Colour separation method
View Patent ↗Separation of a color into values for colorants in colorant space can be done easily when color correction and color separation are done in separate steps, where the color is defined as a set of values in a trajectory space, and trajectories include points for which ink behavior during color separation is known. Colorant values for the color are determined by interpolation using values of the color located in trajectory space, and known values and corresponding colorant values of the trajectory points. Trajectories are preferably color axes dividing the trajectory space into tetrahedral. Colorant values may be corrected using techniques such as colorant replacement or colorant splitting prior to color separation. The color separation process can be calibrated using a test target.
1. Method for separating a color, defined as a set of color values in a color space into colorant values (C 1 , C 2 , . . . , CN), for N colorants in colorant space comprising the steps of:
converting said set of color values in a color correction step into a set of values defining said color in a trajectory space;
defining in said trajectory space at least two trajectories comprising trajectory points (px), each said trajectory point (px) having:
a set of values in trajectory space and
corresponding colorant values (C 1 x , C 2 x , . . . CN x ) for said N colorants in colorant space for said set of values of said trajectory point,
determining for said color said colorant values (C 1 , C 2 , . . . CN) for said N colorants using an interpolation technique based upon:
said set of values in trajectory space for said color,
said set of values in trajectory space and their
corresponding colorant values (C 1 x , C 2 x , . . . CN x ) for said N colorants ot at least two trajectory points (px).
2. The method according to claim 1 wherein said trajectory space is three-dimensional.
3. The method according to claim 2 wherein the number of said trajectories is equal to seven.
4. The method according to claim 1 wherein at least one of the trajectories is a straight line.
5. The method according to claim 4 wherein the trajectories comprise at least one of the following straight lines:
the line connecting black to white;
the line connecting black to yellow;
the line connecting black to magenta;
the line connecting black to cyan;
the line connecting black to red;
the line connecting black to green; or
the line connecting black to blue.
6. The method according to claim 1 wherein said interpolation technique further comprises the following steps:
dividing said trajectory space in which said trajectories are defined into non-overlapping sub-volumes spanned by said trajectories,
determining the location of said color (p) in one of said sub-volumes,
determining from said location at most one trajectory point (px) on each of said trajectories spanning said sub-volume, and
expressing said colorant values for said color as a function of the colorant values (C 1 x ,C 2 x , . . . CN x ) corresponding to said trajectory points (px) determined in the previous step.
7. The method according to claim 6 wherein said function is a weighted mean.
8. The method according to claim 7 wherein weights of said weighted mean are barycentric co-ordinates of the color in a triangle containing said color location (p) and said triangle being specified by three trajectory points (p 1 , p 2 , p 3 ), one on each trajectory spanning said tetrahedron.
9. The method according to claim 8 wherein said triangle is parallel to a tetrahedron plane opposite to an intersection point of the trajectories spanning a tetrahedron.
10. The method according to claim 6 wherein said trajectories are straight lines and said sub-volumes are tetrahedra spanned by these lines.
11. The method according to claim 10 wherein each tetrahedron (T 1 , T 2 , . . . , T 6 ) contains the points representing black and white in trajectory space.
12. The method according to claim 1 wherein said N colorants are Cyan (C), Magenta (H), Yellow (Y) and Black (K).
13. The method according to claim 1 wherein said N colorants are Cyan (C), Magenta CM), Yellow CYL. Black (K), light Magenta (LM) and light Cyan (LC).
14. The method according to claim 1 wherein the N colorants are Cyan (C), Magenta (M), Yellow CY), Black (K), orange (O) and green (C).
15. The method according to claim 1 further comprising the step of replacing at least partially at least one of the values of said N colorants determined for said color by at least one value of a replacement colorant.
16. The method according to claim 15 wherein the replacement of at least one amount of N colorants is color dependant.
17. The method according to claim 1 wherein at least one of said determined colorant values is corrected by means of a one-dimensional gradation correction step.
18. The method according to claim 1 wherein the color correction step is based on a triangulation of a set of calibration points that is ordered in said trajectory space.
19. The method according to claim 1 wherein the color correction step is based on a triangulation of a set of calibration points that is non-ordered in said trajectory space.
20. The method according to claim 1 wherein the color correction step is obtained by means of at least one polynomial that predicts the values, in the trajectory space from the set of corresponding values in said color space.
21. The method according to claim 1 wherein color correction is obtained by inverting at least one polynomial that predicts the set of values for the color in said color space from the corresponding values in said trajectory space.