Reducing registration errors using registration error model

1. A method for correcting color registration errors for a color printer that prints color image data on a continuous web of media, the color image data specifying a pattern of ink coverage as a function of an in-track position and a cross-track position for a plurality of color planes, comprising:

receiving a color registration error model that predicts a color registration error value as a function of ink coverage characteristics, wherein the color registration error model is a parametric model having one or more parameters, wherein the color registration error model accounts for distortions in the web of media that relate to the pattern of ink coverage; and

for a particular in-track position:

analyzing the color image data to determine ink coverage characteristics for an image region including the particular in-track position;

using the color registration error model to determine a predicted color registration error for the particular in-track position responsive to the determined ink coverage characteristics;

determining one or more image data correction values for at least one of the color planes of the color image data responsive to the predicted color registration error; and

printing the color image data for the particular in-track position using the determined image data correction values.

2. The method of claim 1 wherein the image region includes in-track positions corresponding to previously-printed color image data.

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

printing a plurality of color image data patterns, each color image data patterns having different associated ink coverage characteristics;

measuring color registration errors for each of the color image data patterns;

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

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

measuring a residual color registration error value for the printed color image data having the determined ink coverage characteristics; 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.

5. 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.

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

7. 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.

8. The method of claim 7 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.

9. The method of claim 7 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.

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

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

E=a 0 +Σ 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.

12. The method of claim 10 wherein the variables representing the ink coverage characteristics include average ink coverage levels for a plurality of image tiles in the image region.

13. The method of claim 10 wherein the variables representing the ink coverage characteristics include a plurality of ink coverage statistics determined by analyzing the pixel values of the image region.

14. The method of claim 13 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.

15. The method of claim 10 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.

16. The method of claim 1 wherein the image data correction values include 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.

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

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

19. The method of claim 1 further including determining image data correction values for a plurality of in-track positions spaced apart along an in-track direction, and wherein an interpolation process is used to determine interpolated image data correction values for intermediate in-track positions.