N is a natural number of 1 or more,

M is a natural number larger than N,

said first line head and said second line heads each include a plurality of nozzles aligned in a width direction,

said first line head and said second line heads each form images along lines by ejecting ink droplets from said nozzles, the lines being regions extending in said width direction along a surface of said recording medium,

said first line head and said second line heads are arranged to be spaced from each other in said transport direction,

said controller executes a first printing step of making both said first line head group and said second line head group perform said printing process,

said controller makes a switch between said first printing step and a second printing step,

in said first printing step, each of said first line head in number of N performs said printing process on every N-th line out of lines on said recording medium aligned in said transport direction,

in said first printing step, each of said second line heads in number of M performs said printing process on every M-th line out of said lines aligned in said transport direction,

in said first printing step, an image formed by said second line head group has a print resolution in said transport direction higher than that of an image formed by said first line head group,

in said second printing step, both said first line head group and said second line head group perform said printing process, and

in said second printing step, an image formed by said first line head group and an image formed by said second line head group have the same print resolution in said transport direction.

2. The inkjet printing apparatus according to claim 1 , wherein

in said first printing step, an image formed by said second line head group has a print resolution in said transport direction that is M/N times that of an image formed by said first line head group.

3. The inkjet printing apparatus according to claim 2 , wherein

in said first printing step, the number of lines on one page of said recording medium along which said first line head group forms images and the number of lines on one page of said recording medium along which said second line head group forms images are in a ratio N:M.

4. The inkjet printing apparatus according to claim 3 , wherein

said controller makes a switch between said first printing step and a third printing step,

in said third printing step, said second line head group perform said printing process while stopping said printing process by said first line head group, and

a speed of transporting said recording medium employed in said third printing step is higher than that of transporting said recording medium employed in said first printing step.

5. The inkjet printing apparatus according to claim 4 , wherein

a speed of transporting said recording medium employed in said third printing step is M/N times that of transporting said recording medium employed in said first printing step.

6. The inkjet printing apparatus according to claim 3 , wherein

said controller makes a switch between said first printing step and a fourth printing step,

in said fourth printing step, said second line head group perform said printing process while stopping said printing process by said first line head group, and

an image formed by said second line head group in said fourth printing step has a print resolution in said transport direction higher than that of an image formed by said first line head group in said first printing step.

7. The inkjet printing apparatus according to claim 6 , wherein

an image formed by said second line head group in said fourth printing step has a print resolution in said transport direction that is M/N times that of an image formed by said first line head group in said first printing step.

8. The inkjet printing apparatus according to claim 1 , wherein

said second ink is black ink.