Ink discharge complementing method and printing apparatus

1. An ink discharge complementing method which complements an ink discharge defect of a defective nozzle, out of a plurality of nozzles of a discharge head, comprising:

a gradation data acquisition step of obtaining gradation data representing a gradation value of each of a plurality of pixels constituting a print image;

a conversion step of converting the gradation data into defect complemented gradation data by converting the gradation values of the gradation data, using a first conversion rule for the gradation values of the pixels corresponding to normal nozzles other than surrounding nozzles located around the defective nozzle and the defective nozzle, out of the plurality of nozzles, and a second conversion rule for the gradation values of the pixels corresponding to the surrounding nozzles;

a shading correction step of performing a shading correction to correct a variation of an amount of the ink discharged from each of the plurality of nozzles for the defect complemented gradation data;

a halftone step of generating discharge control data by performing a halftone process for the defect complemented gradation data having the shading correction performed therefor; and

an image printing step of printing the print image on a printing medium by the ink discharged from the nozzles by driving the nozzles based on the discharge control data,

the first conversion rule converting the gradation values of the pixels corresponding to the normal nozzles by a first rate lower than 100%, and

the second conversion rule converting the gradation values of the pixels corresponding to the surrounding nozzles by a second rate higher than the first rate.

2. The ink discharge complementing method according to claim 1 , further comprising a defective nozzle detection step of printing a defect detection image on the printing medium by the ink discharged from the nozzles by driving the plurality of nozzles and detecting the defective nozzle, out of the plurality of nozzles, based on a result obtained by reading the defect detection image by an imager, the defective nozzle detection step being performed before the conversion step.

3. The ink discharge complementing method according to claim 1 , further comprising:

a test gradation data preparation step of preparing test gradation data representing a gradation value of each of a plurality of pixels constituting a test image including a plurality of regions having mutually different gradation values;

a test conversion step of converting the test gradation data into defect complemented test gradation data by converting the gradation values of the test gradation data using the first conversion rule and second conversion rule;

a test halftone step of generating test discharge control data by performing the halftone process for the defect complemented test gradation data;

a test image printing step of printing the test image on the printing medium by the ink discharged from the nozzles by driving the nozzles based on the test discharge control data;

a gradation value reading step of reading the gradation values of the test image printed on the printing medium by an imager; and

a correction rule generation step of generating a shading correction rule to correct the variation of the amount of the ink discharged from each of the plurality of nozzles based on the gradation values read in the gradation value reading step;

the test gradation data preparation step, the test conversion step, the test halftone step, the test image printing step, the gradation value reading step and the correction rule generation step being performed before the shading correction step, and

the shading correction being performed using the shading correction rule generated in the correction rule generation step in the shading correction step.

4. The ink discharge complementing method according to claim 1 , wherein:

the second conversion rules are respectively provided for primary adjacent nozzles adjacent to the defective nozzle and for secondary adjacent nozzles adjacent to the primary adjacent nozzles,

the second conversion rule for the primary adjacent nozzles has a primary adjacent rate as the second rate and converts the gradation values of the pixels corresponding to the primary adjacent nozzles by the primary adjacent rate, and

the second conversion rule for the secondary adjacent nozzles has a secondary adjacent rate lower than the primary adjacent rate and converts the gradation values of the pixels corresponding to the secondary adjacent nozzles by the secondary adjacent rate.

5. An ink discharge complementing method which complements an ink discharge defect of a defective nozzle, out of a plurality of nozzles of a discharge head, comprising:

a gradation data acquisition step of obtaining gradation data representing a gradation value of each of a plurality of pixels constituting a print image;

a shading correction step of performing a shading correction to correct a variation of an amount of the ink discharged from each of the plurality of nozzles for the gradation data;

a conversion step of converting the gradation data into defect complemented gradation data by converting the gradation values of the gradation data, which has the shading correction performed therefor, using a first conversion rule for the gradation values of the pixels corresponding to normal nozzles other than surrounding nozzles located around the defective nozzle and the defective nozzle, out of the plurality of nozzles, and a second conversion rule for the gradation values of the pixels corresponding to the surrounding nozzles;

a halftone step of generating discharge control data by performing a halftone process for the defect complemented gradation data; and

an image printing step of printing the print image on a printing medium by the ink discharged from the nozzles by driving the nozzles based on the discharge control data,

the first conversion rule converting the gradation values of the pixels corresponding to the normal nozzles by a first rate lower than 100%, and

the second conversion rule converting the gradation values of the pixels corresponding to the surrounding nozzles by a second rate higher than the first rate.

6. A printing apparatus including a discharge head having a plurality of nozzles and configured to complement an ink discharge defect of a defective nozzle, out of the plurality of nozzles, comprising:

a gradation data acquisitor which obtains gradation data representing a gradation value of each of a plurality of pixels constituting a print image;

a storage which stores a first conversion rule for the gradation values of the pixels corresponding to normal nozzles other than surrounding nozzles located around the defective nozzle and the defective nozzle, out of the plurality of nozzles, and a second conversion rule for the gradation values of the pixels corresponding to the surrounding nozzles;

a data converter which converts the gradation data into defect complemented gradation data by converting the gradation values of the gradation data using the first conversion rule and second conversion rule stored in the storage;

a shading corrector which performs a shading correction to correct a variation of an amount of the ink discharged from each of the plurality of nozzles for the defect complemented gradation data;

a halftone processor which generates discharge control data by performing a halftone process for the defect complemented gradation data having the shading correction performed therefor; and

a drive controller which prints the print image on a printing medium by the ink discharged from the nozzles by driving the nozzles based on the discharge control data,

the first conversion rule converting the gradation values of the pixels corresponding to the normal nozzles by a first rate lower than 100%, and

the second conversion rule converting the gradation values of the pixels corresponding to the surrounding nozzles by a second rate higher than the first rate.

7. A printing apparatus including a discharge head having a plurality of nozzles and configured to complement an ink discharge defect of a defective nozzle, out of the plurality of nozzles, comprising:

a gradation data acquisitor which obtains gradation data representing a gradation value of each of a plurality of pixels constituting a print image;

a shading corrector which performs a shading correction to correct a variation of an amount of the ink discharged from each of the plurality of nozzles for the gradation data;

a storage which stores a first conversion rule for the gradation values of the pixels corresponding to normal nozzles other than surrounding nozzles located around the defective nozzle and the defective nozzle, out of the plurality of nozzles, and a second conversion rule for the gradation values of the pixels corresponding to the surrounding nozzles;

a data converter which converts the gradation data into defect complemented gradation data by converting the gradation values of the gradation data having the shading correction performed therefor, using the first conversion rule and second conversion rule stored in the storage;

a halftone processor which generates discharge control data by performing a halftone process for the defect complemented gradation data; and

a drive controller which prints the print image on a printing medium by the ink discharged from the nozzles by driving the nozzles based on the discharge control data,

the first conversion rule converting the gradation values of the pixels corresponding to the normal nozzles by a first rate lower than 100%, and

the second conversion rule converting the gradation values of the pixels corresponding to the surrounding nozzles by a second rate higher than the first rate.