Marking element registration

1. A method of measuring a relative offset between a first array of marking elements and a second array of marking elements in a printer, the method comprising the steps of:

printing a target by printing a first group of pixels using a plurality of marking elements from the first array and printing a second group of pixels using a plurality of marking elements from the second array, the relative offset being along an offset direction, the target including a first end, a second end, and a center that is positioned midway between the first end and the second end, wherein printing the target including the first group of pixels and the second group of pixels further comprises:

printing a reference pattern including a plurality of first regions spaced apart from one another along the offset direction using the plurality of marking elements from the first array; and

printing a registration pattern including a plurality of second regions successively incrementally displaced along the offset direction from the plurality of first regions using the plurality of marking elements from the second array such that a degree of overlap between the plurality of first regions and the plurality of second regions varies along the target, wherein the optical characteristic varies according to the degree of overlap;

scanning the target to measure an optical characteristic of the target as a function of position along the target;

identifying a position at which an extreme in the optical characteristic of the target occurs; and

comparing the position of the extreme in the optical characteristic to the position of the center of the target.

2. The method of claim 1 , wherein the extreme in the optical characteristic is a maximum in the optical characteristic.

3. The method of claim 1 , wherein the extreme in the optical reflectance is a minimum in the optical reflectance.

4. The method of claim 1 , wherein identifying the position at which the extreme in the optical characteristic occurs includes analyzing a centroid of the optical characteristic.

5. The method of claim 1 , the first array of marking elements and the second array of marking elements each being disposed substantially parallel to a first direction, the relative offset to be measured being parallel to the first direction, wherein scanning the target includes scanning the target along a second direction, the second direction being substantially perpendicular to the first direction.

6. The method of claim 5 , further comprising:

providing a carriage including the first array of marking elements, the second array of marking elements and an optical sensor; and

causing the carriage to move along the second direction, wherein printing of the target occurs as the carriage moves along the second direction, and scanning of the target is accomplished using the optical sensor as the carriage moves along the second direction.

7. The method of claim 6 , the target being printed on a portion of media, the method further comprising:

providing a media advance system that advances the media along the first direction between successive passes of the carriage along the second direction, wherein printing the target includes printing the target during multiple passes of the carriage.

8. The method of claim 1 , further comprising:

disposing the first array of marking elements and the second array of marking elements substantially parallel to a first direction, the relative offset to be measured being along a second direction that is perpendicular to the first direction; and

scanning the target along the second direction.

9. The method of claim 1 , the printer being an inkjet printer, the method further comprising:

printing the first group of pixels with a first ink; and

printing the second group of pixels with a second ink.

10. The method of claim 1 , wherein printing the registration pattern includes printing the plurality of second regions of the registration pattern such that a length of a second region in a direction perpendicular to the offset direction is greater than 3D, where D is a maximum offset that can occur in a direction perpendicular to the offset direction between the first array of marking elements and the second array of marking elements.

11. A registration target comprising:

a first end marker;

a second end marker;

a reference pattern including pixels of a first type located in a plurality of first regions that are spaced apart from one another; and

a registration pattern including pixels of a second type located in a plurality of second regions, the plurality of second regions being successively incrementally offset from the plurality of first regions such that the degree of overlap between the plurality of first regions and the plurality of second regions varies along the target, wherein the nominal position of the extreme in the degree of overlap is located midway between the first end marker and the second end marker.

12. The registration target of claim 11 , wherein the pixels of the first type are a different color than the pixels of the second type.

13. The registration target of claim 11 , the reference pattern including a plurality of first rectangles, and the registration pattern including a plurality of second rectangles, the second rectangles being grouped into a first group and a second group, wherein the first group of the second rectangles substantially overlap the reference pattern, and the second group of the second rectangles substantially do not overlap the reference pattern.

14. The registration target of claim 13 , wherein the first group of rectangles is disposed in a checkerboard pattern.

15. The registration target of claim 13 , the spacing between adjacent pixels of the second type being a distance p, the plurality of first rectangles having a length L=np and a width W=mp, the location of the pixels of the second type having an average offset error of a distance E=xp relative to the location of the pixels of the first type, wherein the resulting position of an extreme in the degree of overlap between rectangles of the second type and rectangles of the first type is shifted by a distance X=nE relative to the nominal position of the extreme in the degree of overlap, the nominal position corresponding to a case of E=0.

16. The registration target of claim 11 , the spacing between adjacent pixels of the second type being a distance p, wherein an amount of incremental offset between the registration pattern and the reference pattern in adjacent regions is p.

17. A printer comprising:

a first array of marking elements;

a second array of marking elements;

a sensor;

a controller configured to control printing patterns of the first array and the second array so that a target can be printed, to receive data from the sensor after the sensor scans the target to measure an optical characteristic of the target as a function of position along the target, to identify a position at which an extreme in the optical characteristic of the target occurs, and to compare the position of the extreme in the optical characteristic of the target to the position of a midpoint of the target.

18. The printer of claim 17 , wherein the controller is configured to adjust actuation of one of the first array and the second array based on an offset calculated by the controller.