Methods and apparatus for converting color values

• US 20040046766A1
• Filed: 05/24/2002
• Published: 03/11/2004
• Est. Priority Date: 05/24/2002
• Status: Active Grant

• Current Assignee: Fiery LLC
• Original Assignee: Electronics For Imaging Incorporated
• Inventors: Falk, Richard
• Granted Patent: US 6,781,596 B2
• Time in Patent Office: Days
• US Class Current: 345/600
• CPC Class Codes: G06F 3/13 Digital output to plotter ; Cooperation and interconnection of the plotter with other functional units
• CPC Class Codes: G09G 2340/02 Handling of images in compressed format, e.g. JPEG, MPEG
• CPC Class Codes: G09G 2340/06 Colour space transformation
• CPC Class Codes: G09G 5/02 characterised by the way in which colour is displayed details of colour display specific for CRTs G09G1/28; specific for flat matrix panels other than liquid crystal displays G09G3/2003; specific for liquid crystal displays G09G3/3607
• CPC Class Codes: G09G 5/06 using colour palettes, e.g. look-up tables
• CPC Class Codes: H04N 1/6019 using look-up tables H04N1/6025 takes precedence

Abstract

Color conversion apparatus and methods are provided for converting a first color value, such as an RGB, L*a*b* or XYZ color value, to a second color value, such as an RGB, CMY, CMKY, L*a*b*, L,C₁C₂, or XYZ color value, without saturation error.

Claims

1. A method for converting a first color value to a second color value, the method comprising:

providing a lookup table comprising a plurality of entries, each entry associating a first value to a second value;

identifying an entry that has a second value equal to a predetermined value, and that is adjacent an entry having a second value ejqual to a number other than the predetermined value;

compressing the second values of a first plurality of the entries to form corresponding intermediate values; and

replacing the second value of the identified entry with a value determined by a regression that includes the intermediate value of the adjacent entry.

2. The method of claim 1, wherein:

identifying further comprises identifying each entry that has a second value equal to a predetermined value, and that is adjacent an entry having a second value equal to a number other than the predetermined value; and

replacing further comprises replacing the second value each of the identified entries with a value determined by a regression that includes the intermediate value of the corresponding adjacent entry.

3. The method of claim 1, wherein each of the second values has a value greater than or equal to zero (“

ZERO”

) and less than or equal to a maximum number (“

MAX”

).

4. The method of claim 3, wherein the predetermined value is ZERO.

5. The method of claim 3, wherein the predetermined value is MAX.

6. The method of claim 1, wherein:

each of the second values has a value greater than or equal to zero (“

ZERO”

) and less than or equal to a maximum number (“

MAX”

);

identifying further comprises identifying a first entry that has a second value equal to ZERO., and that is adjacent a second entry having a second value equal to a number greater than ZERO, and identifying a third entry that has a second value equal to MAX, and that is adjacent a fourth entry having a second value equal to a number less than MAX; and

replacing further comprises replacing the second value of the first entry with a value determined by a regression that includes the intermediate value of the second entry, and replacing the second value of the third entry with a value determined by a regression that includes the intermediate value of the fourth entry.

7. The method of claim 1, further comprising:

identifying a fifth entry that has a first value equal to the first color value; and

setting an intermediate color value equal to the intermediate value of the fifth entry.

8. The method of claim 7, further comprising decompressing the intermediate color value to provide the second color value.

9. The method of claim 1, further comprising:

identifying a second plurality of entries that have first values that surround the first color value; and

setting an intermediate color value equal to a value determined by interpolating the intermediate values of the second plurality of entries.

10. The method of claim 9, further comprising decompressing the intermediate color value to provide the second color value.

11. A method for creating a color conversion lookup table, the method comprising:

creating a plurality of table entries, each entry associating a first value to a second value;

identifying the minimum and maximum values of the plurality of second values;

identifying table entries that have second values equal to the minimum value, and that are adjacent an entry having a second value greater than the minimum value;

identifying table entries that have second values equal to the maximum value, and that are adjacent an entry having a second value less than the maximum value;

compressing the second value of each of the table entries to form corresponding intermediate values; and

replacing the intermediate value of each of the identified entries with a value determined by a regression that includes the intermediate value of the corresponding adjacent entry.

12. The method of claim 11, wherein:

each of the second values has a first bit-length; and

the creating step further comprises converting the second values to values having a second bit length.

13. The method of claim 12, wherein the second bit length is greater than the first bit length.

14. A color conversion system for converting a first color value to a second color value, the system comprising:

a first module comprising an input coupled to the first color value, a lookup table coupled to the input and to an interpolator, the interpolator coupled to an output; and

a second module comprising a decompressor comprising an input coupled to the output of the first module, and an output providing the second color value.

15. The system of claim 14, wherein the first color value comprises a plurality of color values and the second color value comprises a plurality of color values.

16. The system of claim 14, wherein the first color value comprises Red (R), Green (G) and Blue (B) color values.

17. The system of claim 14, wherein the second color value comprises Cyan (C), Magenta (M) and Yellow (Y) color values.

18. The system of claim 14, wherein the decompressor comprises a plurality of one-dimensional lookup tables.

19. The system of claim 18, wherein the second color value comprises a plurality of color values, and each one-dimensional lookup table is coupled to a corresponding one of the plurality of color values.

20. The system of claim 14, wherein the lookup table comprises a plurality of entries, each entry associating a first value to a second value, the second values compressed using a linear compression formula.