Color transformation with black preservation

1. In a computerized color management system having a user-selectable gamut mapping model, a method for transforming a source device colorant value in a source device colorant space to a corresponding destination device colorant value in a destination device colorant space, wherein the source and destination device colorant values each have non-black colorant values and a black colorant value, and wherein the source and destination device colorant spaces are colorimetrically characterized by source and destination device models, the method comprising:

determining, by using a computer, whether all of the non-black colorant values of the source device colorant value are or are not all zero;

transforming the source device colorant value to a colorimetric value using the source device model;

transforming the colorimetric value to a gamut-mapped colorimetric value using the user-selectable gamut-mapping model; and

converting the gamut-mapped colorimetric value to a destination device colorant value, wherein the conversion to the destination device colorant value is based on the determination in the determining step, such that

in a first case where all of the non-black colorant values of the source device colorant value are determined to be all zero, the gamut-mapped colorimetric value is converted so that the black colorant value of the destination device colorant value reproduces a lightness of the gamut-mapped colorimetric value relative to a gamut boundary for the destination device model, and so that all of the non-black colorant values of the destination device colorant value are zero,

and in a second case where all of the non-black colorant values of the source device colorant value are determined to be not all zero, the gamut-mapped colorimetric value is converted so that a combination of the non-black and black colorant values of the destination device colorant value reproduces the gamut-mapped colorimetric value.

2. A method according to claim 1 , wherein a flag value is set based on whether all of the non-black colorant values of the source device colorant value are zero, and wherein the destination device colorant value is selected in accordance with the flag value.

3. A method according to claim 1 , wherein in the first case where all of the non-black colorant values of the source device colorant value are zero, the gamut-mapped colorimetric value is transformed to the destination device colorant value, with the black colorant value of the destination device colorant value reproducing a lightness of the gamut-mapped colorimetric value, and all of the non-black colorant values of the destination device colorant value being zero.

4. A method according to claim 1 , wherein in the first case where all of the non-black colorant values of the source device colorant value are zero, the black colorant value of the destination device colorant value reproduces a lightness of the gamut-mapped colorimetric value by obtaining mapping information from the gamut boundary for the destination device model.

5. A method according to claim 4 , wherein in the mapping information from the gamut boundary for the destination device model is implemented as a table of colors representing a neutral ramp of the destination device model.

6. A method according to claim 1 , wherein the step of transforming the colorimetric value to the gamut-mapped colorimetric value using the user-selectable gamut-mapping model comprises:

transforming the colorimetric value to an appearance space value;

gamut-mapping the appearance space value using the user-selectable gamut-mapping model; and

transforming the appearance space value to the gamut-mapped colorimetric value.

7. A method according to claim 6 , wherein the appearance space value is transformed to the gamut-mapped colorimetric value by using a colorimetric space which has a luminance value.

8. A method according to claim 7 , wherein the colorimetric space which has a luminance value is L*, u*, v*.

9. A method according to claim 1 , wherein the source device colorant space is CMYK and the destination device colorant space is CMYK.

10. A method according to claim 1 , wherein the source device model corresponds to a first color printer and the destination device model corresponds to a second color printer.

11. A method according to claim 1 , wherein

each of plural source color device colorant values in source device colorant value data is transformed to a corresponding destination device colorant value in accordance with claim 1 .

12. A method according to claim 11 , wherein a look-up table is generated to store each of the plural source device colorant values and corresponding destination device colorant values.

13. A method according to claim 1 , wherein in the first case where all of the non-black colorant values of the source device colorant value are determined to be all zero, and wherein the gamut-mapped colorimetric value is converted so that the black colorant value of the destination device colorant value reproduces a lightness of the gamut-mapped colorimetric value relative to a gamut boundary for the destination device model, and so that all of the non-black colorant values of the destination device colorant value are zero, the gamut-mapped colorimetric value is converted by using a neutral ramp of a destination device.

14. Computer-executable process steps retrievably stored on a computer readable medium, said computer-executable process steps for causing a computer to transform a source device colorant value in a source device colorant space to a corresponding destination device colorant value in a destination device colorant space, said computer-executable process steps comprising process steps executable by the computer to perform a method according to any one of claims 1 to 12 .

15. A computer-readable medium which retrievably stores computer-executable process steps, said computer-executable process steps for causing a computer to transform a source device colorant value in a source device colorant space to a corresponding destination device colorant value in a destination device colorant space, said computer-executable process steps comprising process steps executable by the computer to perform a method according to any one of claims 1 to 12 .

16. An apparatus for transforming a source device colorant value in a source device colorant space to a corresponding destination device colorant value in a destination device colorant space, the apparatus comprising:

a program memory for storing process steps executable to perform a method according to any one of claims 1 to 12 ; and

a processor for executing the process steps stored in said program memory.