The present invention relates generally to the field of digital image synthesis and, more particularly to digital color image reproduction, digital color image printing, digital color image display and anti-counterfeiting protection. The invented method and apparatus enable generating digital halftone images by multi-color dithering. The invented method and apparatus are particularly effective for generating images on color printers printing with more colors than cyan, magenta, yellow and black, and for generating images incorporating microstucture patterns (micro-characters, symbols and ornaments) made of a combination of standard and non-standard inks. Non-standard inks comprise non-process colors, opaque inks, and special inks such as metallic ink, variable color inks and fluorescent inks. The generated images are displayed on color displays or printed on physical output devices such as ink-jet printers, electrophotographic printers, offset printers or gravure printers.
The reproduction of color images requires in the general case (1) separating the image colors (for example red, green and blue) into the set of available printable colors (for example cyan, magenta, yellow and black), (2) halftoning each of the printable color layers and (3) printing the halftoned image or visualizing it on a display device.
Most existing color halftoning techniques are based on the independent halftoning of each of the contributing color layers. In offset and in many electrographic printers, the color halftone layers are generated independently at angles of 15, 45, 75 degrees in order to avoid interferences between the cyan, magenta and black layers. Ink-jet printers often use error-diffusion to halftone each of the color layers independently.
Existing approaches for color separation with non-standard inks consider that color layers can be printed independently from one another. This is true as long as the inks are transparent and as long as the dots of each screen are randomly positioned with respect to the dots of the other screens, as assumed by the Neugebauer equations (see G. L. Rogers, “Neugebauer Revisited: Random Dots in Halftone Screening”, Color Research and Applications, Vol. 23, No. 2, pages 104–113, 1998).
If these assumptions are not true, halftoning each of the color layers independently may generate color shifts depending on the amount of superposition between screen dots of individual halftone layers. As long as the inks are transparent, the color shifts are small and the reproduction fidelity can be ensured by calibrating the printing device (see H. R. Kang, Color Technology for Electronic Imaging Devices, SPIE Publication, 1997, chapter 12).
Some applications however require that different inks be always printed side by side without overlapping. This is the case, for example, when printing with opaque inks as taught by inventor H. Kueppers in U.S. Pat. No. 4,812,899, issued Mar. 14, 1989. Different inks must also be printed side by side for some printing devices as taught by J. Gereadts and S. Lenczowski, “Océ's productive colour solution based on the Direct Imaging Technology”, Proceedings of the IS&T International Conf. On Direct Imaging Technology, Vol. NIP-13, 1997, pages 728–733.
Manufacturers of valuable documents, such as banknotes, identity cards and checks make often use of the high registration accuracy of their original printing equipment to create a graphic design which is both visually pleasant and difficult to imitate using standard printing processes (see Rudolf L. VanRenesse, Optical Document Security, Artech House 1998, chapter 6). In addition, they may use non-standard inks (sometimes also called custom inks), i.e. inks which have a color different from process colors Cyan, Magenta, Yellow and Black to reproduce images and graphic designs. Similar anti-counterfeiting techniques are used to print commercial packages which need to be protected against illegal counterfeits (drugs, cosmetics, etc.).
One prior art method of generating a dither pattern for an RGB value, is taught by D. N. Weise and H. Gunter Zieber in U.S. Pat. No. 5,485,558, issued Jan. 16, 1996. That method is limited to applications, mainly display applications, where one can associate to each input color pixel a dither array, for example an 8×8 array, in order to reproduce that color pixel. Since the dither array must remain of small size, that method does not allow to create at the screening layer artistic screen shapes. Another prior art method of color halftoning is to use vector error-diffusion in color space, as taught by J. Sullivan and R. L. Miller in U.S. Pat. No. 5,070,413, issued Dec. 3, 1991. However, error-diffusion has the drawback of inducing considerable dot gain, compared with clustered dot dithering methods. In addition, error-diffusion does not allow to create user-defined screen shapes.
The present invention discloses a new multi-color dithering method which automatically enforces side by side printing of several color and/or metallic ink layers. Thanks to a user definable dither function or dither array of freely chosen size, multi-color dithered images may incorporate identifyable screen shapes, for example artistic screen shapes, which can be used for document authentication. Combined with color separation by tetrahedral interpolation, multi-color dithering enables printing images made of non-standard inks.
The invented method is a non-trivial generalization of ordered dither halftoning, which uses spatial dithering in order to obtain clustered-dot or dispersed-dot binary patterns on output devices (see H. R. Kang, Digital Color Halftoning, SPIE Press/IEEE Press, 1999, chapters 13 and 14).