Color transforms for concave device gamuts
Method for building color transforms for color imaging devices having concave color gamut regions, including forming a device model that relates device control signals for a color imaging device to corresponding device-independent color values in a device-independent color space; forming a device color gamut representing the set of colors in the device-independent color space that are producible by the color imaging device; and forming a convex color gamut by fitting a convex hull to the device color gamut. The method further includes forming an expanded device model that relates device control signals for the color imaging device to expanded device-independent color values in the device-independent color space, such that the expanded device-independent color values fill the convex color gamut; and building a color transform that relates input device-independent color values to corresponding device control signals for the color imaging device using the expanded device model.
1. A method for building color transforms for color imaging devices having color gamuts with one or more concave surface regions, comprising:
a) forming a device model that relates device control signals for a color imaging device to corresponding device-independent color values in a device-independent color space;
b) forming a device color gamut that represents the set of colors in the device-independent color space that are producible by the color imaging device, wherein the device color gamut has one or more concave surface regions;
c) forming a convex color gamut by fitting a convex hull to the device color gamut;
d) forming an expanded device model that relates device control signals for the color imaging device to expanded device-independent color values in the device-independent color space, such that the expanded device-independent color values fill the convex color gamut, wherein the formation of the expanded device model includes:
i) determining the relative position of a device-independent color value within the device color gamut; and
ii) determining an expanded device-independent color value within the three-dimensionally convex color gamut responsive to the relative position of the device-independent color value within the device color gamut; and
e) building a color transform that relates input device-independent color values to corresponding device control signals for the color imaging device using the expanded device model.
2. The method of claim 1 , wherein step e) includes:
i) gamut mapping any input device-independent color values outside the convex color gamut to modified device-independent color values within the convex color gamut; and
ii) determining The device control signals corresponding to The modified device-independent color values using the expanded device model.
3. The method of claim 2 where the device control signals corresponding to The modified device-independent color values are determined using an inverse expanded device model determined from the expanded device model.
4. The method of claim 2 where the gamut mapping step includes finding the device-independent color values in the convex color gamut having the minimum color difference relative to the input device-independent color values.
5. The method of claim 4 where the color difference is determined using the CIE delta E* color difference metric.
6. The method of claim 4 where the color difference is determined using a color difference metric where hue, lightness and chroma differences are weighted independently.
7. The method of claim 6 where weights used for the hue, lightness and chroma differences vary as a function of color.
8. The method of claim 1 where the color imaging device is a digital printing device.
9. The method of claim 8 where the digital printing device is an ink jet printer.
10. The method of claim 1 where the color imaging device is a softcopy display device.
11. The method of claim 1 where the device model is a multi-dimensional look-up table.
12. The method of claim 1 where the device model is formed by fitting measured device-independent color values for a set of patches with known device control signals.
13. The method of claim 1 where the relative position of the expanded device-independent color value within the convex color gamut is equated to the relative position of the device-independent color value within the device color gamut.
14. The method of claim 1 where the relative position of the expanded device-independent color value within the convex color gamut is a function of the relative position of the device-independent color value within the device color gamut.
15. The method of claim 1 where the relative position of the device-independent color value within the device color gamut is determined by projecting from a central color point through the device-independent color value to find an intersection point with the device color gamut, and computing the ratio of the distance between the central color point and the device-independent color value and the distance between the central color point and the intersection point with the device color gamut.
16. The method of claim 15 where the central color point is a neutral color point.
17. The method of claim 16 where the lightness of the neutral is a function of hue angle.
18. The method of claim 15 where the expanded device-independent color value is determined to have the same fractional position relative to the convex color gamut as that calculated for the device-independent color value relative to the device color gamut.
19. The method of claim 1 further including the step of determining the input device-independent color values using an input device color model that relates input device control signals to corresponding input device-independent color values.
20. A computer storage product having at least one computer storage medium having instructions stored therein causing one or more computers to perform the method according to claim 1 .