Document registration using registration error model

1. A method for correcting color registration errors while printing a print job on a print media using a color printer, the print job including one or more documents having pixel values specifying ink coverage for a plurality of color planes, comprising:

receiving a color registration error model that predicts a color registration error value for a document as a function of ink coverage characteristics for the document, wherein the color registration error model is a parametric model having one or more parameters; and

for each document within the print job:

determining the ink coverage characteristics for the document;

using the color registration error model to determine a predicted color registration error for the document responsive to the determined ink coverage characteristics;

determining an image plane correction value for at least one of the color planes of the document responsive to the predicted color registration error; and

printing the document using the determined image plane correction value.

2. The method of claim 1 wherein color registration model is determined by:

printing a plurality of documents, each document having associated ink coverage characteristics;

measuring color registration errors for each of the documents;

determining one or more parameters for the color registration model by fitting the measured color registration errors as a function of the ink coverage characteristics.

3. The method of claim 1 wherein the color registration model is updated while the print job is being printed by:

measuring a residual color registration error value for one or more of the documents that have already been printed; and

updating one or more of the parameters of the color registration error model responsive to the measured residual color registration error values and the corresponding ink coverage characteristics of the printed documents.

4. The method of claim 1 wherein the determination of the predicted color registration error is also responsive to a media characteristic, a print speed, a dryer setting or a measured environmental characteristic.

5. The method of claim 1 wherein different color registration models are provided for different types of print media, different printing speeds or different dryer settings.

6. The method of claim 1 wherein the ink coverage characteristics are determined responsive to an ink coverage profile representing a distribution of ink across a width dimension of the print media.

7. The method of claim 6 wherein ink coverage profiles are determined for a plurality of the color planes, and wherein the ink coverage characteristics are determined responsive to the ink coverage profiles for the plurality of color channels.

8. The method of claim 6 further including representing the ink coverage profile using a series expansion based on a set of orthogonal basis functions, and wherein the ink coverage characteristics include coefficients for a plurality of the basis functions.

9. The method of claim 1 wherein the color registration model determines the predicted color registration error as a function of a plurality of variables representing the ink coverage characteristics.

10. The method of claim 9 wherein the color registration model has the form:

E

=

a

0

+

∑

n

=

1

N

⁢

a

n

⁢

C

n

where C n is the n th variable representing the ink coverage characteristics, N is the number of variables, a n is a vector of weighting coefficients for the n th variable, and E is a vector of predicted color registration errors.

11. The method of claim 9 wherein the variables representing the ink coverage characteristics include average ink coverage levels for a plurality of image tiles in the document.

12. The method of claim 9 wherein the variables representing the ink coverage characteristics include a plurality of ink coverage statistics determined by analyzing the pixel values of the document.

13. The method of claim 12 wherein the ink coverage statistics include an average ink coverage, a standard deviation of the ink coverage or a statistic characterizing the asymmetry of the ink coverage.

14. The method of claim 9 further including:

determining an ink coverage profile representing a distribution of ink across a width dimension of the print media; and

representing the ink coverage profile using a series expansion based on a set of orthogonal basis functions;

wherein the variables representing the ink coverage characteristics include coefficients determined for a plurality of the basis functions.

15. The method of claim 1 wherein the image plane correction value is an in-track shift value, a cross-track shift value, an in-track magnification factor, a cross-track magnification factor, or an image rotation value.

16. The method of claim 1 wherein the image plane correction values are used to modify the color image data which is printed.

17. The method of claim 1 wherein the image plane correction values are used to control a web-transport system that moves the continuous web of media through the color printer.