Method and a device for converting a color coordinate set

• US 5,268,754 A
• Filed: 02/27/1992
• Issued: 12/07/1993
• Est. Priority Date: 03/01/1991
• Status: Expired due to Term

• Current Assignee: Barco Graphics nv
• Original Assignee: Barco Graphics nv
• Inventors: Debaere, Eddy, Van de Capelle, Jean-Pierre, Pletinckx, Daniel, Roose, Karel
• Primary Examiner(s): Coles, Sr., Edward L.
• Assistant Examiner(s): VU, KIM Y
• Application Number:
  US07/842,541
• Time in Patent Office: 649 Days
• Field of Search: 358/75, 358/78, 358/80, 358/22, 358/28, 395/131, 382/17
• US Class Current: 358/527
• CPC Class Codes: G01J 3/46 Measurement of colour; Colour measuring devices, e.g. colorimeters measuring colour temperature G01J5/60
• CPC Class Codes: G01J 3/462 Computing operations in or between colour spaces; Colour management systems
• CPC Class Codes: H04N 1/6022 Generating a fourth subtractive colour signal, e.g. under colour removal, black masking
• CPC Class Codes: H04N 1/6058 Reduction of colour to a range of reproducible colours, e.g. to ink- reproducible colour gamut

Abstract

A method and a device for converting a color defined in a first n-dimensiontal device dependent color space to a corresponding color defined in a second m-dimensional (m≧n; n≧p; m>p; p≧3) device dependent color space, wherein for each (m-p)^th color dimension from the second device dependent color space there is each time defined a color value and said color coordinate set of the second device dependent color space is determined on the basis of said color value.

Claims

1. A method for converting a color defined in a first n-dimensional device dependent color space to a corresponding color defined in a second m-dimensional (m≧

n;

n≧

p;

m>

p;

p≧

3) device dependent color space, wherein said color is first converted from said first device dependent color space to a first device independent color space and thereafter from said first device independent color space to a second device independent color space in order to be there after converted to said second device dependent color space, and wherein for each conversion from one color space to another there is determined a respective mapping algorithm mapping a color coordinate set from said one color space to said other color space, and wherein each device dependent color space comprises a color gamut and said color gamut of said first respectively second device dependent color space is each time by means of said respective mapping algorithm mapped to said first respectively second device independent color space in order to each time define therein a first respectively a second color gamut which is correlated with said first respectively said second device dependent color space and subsequently there being verified if said color coordinate set converted from said first to said second device independent color space is within said correlated color gamut defined within said second device independent color space, and wherein for each (m-p)^th color dimension from said second device dependent color space there is each time defined a color value and said color coordinate set of said second device dependent color space is determined on the basis of said color value, which color value defines a further color gamut within said color gamut of said second device dependent color space, which further color gamut is mapped by means of said respective mapping to said second device independent color space in order to define therein a third color gamut and if by said verifying there is established that said converted color coordinate set is outside said third but inside respectively outside said correlated second color gamut that color coordinate set is substituted by a first respectively a second color coordinate set.

2. A method as claimed in claim 1, wherein for said color value, a value is chosen determined in function of said color values of remaining color dimensions.

3. A method as claimed in claim 2, wherein for said color value, a value is chosen in function of a minimum value of remaining p color coordinates.

4. A method as claimed in claim 1, wherein said second color coordinate set is determined by projection of said color coordinate set, obtained by mapping to said second device independent color space, on a boundary plane of said correlated second color gamut.

5. A method as claimed in claim 4, wherein said projection is carried out in a plane with constant hue.

6. A method as claimed in claim 4, wherein said second color coordinate set is obtained by determining for at least one of said color parameters hue, luminance and saturation, a difference between said second color coordinate set obtained by projection and said color coordinate set obtained by mapping to said second device independent color space.

7. A method as claimed in claim 1, wherein said second color coordinate set is obtained by minimizing a difference between a color coordinate set obtained by mapping to said second device independent color space and a further color coordinate set in said second device independent color space obtained by using said color value.

8. A method as claimed in claim 7, wherein for said minimization said color value is respected within predetermined restrictions.

9. A method as claims in claim 7 or 8, wherein said minimization comprises a minimization of a color distance in an Euclidian space.

10. A method as claimed in claim 7 or 8, wherein said first color coordinate set is determined by defining for each (m-p)^th color dimension of said second device dependent color space a further color value, and wherein on a basis of said further color value there is determined in said second device independent color space a fourth color gamut which is included within said second color gamut and which comprises at least partially said third color gamut.

11. A method as claimed in claim 7 or 8, wherein said first color coordinate set is determined by defining for each (m-p)^th color dimension of said second device dependent color space a further color value, wherein on a basis of said further color value there is determined in said second device independent color space a fourth color gamut which is included within said second color gamut and which comprises at least partially said third color gamut and wherein for determining said further color value an inverse mapping is made of said color coordinate set obtained by mapping from said second device independent color space to said second device dependent color space and wherein for said further color value a value is chosen determined on the basis of a result of said inverse mapping.

12. A method as claimed in claim 1, wherein by converting a first color coordinate set of said first device dependent color space to a corresponding color coordinate set of said second device dependent color space, use is made of an interpolation between predetermined color coordinates.

13. A method as claimed in claim 12, wherein to determine said predetermined color coordinates, said first device dependent color space is subdivided in geometrically each time substantially uniform subspaces wherein each subspace is delimited by corner points, and for each corner point a corresponding color coordinate set of said second device dependent space is stored into a memory.

14. A method as claimed in claim 13, wherein said subspaces are formed by rectangular subspaces.

15. A method as claimed in claim 13, wherein said subspaces are formed by tetrahedral subspaces.

16. A method as claimed in anyone of the claims 13, 14 or 15, wherein for each first color coordinate set to be converted a subspace wherein said first color coordinate set is located, is identified and wherein for said interpolation use is made of said second color coordinates of said corner points of said identified subspace which second color coordinates are stored in said memory.

17. A method as claimed in anyone of the claims 13, 14, or 15 wherein for each first color coordinate set to be converted a subspace wherein said first color coordinate set is located, is identified and wherein for said interpolation use is made of said second color coordinates of said corner points of said identified subspace which second color coordinates are stored in said memory and wherein for said identifying of said subspace use is made of most significant bits of said color coordinate set to be converted in order to determine a first corner point of said subspace to be identified and wherein remaining corner points of the subspace to be identified are identified by incrementation of said most significant bits.

18. A method as claimed in claim 15 wherein for said identifying of said subspace use is made of most significant bits of said color coordinate set to be converted in order to determine a first corner point of said subspace to be identified and wherein the remaining corner points of the subspace to be identified are identified by incrementation of said most significant bits and wherein for identifying a tetrahedron there is first checked on the basis of said most significant bits in which rectangular subspace said color coordinate set is located and thereafter there is checked on the basis of said least significant bits in which tetrahedron belonging to said rectangular subspace said color coordinate set is located.

19. A method as claimed in claim 18, wherein on the basis of said most significant bit addresses are formed for addressing said memory.

20. A method as claimed in anyone of the claims 13, 14 or 15 wherein for each first color coordinate set to be converted a subspace wherein said first color coordinate set is located, is identified and wherein for said interpolation use is made of said second color coordinates of said corner points of said identified subspace which second color coordinates are stored in said memory and wherein for said identifying of said subspace use is made of most significant bits of said color coordinate set to be converted in order to determine a first corner point of said subspace to be identified and wherein remaining corner points of the subspace to be identified are identified by incrementation of said most significant bits and wherein on the basis of said most significant bit addresses are formed for addressing said memory.

21. A method as claimed in anyone of the claims 13, 14, 15, wherein for each first color coordinate set to be converted a subspace wherein said first color coordinate set is located, is identified and wherein for said interpolation use is made of said second color coordinates of said corner points of said identified subspace which second color coordinates are stored in said memory and wherein on the basis of least significant bits of said color coordinate set to be converted weight coefficients are selected which represent a contribution of said color coordinate set to each second color coordinates which are related to said corner points of said identified subspace.

22. A method as claimed in anyone of the claims 13, 14 or 15 wherein for each first color coordinate set to be converted a subspace wherein said first color coordinate set is located, is identified and wherein for said interpolation use is made of said second color coordinates of said corner points of said identified subspace which second color coordinates are stored in said memory and wherein for said identifying of said subspace use is made of most significant bits of said color coordinate set to be converted in order to determine a first corner point of said subspace to be identified and wherein remaining points of the subspace to be identified are identified by incrementation of said most significant bits and wherein on the basis of said most significant bit addresses are formed for addressing said memory and wherein on the basis of least significant bits of said coordinate set to be converted weight coefficients are selected which represent a contribution of said color coordinate set to each second color coordinates which are related to said corner points of said identified subspace and wherein said interpolation is realized on the basis of said second color coordinate set and said weight coefficients.

23. A device for application of a method as claimed in claim 1, which is provided with mapping means for converting the color from said first device dependent color space to said first color independent color space and thereafter via said second device independent color space to said second device dependent color space, which device further comprises verification means provided for verifying if said color coordinate set transformed from said first to said second device independent color space is within said color gamut defined within said second device independent color space, wherein said device comprises a first memory provided for each time storing a color value, for each (m-p)^th color dimension of said second device dependent color space, which first memory is connected with a color coordinate set generator provided for generating said color coordinate set of said second device dependent color space on the basis of said color value, which color value defines within the color gamut of said second device dependent color space a further color gamut, which mapping means are further provided for mapping by means of said respective mapping said further color gamut to said second device independent color space in order to define there a third color gamut, and wherein said verification means are connected with substitution means provided for substituting said color coordinate set by a first respectively a second color coordinate set when it has been established that said converted color coordinate set is outside said third but inside respectively outside said correlated second gamut.

24. A device as claimed in claim 23, wherein said substitution means are provided for generating said second color coordinate set by means of a projection of said color coordinate set obtained by mapping to said second device independent color space on a boundary surface of said correlated second color gamut.

25. A device as claimed in claim 23, wherein said substitution means are provided for generating said second color coordinate set by determining for at least one of the color parameters hue, luminance and saturation a difference between said second color coordinate set obtained by projection and said color coordinate set obtained by mapping and by minimizing thereafter said difference.

26. A device as claimed in any one of the claims 23, 24 or 25, wherein said substitution means are provided for generating said first color coordinate set by determining a difference between said color coordinate set obtained by mapping to said second device independent color space and a further color coordinate set in said second device independent color space obtained by using said color value and by means of thereafter minimizing said difference.

27. A device for performing a method as claimed in claim 13, wherein said device comprises a second memory provided for storing predetermined conversion values for said corner points and a third memory provided for storing weight coefficients, which second memory comprises an address input connected with a first combinative circuit which is provided for determining from a supplied first color coordinate set in which subspace said first color coordinate set is located and for forming an address for addressing in said memory said conversion values belonging to said determined subspace, which third memory comprises an address input connected with an output of a second combinatorial circuit which is provided for selecting from a supplied first color coordinate set a set of weight coefficient, which third memory is connected to a first input of an interpolation unit which second input is connected with a data output of said memory.