Color calibration of displays

• US 6,690,383 B1
• Filed: 01/24/2000
• Issued: 02/10/2004
• Est. Priority Date: 01/25/1999
• Status: Expired due to Term

• Current Assignee: AU Optronics Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Braudaway, Gordon Wesley, Christensen, James Edward, Mintzer, Frederick Cole, Cordes, Michael James, DeCain, Donald Michael, Cazes, Albert Nessim, Wright, Steven Lorenz, Lien, Shui-Chih Alan
• Primary Examiner(s): Razavi, Michael
• Assistant Examiner(s): HAVAN, THU THAO
• Application Number:
  US09/490,545
• Time in Patent Office: 1,478 Days
• Field of Search: 345/3.1, 345/10, 345/22, 345/589-605, 345/690, 345/3.2, 345/38, 345/50, 345/72-89, 345/104, 382/162-167
• US Class Current: 345/600
• CPC Class Codes: G09G 2320/0693 Calibration of display systems
• CPC Class Codes: G09G 5/006 Details of the interface to the display terminal specific for a display terminal using a CRT G09G1/167; using a flat panel G09G3/2096; circuits for interfacing with colour displays G09G5/04
• CPC Class Codes: G09G 5/04 using circuits for interfacing with colour displays
• CPC Class Codes: H04N 17/02 for colour television signals
• CPC Class Codes: H04N 17/04 for receivers

Abstract

The present invention provides methods and apparatus for the color calibration of displays. These methods are particularly useful for displays having properties different than those associated with cathode ray tubes (CRT). Some displays, for example liquid crystal displays (LCD), have a high luminance black, the chromas of their primaries are not independent of their luminance levels, the intensities of their primaries are not strictly monotonically increasing with respect to their RGB levels, the color additivity of their subpixels is affected by crosstalk or leakage and their color measurements are dependent on the view angle of the observer. Therefore, the usage of CRT methods is deficient for such displays. The methods and apparatus of the present invention overcome several deficiencies generally found in non-CRT display devices by employing techniques associated with the use of restricted ranges for the primaries, subtracting the color black from all color measurements, forming a modified display matrix and/or creating tables which correct errors due to the non-linearities or color leakages of these display devices. These enable images to be processed using algorithms of different complexity and precision.

Claims

1. A method for color calibrating an LCD display, said method comprising:

measuring color patches of primaries and grays of said LCD display to obtain measurement data;

wherein said step of measuring includes;

determining useful display-RGB ranges of the primaries by rendering uniform color patches on said LCD display;

measuring CIE XYZ coordinates of each of said uniform color patches;

using said display-RGB ranges to determine sample points for the primaries and related grays;

rendering said sample points;

measuring and storing results in a display profile;

creating said display profile from the measurement data;

wherein said step of creating said display profile includes

reading the CIE XYZ measurements from the display profile;

subtracting black from all the CIE XYZ measurements;

computing a display matrix Md and its inverse Mdi matrix from the CIE XYZ measurements corresponding to the maximum levels of the primaries;

transforming all the measurements from CIE XYZ to matrix-RGB coordinates using the inverse Mdi; and

computing non-linear errors and color leakages for the primaries using the matrix-RGB coordinates of grays and primaries and

calibrating said LCD display using the display profile in conjunction with display specific corrections for said display in forming calibration results.

2. A method as in claim 1, further comprising forming a model for said display LCD.

3. A method as in claim 1, wherein the step of calibrating includes:

creating color correction tables; and

computing two sets of gamma tables.

4. A method as in claim 3, wherein the step of creating color correction tables includes using linear interpolation.

5. A method as in claim 3, wherein the step of computing two sets of gamma tables includes using log-log linear interpolation.

6. A method as in claim 3, further comprising processing images using the calibration results, wherein the step of processing includes converting the CIE XYZ coordinates of pixel data stored in an image file into display-RGB using the inverse matrix Mdi, average black, correction and gamma tables.

7. A method as in claim 6, wherein the step of converting the CIE XYZ coordinates of pixel data stored in an image file into display-RGB employs a Simple-algorithm.

8. A method as in claim 6, wherein the step of converting the CIE XYZ coordinates of pixel data stored in an image file into display-RGB employs a Full-algorithm.

9. A method as in claim 1, wherein the step of determining useful display-RGB ranges includes:

determining restricted display-RGB ranges for the red, green and blue primaries of an LCD, by;

performing photometric measurements on the display by;

rendering three sets of primary patches from a display-RGB low level to a display-RGB maximum level;

reading a CIE XYZ measurement for each patch;

creating a two dimensional graph for each set;

determining a maximum CIE X for the red patches such that all lower intensities are monotonically decreasing from said maximum CIE X, and saving a display-RGB red intensity level corresponding with said CIE X;

determining a maximum CIE Z for the blue patches such that all lower intensities are monotonically decreasing from said maximum CIE Z, and saving a display-RGB blue intensity level corresponding with said CIE Z;

forming a restricted red range being from a minimum red intensity level to said display-RGB red intensity level;

forming a restricted green range being from a minimum green intensity level to said display-RGB green intensity level; and

forming a restricted blue range being from a minimum blue intensity level to said display-RGB blue intensity level,

wherein said restricted red range, said restricted green range and said restricted blue range form said useful display-RGB ranges.

10. A method as in claim 9, wherein at least one of the restricted ranges is an entire display-RGB range for said primary.

11. A method as in claim 1, further comprising processing images using the calibration results.

12. A method as in claim 1, wherein the display specific corrections overcome at least one deficiency taken from a group of deficiencies which includes high intensity display black, non additivity of primaries, leakage between sub-pixels, other non linearities.

13. A method as in claim 1, wherein the display profile includes 3×

M CIE XYZ arrays for red, green, blue and gray, where M is a number of color patches rendered for red, green, blue and gray, and the display specific corrections overcomes a high intensity display black by;

computing an average 1×

3 black vector for the display;

subtracting the black vector from each column vector of the 3×

M CIE XYZ arrays for red, green, blue and gray.

14. A method as in claim 1, wherein the display specific corrections overcomes non linear errors in the display by:

computing the display matrix;

using the display matrix to transform data from CIE XYZ to matrix-RGB coordinates;

decomposing a gray array and total leakage arrays into each array'"'"'s primary components;

computing the non-linear error arrays using said each array'"'"'s primary components; and

employing the non-linear error arrays in an image processing display algorithm.

15. A method as in claim 14, further comprising computing a total leakage array.

16. A method as in claim 15, wherein the step of computing a total leakage array includes employing leakage coefficients for each pixel for each primary.

26. A method as in claim 1, wherein the step of calibrating includes creating color correction tables, and computing sets of gamma tables, and further comprises employing the matrix Mdi, average black, correction tables and gamma tables developed for one display, for calibration of at least one other display.

27. A method as in claim 26, wherein said at least one other display is manufactured in the same production run as said one display.

28. An article of manufacture comprising a computer usable medium having computer readable program code means embodied therein for causing color calibration of a display, the computer readable program code means in said article of manufacture comprising computer readable program code means for causing a computer to effect the steps of claim 1.

30. A computer program product comprising a computer usable medium having computer readable program code means embodied therein for causing color calibration of a display, the computer readable program code means in said computer program product comprising computer readable program code means for causing a computer to effect the steps of claim 1.

31. A program storage device readable by machine, tangibly embodying a program of instructions executable by the machine to perform method steps for color calibrating a display, said method steps comprising the steps of claim 1.

17. A method comprising:

determining restricted display-RGB ranges for the red, green and blue primaries of an LCD, by;

performing photometric measurements on the display by;

rendering three sets of primary patches from a display-RGB low level to a display-RGB maximum level;

reading a CIE XYZ measurement for each patch;

creating a two dimensional graph for each set;

determining a maximum CIE X for the red patches such that all lower intensities are monotonically decreasing from said maximum CIE X, and saving a display-RGB red intensity level corresponding with said CIE X;

determining a maximum CIE Y for the green patches such that all lower intensities are monotonically decreasing from said maximum CIE, and saving a display-RGB green intensity level corresponding with said CIE Y;

determining a maximum CIE Z for the blue patches such that all lower intensities are monotonically decreasing from said maximum CIE Z, and saving a display-RGB blue intensity level corresponding with said CIE Z;

forming a restricted red range being from a minimum red intensity level to said display-RGB red intensity level;

forming a restricted green range being from a minimum green intensity level to said display-RGB green intensity level; and

forming a restricted blue range being from a minimum blue intensity level to said display-RGB blue intensity level.

18. A method for determining a color profile for a display, said method comprising:

rendering on the display one at a time, one of M red patches, one of M green patches, one of M blue patches, and one of M gray patches,

separately dividing each intensity level range of the red, the green, and the blue into M sample points;

determining a related gray for the M sample points of each of the corresponding red, green, and blue intensity levels;

forming separate 3×

M display-RGB arrays for the red patches, the green patches, the blue patches, and the gray patches resulting in a corresponding separate 3×

M array for the red, the green, the blue and the gray;

performing a color patch measurements for each of the 4×

M patches and obtaining separate red, green, blue and grey measured 3×

M corresponding CIE XYZ arrays; and

storing the display-RGB arrays and the corresponding CIE XYZ arrays to form said color profile of the display.

19. A method as in claim 18, further comprising finding useable ranges of display RGB for each primary.

20. A method as in claim 19, wherein the step of finding includes:

employing display-RGB coordinates and determining a maximum CIE X such that all lower intensities are monotonically decreasing from said maximum CIE X;

employing display-RGB coordinates and determining a maximum CIE Y such that all lower intensities are monotonically decreasing from said maximum CIE Y; and

employing display-RGB coordinates and determining a maximum CIE Z such that all lower intensities are monotonically decreasing from said maximum CIE Z.

24. A method as recited in claim 19, wherein each patch has an area of 10×

10 cm.

25. A method as recited in claim 18, wherein the display is in display-RGB coordinates.

29. An article of manufacture comprising a computer usable medium having computer readable program code means embodied therein for causing determination of a color profile for a display, the computer readable program code means in said article of manufacture comprising computer readable program code means for causing a computer to effect the steps of claim 18.

32. A program storage device readable by machine, tangibly embodying a program of instructions executable by the machine to perform method steps for determining a color profile for a display, the computer readable program code means in said article of manufacture comprising computer readable program code means for causing a computer to effect the steps of claim 18.

21. A method to obtain calibration operators for a display, the method comprising:

obtaining a color profile and an average black CIE XYZ vector for the display;

forming a sum primary array from the red, green and blue modified 3×

M arrays;

subtracting the gray modified 3×

M array from the sum primary array forming a sum leakage array for the display;

forming Md and Mdi matrixes from the red, green and blue modified 3×

M arrays;

employing the Mdi matrix to transform the modified 3×

M arrays, the sum primary array, the sum leakage array, the gray arrays and the average black array into matrix-RGB arrays;

decomposing the matrix-RGB gray array into its red, green and blue components and forming 3×

M arrays of these components;

decomposing the matrix-RGB sum leakage array into its red, green and blue components and forming 3×

M arrays of these components;

computing the red, green and blue 3×

M non-linear arrays using the primary arrays, the leakage component arrays and the gray component arrays;

creating a model from the 3×

M primary arrays, sum primary array, gray array, gray component arrays, non-linear error arrays, sum leakage array and leakage component arrays;

interpolating 3×

M non-linear error arrays into 3×

(Scale+1) non-linear tables;

interpolating 3×

M leakage component arrays into 1×

(Scale+1) non-linear tables;

computing 3×

M normalized gray array using gray array;

interpolating 3×

M normalized gray array into 1×

(Scale+1) normalized gray tables;

interpolating two sets of gamma tables;

computing red, green and blue coefficients to determine the leakage of a pixel.

22. A method as in claim 21, further comprising subtracting the average black vector from each column vector of the four measured 3×

M CIE XYZ arrays in the color profile forming red, green, blue and grey modified 3×

M arrays.

23. A method as in claim 21 wherein the Md matrix is maximum levels of each modified array.