Color halftone processing with mapping color values to NPac values

1. A method of color halftone processing comprising:

accessing a plurality of color mappings, each color mapping providing a mapping from a color value in a color space to a set of Neugebauer Primary area coverage (NPac) values in an NPac space of an imaging device;

for each color value of a plurality of color values in the color space:

determining a plurality of NPac values that result from each of the plurality of color mappings;

obtaining imaging metrics for each NPac value; and

selecting one of the plurality of NPac values for the color value based on a comparison of the obtained imaging metrics to each other,

wherein the selected one of the plurality of NPac values is used as an output NPac value for the color value for the plurality of color mappings;

mapping each input color value of a plurality of input color values of an input image to a corresponding output color value of a plurality of output color values, the corresponding output value being the selected output NPac value for the input color value; and

outputting the input image, by the imaging device, using the output color values to which the input color values of the input image have been mapped.

2. The method of claim 1 , wherein determining the plurality of NPac values comprises:

determining color space region comprising the given color value, the color space region having one or more uniform color properties; and

determining a plurality of NPac values that map to the color space region.

3. The method of claim 1 , wherein:

each color mapping is modeled using a color-space-to-NPac look-up table,

the given color value comprises a node within each look-up table, and

the selected one of the plurality of NPac values provides a custom mapping from each of the nodes.

4. The method of claim 3 , wherein determining the plurality of NPac values comprises:

determining a color-space-to-NPac sampling that has a resolution lower than that of one or more of the color-space-to-NPac look-up tables;

determining a set of color space values that map to a common lower-resolution color space value; and

determining a plurality of NPac values that correspond to the set of color space values.

5. The method of claim 1 , wherein the color space comprises one of: a Red, Green, Blue (RGB)-based color space; a Cyan, Magenta, Yellow and Black (CMYK)-based color space; an International Commission on Illumination (CIE) Lab-based color space and a CIE XYZ-based color space.

6. The method of claim 1 , wherein the imaging metric comprises one or more of:

an ink use metric;

a grain appearance metric; and

a spectral reflectance metric.

7. The method of claim 1 , wherein the plurality of color mappings represent a respective plurality of color separations and the method is repeated for a plurality of given color values in the color space to provide a plurality of selected NPac values that represent a color mapping for a combination of the plurality of color separations.

8. The method of claim 1 , wherein the imaging device is a printing device, and outputting the input image comprises printing the input image.

9. The method of claim 8 , wherein the printing device is an inkjet-printing device.

10. The method of claim 1 , wherein the imaging device is a display device, and outputting the input image comprises displaying the input image.

11. A method of combining a plurality of color separations comprising:

determining a set of color space regions in a color space of an imaging device, each color space region having a uniform color property;

for each color space region, determining a per-region set of Neugebauer Primary area coverage (NPac) vectors for any color values that are present in the plurality of color separations to be combined;

obtaining imaging metrics for each NPac vector in each per-region set;

selecting one NPac vector for each color space region based on a comparison of the imaging metrics to one another, each selected NPac vector being representative of a color mapping for the plurality of color separations to be combined;

mapping a plurality of input color values of an input image to a plurality of output color values using the color mapping of which each selected NPac vector is representative; and

outputting the input image, by the imaging device, using the output color values to which the input color values of the input image have been mapped.

12. The method of claim 11 , wherein the one or more uniform color properties comprises a uniform CIE Lab value.

13. The method of claim 11 , wherein the imaging metric comprises one or more of:

an ink use metric;

a grain appearance metric; and

a spectral reflectance metric.

14. The method of claim 11 , wherein the color space is a colorimetric color space and each color separation represents a colorimetric-to-NPac color mapping.

15. A non-transitory machine-readable storage medium encoded with instructions for color halftone processing, the instructions executable by a processor of a system to cause the system to:

access a plurality of color mappings, each color mapping providing a mapping from a color value in a color space to a set of Neugebauer Primary area coverage (NPac) values in an NPac space of an imaging device;

for each color value of a plurality of color values in the color space:

determine a plurality of NPac values that result from each of the plurality of color mappings;

obtain imaging metrics for each NPac value; and

select one of the plurality of NPac values based on a comparison of the obtained imaging metrics to each other,

wherein the selected one of the plurality of NPac values is used as an output NPac value for the color value for the plurality of color mappings;

mapping each input color value of a plurality of input color values of an input image to a corresponding output color value of a plurality of output color values, the corresponding output value being the selected output NPac value for the input color value;

outputting the input image, via the imaging device, using the output color values to which the input color values of the input image have been mapped.

16. The machine-readable storage medium of claim 15 , wherein the instructions are executable by the processor of the system to cause the system to:

determine a color space region comprising the given color value, the color space region having one or more uniform color properties; and

determine a plurality of NPac values that map to the color space region.

17. The machine-readable storage medium of claim 15 , wherein the instructions are executable by the processor of the system to cause the system to:

determine a color-space-to-NPac sampling that has a resolution lower than that of one or more of color-space-to-NPac look-up tables;

determine a set of color space values that map to a common lower-resolution color space value; and

determine a plurality of NPac values that correspond to the set of color space values.

18. The machine-readable storage medium of claim 15 , wherein the imaging metric comprises one or more of:

an ink use metric;

a grain appearance metric; and

a spectral reflectance metric.