Color correction of a compressed image

• US 6,097,838 A
• Filed: 05/06/1999
• Issued: 08/01/2000
• Est. Priority Date: 12/19/1996
• Status: Expired due to Fees

• 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): Boudreau, Leo H.
• Assistant Examiner(s): Novik, Dmitry A.
• Application Number:
  US09/306,162
• Time in Patent Office: 453 Days
• Field of Search: 382/162, 382/164, 382/166, 382/232, 382/233, 382/167, 382/235, 382/244, 382/245, 382/246, 382/248, 382/250, 358/518, 358/539, 358/426, 358/261.1, 358/261.2, 358/261.3, 358/427, 358/432, 358/433, 348/390, 348/391, 348/395, 348/403, 348/386, 348/717, 348/577
• 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 in the form of a digital document comprising at least one compressed unit, said method comprising the steps of:

retrieving from memory a compressed unit of digital image data;

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

performing color correction on the partially decompressed unit, to produce a corrected unit in the first color space, by mapping at least a first color of the region in the first color space to a corresponding second color in the first color space using an interpolation process to complete the color correction transformation;

subsequently partially decompressing the corrected unit to produce decompressed, corrected image data having the form of a device dependent color space;

integrating said partially decompressed device dependent image data with the remaining compressed data before completely decompressing this result to form a fully decompressed data block; and

performing a second phase color correction to convert said fully decompressed data block into device dependent signals.

2. The method of claim 1, further comprising the step of converting the decompressed, corrected image data into a second color space image for rendering.

3. The method of claim 1, wherein the compressed digital image is generated using a lossless compression method.

4. The method of claim 3 wherein the lossless compression method is selected from the group consisting of:

Huffman encoding;

Lempel-Ziv encoding; and

run-length encoding.

5. The method of claim 1, wherein the compressed digital image is generated using a lossy compression method.

6. The method of claim 5 wherein the region comprises a plurality of pixels and where the lossy compression method uses discrete cosine transform.

7. The method of claim 6, 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.

8. The method of claim 7, 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.

9. The method of claim 8, wherein the correcting step comprises a matrix transformation using the transformed coefficients as an input.

10. The method of claim 8, 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.

11. The method of claim 8, 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.

12. The method of claim 1 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, the method further comprising the steps of:

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

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

temporarily storing the second compressed unit in a memory.

13. The method of claim 1 wherein the region comprises a plurality of pixels.