METHOD OF GENERATING A PALETTE FOR USE IN A PRINTING PROCESS AND PRINTING METHOD

1 . A method of generating a palette for use in a printing process using a printing apparatus, the method comprising:

for each of a first plurality of images printed on a substrate surface or within a substrate by varying one or more parameters of the printing apparatus, obtaining at least one color value in a color space;

performing a clustering of the color values of the first plurality of images in the color space to obtain a predetermined number (M) of clusters of color values;

for each of the predetermined number (M) of clusters, selecting at least one representative image to obtain a set of images; and

for each image in the set of images, storing the at least one color value of the image in association with the one or more parameters used to print the image to form the palette.

2 . The method of claim 1 , further comprising:

determining a convex hull of the at least one color values of the first plurality of images in the color space;

for each of the predetermined number (M) of clusters, performing the following steps:

determining whether at least one of the color values lies on the convex hull;

if yes, selecting an image with its at least one color value on the convex hull as the at least one representative image of the cluster;

if no, selecting an image with its at least one color value having a smallest color distance to a centroid of the cluster as the at least one representative image of the cluster.

3 . The method of claim 2 , further comprising:

if, for a given cluster, two or more of the at least one color values lie on the convex hull, at least one of:

(i) selecting an image with its at least one color value on the convex hull and having a smallest color distance to a centroid of the cluster as the at least one representative image of the cluster; or

(ii) determining whether a color distance of two of the color values is greater than a threshold, and, if the color distance is greater than the threshold, selecting the associated images as the at least one representative image of the cluster.

4 . The method of claim 1 , wherein the clustering is an agglomerative clustering, comprising:

a) starting with each of the at least one color values of the first plurality of images as an initial cluster;

b) determining a color distance between each pair of clusters, the color distance being defined by a color distance between respective centroids of the clusters;

c) merging the two clusters having the smallest color distance to form a new cluster; and

d) repeating steps b) and c) until the predetermined number (M) of clusters is obtained.

5 . The method of claim 1 , wherein a single representative image is selected for each cluster.

6 . The method of claim 1 , further comprising:

printing the first plurality of images using the printing apparatus; and

obtaining the color values of the first plurality of images by measurement;

wherein the printing apparatus is a laser marking apparatus, and the parameters include one or more of: laser power, scanning speed, laser focus, laser spacing, repetition rate, laser polarization, laser wavelength, or pulse duration.

7 . The method of claim 1 , wherein the predetermined number (M) is between 10 and 300.

8 . The method of claim 1 , wherein each of the images is a multiplexed image that can be observed in a plurality of modes and has different color values in the plurality of modes, the method comprising:

obtaining the color values of each of the plurality of images in the plurality of modes; and

performing the clustering in a combined color space defined by the color values in the plurality of modes.

9 . The method of claim 8 , further comprising:

reducing a number of dimensions of the combined color space by appropriate processing of the color values in each of the plurality of modes.

10 . The method of claim 8 , further comprising:

performing a change of basis for at least one of the plurality of modes;

transforming the color values in the at least one of the plurality of modes to the changed basis; and

performing the clustering in the combined color space defined at least in part by the at least one changed basis.

11 . The method of claim 10 , further comprising:

performing a principal component analysis in the at least one of the plurality of modes to obtain the combined color space.

12 . The method of claim 10 , further comprising:

performing a principal component analysis in the at least one of the plurality of modes, in combination with reducing a number of dimensions, to obtain the combined color space.

13 . A method of printing an input image using a palette generated in accordance with the method of claim 1 , the method comprising:

receiving the input image;

retrieving the palette from a memory;

performing gamut mapping to map color values of the input image to color values obtainable from the palette; and

printing an output image by halftoning using the palette and the gamut-mapped color values of the input image.

14 . A method of printing a multiplexed input image using a palette generated in accordance with the method of claim 8 , the method comprising:

receiving the multiplexed input image;

retrieving the palette from a memory, the palette being a multiplexed palette including a predetermined number (N) of color values in each mode;

determining a maximum enclosed box enclosed in a convex hull of the palette in the combined color space;

performing gamut mapping to map color values of the input image to color values inside the box; and

printing an output image by halftoning using the palette and the gamut-mapped color values of the input image.

15 . The method of claim 14 , further comprising:

transforming color values of the input image in each mode to respective bases forming the combined color space prior to performing the gamut mapping to obtain gamut-mapped color values;

transforming the gamut-mapped color values of the input image back to the original color values; and

performing halftoning using the palette and the transformed gamut-mapped color values.

16 . A printing apparatus comprising:

a memory; and

a control unit configured to perform the method of claim 1 .

17 . The printing apparatus of claim 16 . wherein the printing apparatus is a laser marking apparatus.