Automated image calibration

• US 5,185,673 A
• Filed: 06/12/1991
• Issued: 02/09/1993
• Est. Priority Date: 06/12/1991
• Status: Expired due to Term

• Current Assignee: Hewlett-Packard Company (HP Inc.)
• Original Assignee: Hewlett-Packard Company (HP Inc.)
• Inventors: Sobol, Robert E.
• Primary Examiner(s): Fuller, Benjamin R.
• Assistant Examiner(s): Rogers, Scott A.
• Application Number:
  US07/714,392
• Time in Patent Office: 608 Days
• Field of Search: 358/296, 358/300, 358/298, 358/406, 358/447, 358/448, 358/455, 358/461, 358/463, 358/464, 358/486, 358/488, 358/80, 358/78, 395/109
• US Class Current: 358/296
• CPC Class Codes: H04N 1/4078 using gradational references, e.g. grey-scale test pattern analysis
• CPC Class Codes: H04N 1/6033 using test pattern analysis H04N1/6055 takes precedence

Abstract

A document processing system (DPS) having a closed feedback loop for improving the quality of the printed form of images that have been scanned into the DPS by scanners. The DPS includes a print path. The closed feedback loop automatically calibrates the print path such that printed documents have a true representation of the scanned image. The closed feedback loop operates as follows. First, a calibration image for the print path is created. Then, the calibration image is processed in the DPS. Then, distortions in the calibration image, caused by the processing, are measured. Then, calibration coefficients for offsetting the distortions are calculated. Finally, the image of the original document is precompensated by modifying the image according to the calibration coefficients.

Claims

1. A method for automatically calibrating a print path in a document processing system, the system receiving image data of an original document generated by an image capture device, such that the system generates a printed document having a true representation of the original document, the method comprising the steps of:

(a) creating a calibration image for the print path;

(b) processing said calibration image in the document processing system;

(c) measuring distortions in said calibration image caused by said processing;

(d) calculating calibration coefficients for offsetting said measured distortions; and

(e) precompensating an image of the original document by modifying the image according to said calculated calibration coefficients.

2. The method of claim 1, wherein said input capture device comprises a scanner having a scanner bed, said method further comprising the steps of:

determining a location of a printed calibration image on the scanner bed.

recording information regarding a print path calibration associated with said printed calibration image;

compensating for dotgain in said printed calibration image;

reducing noise in said printed calibration image; and

identifying boundaries in said calibration image.

3. The method of claim 2, wherein said calibration image further comprises a color block having a multitude of colors.

4. The method of claim 3, wherein the step of measuring distortions in said calibration image caused by said processing comprises the step of creating a lookup table having entries for said colors in said color block, wherein said entries are associated with original colors and response colors, and wherein said original colors represent said colors in said color block.

5. The method of claim 4, wherein the step of calculating calibration coefficients for offsetting said distortions comprises the steps of:

(1) creating a compensation table having entries associated with desired colors;

(2) selecting one of said entries;

(3) using said original and response colors for determining a stimulus color for said desired color associated with said selected entry;

(4) saving said stimulus color in said selected entry; and

(5) repeating steps (1)-(4) for each of said entries.

6. The method of claim 5, wherein said desired colors outnumber said stimulus colors.

7. The method of claim 5, wherein said step for using said original and response colors for determining a stimulus color for said desired color associated with said selected entry comprises the steps of:

(i) clearing a stimulus accumulator;

(ii) initializing an error;

(iii) generating a mixed color by adding said error to said desired color;

(iv) determining a closest color to said mixed color among said response colors;

(v) identifying one of said original colors associated with said closest color;

(vi) adding said identified original color with values in said stimulus accumulator;

(vii) updating said error by determining a difference between said desired color and closest color;

(viii) updating said mixed color by adding said error to said desired color;

(ix) updating said closest color by determining which of said response colors is closest to said mixed color; and

(x) repeating steps (v)-(ix) for a finite number of iterations such that said stimulus accumulator converges upon said stimulus color.

8. The method of claim 7, wherein steps (v)-(ix) are performed 100 times.

9. The method of claim 5, wherein the step of precompensating the image of the original document by modifying the image according to said calibration coefficients comprises the steps of:

(i) receiving a pixel color associated with a pixel, wherein said pixel color has a multitude of color bits;

(ii) retrieving one of said stimulus colors in said compensation table from a position identified by said color bits;

(iii) retrieving adjacent stimulus colors from said compensation table;

(iv) generating a high resolution stimulus color by using all of said color bits to interpolate between said retrieved stimulus color and said adjacent stimulus color;

(v) replacing said pixel color with said high resolution stimulus color; and

(vi) repeating steps (i)-(v) for all pixels in the image.

10. The method of claim 2, wherein said calibration image further comprises a grayscale block having a multitude of grays.

11. The method of claim 10, wherein the step of measuring distortions in said calibration image caused by said processing comprises the step of creating a lookup table having entries for said grays, wherein each of said entries is associated with a desired or stimulus gray and a response gray.

12. The method of claim 11, wherein the step of calculating calibration coefficients for offsetting said distortions comprises the steps of:

(1) selecting one of said entries;

(2) for said desired gray associated with said selected entry, identifying a closest gray among said response grays in any of said entries;

(3) saving said stimulus gray associated with said closest gray in a compensation table at a position indexed by said desired gray associated with said selected entry; and

(4) repeating steps (1)-(3) for each of said entries.

13. The method of claim 12, wherein the step of precompensating the image of the original document by modifying the image according to said calibration coefficients comprises the steps of:

(1) selecting a gray in the image;

(2) retrieving one of said stimulus grays in said compensation table from a position indexed by said selected gray;

(3) replacing said selected gray with said retrieved stimulus gray in said image; and

(4) repeating steps (1)-(3) for all grays in the image.

14. The method of claim 2, further comprising the step of capturing an image of said processed calibration image comprising the steps of:

scanning raster scan lines in said scanner bed until said scanner locates a series of black pixels associated with said registration means;

scanning said raster scan lines until said scanner locates a series of white pixels at a first edge of said registration means;

determining a skewed direction based on whether said first edge is a left or a right edge of said registration means;

scanning said raster scan lines until said scanner locates a series of white pixels at a second edge of said registration means;

determining a distance between where said first and second edges turned white;

calculating a skewed angle based on said distance; and

using said tracking means to scan said image for data.

15. The method of claim 1, wherein said step for precompensating comprises dotgain compensation.

16. A document processing system comprising:

(1) a print path comprising;

(a) image capture means for capturing an image of an original document;

(b) means for controlling said image capture means;

(c) one or more application programs;

(d) printing means for generating a printed document having said image;

(e) means for controlling said printing means;

(2) means for automatically calibrating the print path such that said printed document has a true representation of said image, said means for automatically calibrating comprising;

(a) means for generating calibration coefficients for offsetting distortions caused by said print path; and

(b) means for precompensating said image of said original document by modifying said image according to said calibration coefficients.

17. The document processing system of claim 16, wherein said means for generating calibration coefficients for offsetting distortions caused by said print path comprises:

means for creating a calibration image for said print path;

means for processing said calibration image in said print path;

means for measuring distortions in said calibration image caused by said processing; and

means for calculating calibration coefficients for offsetting said distortions.