Color correction of a compressed image

• US 6,009,192 A
• Filed: 12/19/1996
• Issued: 12/28/1999
• Est. Priority Date: 12/19/1996
• Status: Expired due to Term

• Current Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Original Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Inventors: Klassen, R. Victor, Harrington, Steven J., de Queiroz, Ricardo L., Balasubramanian, Thyagarajan
• Primary Examiner(s): Tadayon, Bijan
• Assistant Examiner(s): NOVIK, DMITRY A
• Application Number:
  US08/770,765
• Time in Patent Office: 1,104 Days
• Field of Search: 382/162, 382/167, 382/233, 382/235, 382/244, 382/245, 382/246, 382/248, 382/250, 358/539, 358/426, 358/261.1, 358/432, 358/518, 358/261.3, 348/391, 348/415, 348/416, 348/387, 348/403, 348/394, 348/395, 348/396
• US Class Current: 382/167
• CPC Class Codes: H04N 1/60 Colour correction or control H04N1/54 takes precedence
• CPC Class Codes: H04N 1/64 Systems for the transmission or the storage of the colour picture signal; Details therefor, e.g. coding or decoding means therefor H04N19/00 takes precedence

Abstract

The present invention is a method for the color correction of digital images that have been compressed. In a preferred embodiment, aspects of the color correction are carried out on the compressed image data to improve computational efficiency. One of a number of alternative methods is employed to accomplish the color correction on lossy or losslessly compressed images. The color transformation process accomplishes color correction on compressed image data in conjunction with compression or decompression operations. A second, simplified phase of the color correction may be applied subsequently to the decompressed image data in certain embodiments. Accordingly, the technique has application to any number of color imaging systems, including digital printers and copiers where there is a necessity to color correct compressed digital images.

Claims

1. A method for color correcting a compressed digital image, wherein the compressed digital image is generated using a lossy compression method employing a discrete cosine transform, comprising the steps of:

retrieving from memory a compressed unit of image data;

partially decompressing the compressed unit to produce a partially decompressed unit representing the color of a region of the image in a first color space, wherein the region comprises a plurality of pixels, wherein the step of partially decompressing the compressed unit produces a set of DCT coefficients and where the step of performing color correction comprises;

determining the DCT coefficients for each of a plurality of color separations;

inverse discrete cosine transforming the DCT coefficients to produce transformed coefficients including a DC term; and

using at least the DC term of the transformed coefficients for each of a plurality of color separations, correcting at least one of the transformed coefficients;

performing color correction on the partially decompressed unit to produce a corrected unit in the first color space; and

subsequently decompressing the entire corrected unit to produce decompressed, corrected image data.

2. The method of claim 1, wherein the transformed coefficients further include a plurality of AC terms and where the correcting step uses the DC term of the transformed coefficients and at least one low-order AC term of the transformed coefficients.

3. The method of claim 2, wherein the correcting step comprises a matrix transformation using the transformed coefficients as an input.

4. The method of claim 2, wherein the number of transformed coefficients is determined as a function of the value of the DC term of the DCT coefficient for each of the color separations.

5. The method of claim 2, wherein the number of AC terms of the transformed coefficients is determined as a function of the values of a plurality of low order AC terms of the DCT coefficients for each of the color separations.

6. A method for color correcting a compressed digital image, comprising the steps of:

retrieving from memory a compressed unit of image data;

partially decompressing the compressed unit to produce a partially decompressed unit representing the color of a region of the image in a first color space;

performing color correction on the partially decompressed unit to produce a corrected unit in the first color space, wherein the steps of partially decompressing the compressed unit and performing color correction on the partially decompressed unit are carried out upon only a subset of the compressed data representing the region of the image;

merging the subset of color space transformed data with the partially decompressed unit;

recompressing the partially decompressed unit to generate a second compressed unit;

temporarily storing the second compressed unit in a memory; and

subsequently decompressing the second compressed unit to produce decompressed, corrected image data.

7. A method for color correcting a digital image in conjunction with the compression of the image, comprising:

retrieving from memory digital image data representing the image in a first color space;

partitioning the data into compressible units representing regions of the image;

at least partially compressing the data within each compressible unit to produce compressed data representative of each region of the image, wherein the step of partially compressing the data comprises discrete cosine transforming the compressible unit to produce a plurality of DCT coefficients;

performing color correction on the compressed data for each compressible unit, resulting in the alteration of at least one of the DCT coefficients, to produce a corrected unit of compressed data; and

storing the compressed data for each of the regions to produce a compressed image.

8. The method of claim 7, wherein the step of performing a color correction further comprises:

determining the DCT coefficients for each of a plurality of color separations;

inverse discrete cosine transforming the DCT coefficients to produce transformed coefficients including a DC term; and

using at least the DC term of the transformed coefficients for each of a plurality of color separations, correcting at least one of the transformed coefficients.

9. The method of claim 8, wherein the transformed coefficients further include a plurality of AC terms and where the correcting step uses as inputs the DC term of the transformed coefficients and at least one low-order AC term of the transformed coefficients.

10. The method of claim 9, wherein the correcting step comprises a matrix transformation using the transformed coefficients as an input.

11. The method of claim 9, wherein the number of transformed coefficients is determined as a function of the value of the DC term of the DCT coefficients for each of the color separations.

12. The method of claim 9, wherein the number of AC terms of the transformed coefficients is determined as a function of the values of a plurality of low order AC terms of the DCT coefficients for each of the color separations.

13. A method for color correcting a compressed digital image in conjunction with the decompression thereof, comprising:

retrieving from memory a compressed unit of digital image data, each compressed unit representing a region of the image comprising a plurality of pixels, wherein the compressed unit was generated using a lossy compression method employing a discrete cosine transform;

performing color correction on the compressed data for each compressible unit to produce a corrected unit of compressed data, wherein the step of performing color correction includes

partially decompressing the data within each compressed unit to produce a set of DCT coefficients;

determining the DCT coefficients for each of a plurality of color separations;

inverse discrete cosine transforming the DCT coefficients to produce transformed coefficients including a DC term; and

using at least the DC term of the coefficients for each of a plurality of color separations, correcting at least one of the transformed coefficients; and

completely decompressing the compressed unit to produce a decompressed image wherein the decompressed image is a color corrected version of an image used to generate the compressed digital image.

14. The method of claim 13, wherein the transformed coefficients further include a plurality of AC terms and where the correcting step uses as inputs the DC term of the transformed coefficients and at least one low-order AC term of the transformed coefficients.

15. The method of claim 14, wherein the correcting step comprises a matrix transformation using the transformed coefficients as an input.

16. The method of claim 14, wherein the number of transformed coefficients is determined as a function of the value of the DC term of the DCT coefficients for each of the color separations.

17. The method of claim 14, wherein the number of AC terms of the transformed coefficients is determined as a function of the values of a plurality of low order AC terms of the DCT coefficients for each of the color separations.