Color calibration in a printing system

1. A method of adjusting a printing system, the method comprising:

obtaining a lookup table, LUT, comprising a plurality of nodes that map a plurality of input first color space values to a plurality of Neugebauer Primary area coverage, NPac, vectors in a second color space, wherein each NPac vector comprises at least one Neugebauer Primary, NP;

adjusting each NPac vector in the LUT by:

determining a colorimetric value of the NPac vector;

modifying the NPac vector based on a characteristic of the corresponding at least one NP and the colorimetric value such that the modification is specific to the NPac vector;

updating the LUT with the modified NPac vector; and

storing the updated LUT for referral by the printing system in any future printing.

2. The method of claim 1 , wherein for each NPac vector:

determining a weighting based on a difference between the characteristic of the at least one NP and the colorimetric value of the NPac vector, such that the weighting is an adjustment parameter specific to the NPac vector and reduces the difference between the characteristic of the at least one NP and the colorimetric value of the NPac vector; and

applying the weighting to the at least one NP.

3. The method of claim 2 , wherein the weighting is a numerical value that corresponds to the determined difference.

4. The method of claim 3 , wherein the weighting causes either an increase or a decrease in the amount of the NP.

5. The method of claim 1 , wherein the modifying comprises adding another, different NP to the NPac vector.

6. The method of claim 1 , wherein the characteristic of the corresponding at least one NP is a colorimetric value of the NP.

7. The method of claim 1 , further comprising: determining the characteristic of the of the corresponding at least one NP.

8. The method of claim 1 , wherein determining the colorimetric value of the NPac vector comprises using a predictive colour model that estimates the colorimetric value based on the NPac of an ink vector associated with the LUT.

9. A print system comprising:

a printing device to apply print material to a print medium in a printing process;

a memory arranged to store computer-readable instructions; and

a processor configured to execute the computer-readable instructions to:

retrieve a lookup table, LUT, comprising a mapping of first color space to Neugebauer Primary area coverage, NPac, vectors in a second color space, wherein each NPac vector comprises at least one Neugebauer Primary, NP;

change each NPac vector by modifying the respective at least one NP, wherein the modifying is determined independently from modifications to other NPac vectors and is based on a relative characteristic between the at least one NP and the NPac;

generate an updated LUT comprising each changed NPac vector; and

provide the updated LUT to the printing device such that the print material is applied in accordance with the updated LUT to print the image on the print medium.

10. The system of claim 9 , wherein the processor is configured to execute the computer readable instructions to:

determine a weighting based on a difference between the characteristic of the at least one NP and the colorimetric value of the NPac vector, such that the weighting is an adjustment parameter specific to the NPac vector and reduces the difference between the characteristic of the at least one NP and the colorimetric value of the NPac vector; and

apply the weighting to the at least one NP.

11. The system of claim 10 , wherein the weighting is a numerical value that corresponds to the determined difference.

12. The system of claim 11 , wherein the weighting causes either an increase or a decrease in the amount of the NP.

13. The system of claim 9 , wherein the modifying comprises adding another, different NP to the NPac vector.

14. The system of claim 9 , wherein the characteristic of the corresponding at least one NP is a colorimetric value of the NP.

15. A non-transitory computer readable medium comprising a set of instructions, that, when executed by a processor of a printing system cause the processor to:

obtain a lookup table, LUT, that maps input first color space values to Neugebauer Primary area coverage, NPac, vectors in a second color space, wherein each NPac vector comprises at least one Neugebauer Primary, NP and the NPac vectors form the basis of a printing instruction for the image;

adjust each NPac vector of the LUT based on a characteristic of the corresponding at least one NP relative to that of the NPac vector, such that the modification of each NPac vector is specific to the NPac vector;

update the LUT with the adjusted NPac vectors; and

change the printing instruction for the image based on the updated LUT.