System and method of gamut mapping image data

• US 20040170319A1
• Filed: 02/28/2003
• Published: 09/02/2004
• Est. Priority Date: 02/28/2003
• Status: Abandoned Application

• Current Assignee: Hewlett-Packard Development Company, L.P. (HP Inc.)
• Original Assignee: Hewlett-Packard Development Company, L.P. (HP Inc.)
• Inventors: Maurer, Ron P.
• Application Number:
  US10/377,911
• Publication Number:
  US 20040170319A1
• Time in Patent Office: Days
• US Class Current: 382/167
• CPC Class Codes: H04N 1/6027 Correction or control of colour gradation or colour contrast H04N1/6058 takes precedence
• CPC Class Codes: H04N 1/6058 Reduction of colour to a range of reproducible colours, e.g. to ink- reproducible colour gamut

Abstract

A system and method of gamut mapping/clipping is described. The system and method applies to original image data color values in an original color space that have been transformed to obtain an original luminance value having an associated luminance/chrominance based color space and original chrominance values. The original luminance value is processed to obtain a processed color value having a processed luminance value and the original chrominance values. According to the system and method, the processed color value is mapped back into the original color gamut by selecting a target color value on a luminance axis. A different target color value is selectable for each of a plurality of color values of the image data. A mapped color value is then determined by linearly projecting from the processed color value towards the target color value. The mapped color value is located at the intersection of the gamut boundary of the original color space and the projection.

Claims

1. A method of processing image data having a plurality of color values in an original color space, wherein an original color value of the plurality of color values is transformed to obtain an original luminance value, the original luminance value having an associated luminance/chrominance based color space and original chrominance values, and wherein the original luminance value is processed to obtain a processed color value having a processed luminance value and the original chrominance values, the method for mapping the processed color value into the original color gamut comprising:

selecting a target color value on a luminance axis, wherein a different target color value is selectable for each of a plurality of color values of the image data;

determining a mapped color value by linearly projecting from the processed color value towards the target color value, the mapped color value being located at the intersection of a gamut boundary of the original color space and the projection.

2. The method as described in claim 1 wherein the target color value has a corresponding luminance component and chrominance component equal to zero and wherein selecting the target color value is based on setting the luminance component to one of the group of the original luminance value, zero, the luminance component of the processed color value, and a background luminance value.

3. The method as described in claim 1 wherein the original color space is the RGB color space and the luminance/chrominance based color space is a YCC type color space wherein the transformation between the RGB and YCC color spaces is linear.

4. The method as described in claim 1 wherein the luminance/chrominance based color space has an associated luminance component and two chrominance components and wherein the original color space and the luminance/chrominance based color space have an associated invertible transformation relationship such that the sum of the weighting coefficients of the transform of the luminance component equals 1 and the sum of each of the weighting coefficients of the transform of each of the chrominance components equals 0.

5. The method as described in claim 1 wherein the selected target color value is selectable by one of a user and other control sources.

6. The method as described in claim 1 further comprising selecting by one of manual selection and automatic selection.

7. The method as described in claim 1 wherein determination of the mapped color value is based on the following mapping expression:

8. The method as described in claim 1 further comprising using the original color value to predetermine whether the original color value, once processed, will be out-of-gamut from the original color space.

9. The method as described in claim 8 wherein out-of-gamut pre-determining is performed according the following mathematical expression:

out_of_gamut=(Δ

Y>

0 AND max(R, G, B)>

M−

Δ

Y) OR (Δ

Y<

0 AND min(R, G, B)<

−

Δ

Y)

10. The method as described in claim 9 further comprising using the original color value to predetermine whether the original color value, once processed, will be out-of-gamut from the original color space, wherein out-of-gamut pre-determining is performed according the following mathematical expression:

out_of_gamut=(Δ

Y>

0 AND max(R, G, B)>

M−

Δ

Y) OR (Δ

Y<

0 AND min(R, G, B)<

−

Δ

Y)

and wherein max(R′

,G′

,B′

) and min(R′

,G′

,B′

) can be obtained from the determination of max(R,G,B), min(R,G,B).

11. A method of out-of-gamut predetermination in which an original RGB color value, having a corresponding gamut range M, is transformed to obtain an original luminance value, the original luminance value having associated original chrominance value, and wherein the original luminance value is processed to obtain a processed color value having a processed luminance value and the original chrominance values, the method comprising:

determining a difference between the processed luminance value and the original luminance value;

making the out-of-gamut predetermination using only the original RGB color value, the gamut range M, and the difference.

12. The method as described in claim 11 wherein making the out-of-gamut predetermination is performed according to the following mathematical expression:

out_of_gamut=(Δ

Y>

0 AND max(R, G, B)>

M−

Δ

Y) OR (Δ

Y<

0 AND min(R, G, B)<

−

Δ

Y)

13. A system of processing image data having a plurality of color values in an original color space, wherein an original color value of the plurality of color values is transformed to obtain an original luminance value, the original luminance value having an associated luminance/chrominance based color space and original chrominance values, and wherein the original luminance value is processed to obtain a processed color value having a processed luminance value and the original chrominance values, the system for mapping the processed color value into the original color gamut comprising:

a selector for selecting a target color value on a luminance axis, wherein a different target color value is selectable for each of the plurality of color values;

a color value mapper for determining a mapped color value by linearly projecting from the processed color value towards the target color value, the mapped color value being located at the intersection of a gamut boundary of the original color space and the projection.

14. The system as described in claim 13 wherein the target color value has a corresponding luminance component and a corresponding chrominance component equal to zero and the target color value is based on setting the luminance component to one of the group of the original luminance value, zero, the luminance component of the processed color value, and a background luminance value.

15. The system as described in claim 13 wherein the original color space is the RGB color space and the luminance/chrominance based color space is a YCC type color space wherein the transformation between the RGB and YCC color spaces is linear.

16. The system as described in claim 13 wherein the original color space is the RGB color space and the luminance based color space is a generalized based color space having an associated luminance color channel and two chrominance color channels.

17. The system as described in claim 13 wherein the luminance/chrominance based color space has an associated luminance component and two chrominance components and wherein the original color space and the luminance/chrominance based color space have an associated invertible transformation relationship such that the sum of the weighting coefficients of the transform of the luminance component equals 1 and the sum of each of the weighting coefficients of the transform of each of the chrominance components equals 0.

18. The system as described in claim 13 further including an input for allowing a user to select the target color value.

19. The system as described in claim 13 further including an input for one of automatic and manual selection.

20. The system as described in claim 13 wherein the color value mapper implements the following mathematical expression:

21. The method as described in claim 20 further comprising an out-of-gamut predeterminer for determining, using the original color value, whether the original color value, once processed, will be out-of-gamut from the original color space, wherein out-of-gamut pre-determining is performed according the following mathematical expression:

out_of_gamut=(Δ

Y>

0 AND max(R, G, B)>

M−

Δ

Y) OR (Δ

Y<

0 AND min(R, G, B)<

−

Δ

Y)

and wherein determination of max(R′

,G′

,B′

), min(R′

,G′

,B′

), max(R,G,B), and min(R,G,B are performed using, in part, the same processing steps by the predeterminer.

22. A method of processing image data having a plurality of color values in an original color space to obtain a mapped color value comprising:

transforming an original color value of the plurality of color values to obtain an original luminance value, the original luminance value having an associated luminance/chrominance based color space and original chrominance values;

processing the original luminance value to obtain a processed color value having a processed luminance value and the original chrominance values;

selecting a target color value on a luminance axis, wherein a different target color value is selectable for each of a plurality of color values of the image data;

determining a mapped color value by linearly projecting from the processed color value towards the target color value, the mapped color value being located at the intersection of a gamut boundary of the original color space and the projection.

23. A system of processing image data having a plurality of color values in an original color space to obtain a mapped color value comprising:

color value transformer for transforming an original color value of the plurality of color values to obtain an original luminance value, the original luminance value having an associated luminance/chrominance based color space and original chrominance values;

an image data processor processing the original luminance value to obtain a processed color value having a processed luminance value and the original chrominance values;

a selector for selecting a target color value on a luminance axis, wherein a different target color value is selectable for each of the plurality of color values;

a color value mapper for determining a mapped color value by linearly projecting from the processed color value towards the target color value, the mapped color value being located at the gamut intersection of a boundary of the original color space and the projection.

24. A computer readable medium for causing a processor in a computer system to perform processing instructions for mapping a processed color value into an original color gamut, the processed color value having an associated original color value of a plurality of color values of image data in the original color space comprising:

selecting a target color value on a luminance axis, wherein a different target color value is selectable for each of a plurality of color values of the image data;

determining a mapped color value by linearly projecting from the processed color value towards the target color value, the mapped color value being located at the intersection of a gamut boundary of the original color space and the projection.