Multilevel print masking method

1. A method of computing swath data in response to a digital image having a plurality of rows and columns of pixels, each pixel having a multitone code value for one or more color channels, each multitone code value having a plurality of possible values corresponding to different density levels, the swath data suitable for commanding an inkjet printer containing at least one printhead having a plurality of nozzles, wherein the inkjet printer is capable of ejecting ink drops in response to the swath data, comprising the steps of:

a) providing a print mask for a particular color channel having a plurality of mask planes, each mask plane corresponding to a different multitone code value for the particular color channel and wherein each mask plane contains a plurality of mask elements, each mask element having a mask value being one of at least a first value corresponding to no ejection of an ink drop, and a second value corresponding to an ejection of an ink drop;

b) selecting a mask plane in response to the multitone code value;

c) selecting a mask element from the selected mask plane in response to a pixel row index and a pixel column index, and;

d) computing a swath data value for the particular color channel in response to the mask value of the selected mask element.

2. The method of claim 1 wherein each of the plurality of mask planes for the particular color channel is independent from each of the other of the plurality of mask planes for the particular color channel.

3. The method of claim 1 wherein the number of mask planes is equal to the number of possible multitone code values for the particular color channel.

4. The method of claim 1 wherein the inkjet printer is commanded such that N drops of ink for the particular color channel are placed at a given pixel having a corresponding multitone code value of N.

5. The method of claim 1 wherein the plurality of mask elements for each mask plane contains at least one mask element corresponding to each active nozzle for the corresponding color channel in the printhead.

6. The method of claim 1 wherein each mask plane is represented as a two dimensional array indexed along a first dimension by the pixel row index, and indexed along a second dimension by a pixel column index.

7. The method of claim 6 wherein the pixel column index is computed by taking the pixel column number and performing a modulo operation with an integer mask width M w .

8. The method of claim 6 wherein the pixel row index is computed by taking the pixel row number and performing a modulo operation with an integer mask height M h .

9. The method of claim 8 wherein the integer mask height M h is equal to the number of nozzles in the printhead.

10. The method of claim 1 wherein step d) includes setting the swath data value for the particular color channel equal to the value of the selected mask element.

11. The method of claim 5 wherein a set of all mask elements within a mask plane that correspond to a nozzle new one of the ends of the printhead has fewer occurrences of the second mask value than a set of all mask elements within the mask plane than correspond to a nozzle near the center of the printhead.

12. The method of claim 5 wherein at least a first mask plane corresponding to a first multitone code value has a non-uniform duty cycle profile.

13. The method of claim 12 wherein at least a second mask plane corresponding to a second multitone code value has a substantially uniform duty cycle profile.

14. The method of claim 13 wherein the second multitone code value is larger than the first multitone code value.

15. A method of computing swath data in response to a digital image having a plurality of rows and columns of pixels, each pixel having a multitone code value for one or more color channels, each multitone code value having a plurality of possible values corresponding to different density levels, the swath data suitable for commanding an inkjet printer containing at least one printhead having a plurality of nozzles, wherein the inkjet printer is capable of ejecting ink drops in response to the swath data, comprising the steps of:

a) providing a print mask for a particular color channel containing a multidimensional array of mask elements having at least three dimensions, wherein a first dimension of the multidimensional array corresponds to a pixel row index, a second dimension corresponds to a pixel column index, and a third dimension corresponds to the multitone code value for the particular color channel;

b) selecting a mask element from the print mask responsive to the pixel row index, the pixel column index, and the multitone code value;

c) computing a swath data value for the particular color channel in response to the value of the selected mask element.

16. A method of claim 1 wherein an inkjet printer is a color inkjet printer in which the method is applied to at least one color channel corresponding to at least one ink color.

17. A method of claim 1 wherein an inkjet printer is a color inkjet printer in which the method is applied to at least a first color channel corresponding to a first ink and a second color channel corresponding to a second ink, and wherein at least a first mask plane of a first print mask corresponding to the first color channel has a concave up duty cycle, and at least a second mask plane of a second print mask corresponding to the second color channel has a concave down duty cycle.

18. The method of claim 17 wherein at least one of the first or second inks is a clear ink.

19. The method of claim 1 wherein the print mask has N mask planes corresponding to N multitone levels, and the inkjet printer prints the digital image using a printmode at least N-1 passes.

20. A method of computing swath data in response to a digital image having a plurality of rows and columns of pixels, each pixel having a multitone code value for one or more color channels, each multitone code value having a plurality of possible values corresponding to different density levels, the swath data suitable for commanding an inkjet printer containing at least one printhead having a plurality of nozzles, wherein the inkjet printer is capable of ejecting ink drops in response to the swath data, comprising the steps of:

a) providing a print mask for a particular color channel having a plurality of mask planes, each mask plane corresponding to a different multitone code value for the particular color channel and wherein each mask plane contains a plurality of mask elements, each mask element having a mask value being one of at least a first value corresponding to no ejection of an ink drop, and a second value corresponding to an ejection of an ink drop;

b) selecting a mask element from the print mask in response to a nozzle number, a pixel column, and the multitone code value for the particular color channel for selected pixel; and

c) computing a swath data value for the particular color channel for the selected pixel in response to the value of the selected mask element.

21. A method of printing a digital image with an ink jet print head including a plurality of nozzles, the digital image having a plurality of rows and columns of pixels, each pixel of the digital image having a multitone code value for one or more color channels, each multitone code value having a plurality of possible values corresponding to different density levels, the method comprising the steps of:

a) providing a print mask for a particular color channel having a plurality of mask planes, each mask plane corresponding to a different multitone code value for the particular color channel and wherein each mask plane contains a plurality of mask elements, each mask element having a mask value being one of at least a first value corresponding to no ejection of an ink drop, and a second value corresponding to an ejection of an ink drop;

b) selecting a mask element for each pixel from the print mask in response to a nozzle number, a pixel column, and the multitone code value of the pixel for the particular color channel;

c) computing a swath data value for the particular color channel for each pixel in response to the value of the selected mask element, and

d) printing ink drops for the particular color channel with the print head in response to the swath data value computed for each pixel.

22. The method of claim 21 wherein the values of the mask elements in each of the plurality of mask planes are independent of the values of the mask elements of any others of the plurality of mask planes.