Image processing method and image processing apparatus for reducing the bad effects of seam lines that appear at the boundary portions for each printing scan in a serial type printer

An image processing apparatus for processing an image by ejecting ink to print dots on a print medium, comprising:

• a scanning unit configured to cause a print head having a nozzle array in which a plurality of nozzles for ejecting ink are arranged in an arraying direction to perform a plurality of scans across a unit area of the print medium in a scanning direction crossing the arraying direction;
• a conveying unit configured to convey the print medium a predetermined length in a conveying direction crossing the scanning direction between scans of the print head such that each of nozzle blocks is able to eject ink to the unit area in the plurality of scans, wherein each of the nozzle blocks is formed by a predetermined number of nozzles arrayed in the arraying direction continuously, and wherein the predetermined length corresponds to a length of each of the nozzle blocks in the arraying direction;
• an obtaining unit configured to obtain image data having a gradation value corresponding to pixel area areas in the unit area, wherein each of the pixel areas consists of a plurality of dot areas;
• a generating unit configured to generate a plurality of print data which determines printing or non-printing of a dot for each of dot areas based on the image data, wherein each of the plurality of the print data corresponds to one of the plurality of scans of the print head;
• and
• a controlling unit configured to control ink ejecting so as to eject ink according to each of the plurality of print data generated by the generating unit in each of the plurality of scans of the print head by the scanning unit,
• wherein the generating unit generates the plurality of print data such that (i) in a case that the obtaining unit obtains image data having a first gradation value, the amount of ink ejection to a first pixel area which is positioned at one end position in the conveying direction in the unit area and the amount of ink ejection to a second pixel area which is positioned at a different position from the first pixel area in the conveying direction in the unit area are substantially equal, and (ii) in a case that the obtaining unit obtains image data having a second gradation value which is greater than the first gradation value, the amount of ink ejection to the first pixel area is greater than the amount of ink ejection to the second pixel area.