Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
2000-06-14
2004-09-14
Lee, Thomas D. (Department: 2626)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S003170, C358S534000, C382S237000
Reexamination Certificate
active
06791718
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to halftone images, and more particularly, to methods for generating dither matrices for use in printing halftone images by utilizing cluster filters.
BACKGROUND OF THE INVENTION
Electro-photographic (EP) printers generally do not render single dots well. When attempting to print a single dot, EP prints may print a very light dot or no dot at all. Moreover, in a single dot system, neighboring dots interact in the EP development process, thereby injecting difficulties in this design process. Accordingly, the default halftone for most EP printers is a regular clustered-dot halftone. “An Optimum Algorithm for Halftone Generation for Displays and Hard Copies,” written by Thomas M. Holladay, and published in the Proceedings of the SID, Vol. 21/2, 1980, generally describes an exemplary clustered-dot approach. Unfortunately, the halftone images that are rendered by using dither matrices designed by the regular clustered-dot process suffer from moire problems, especially when the original image contains a periodic structure. A general discussion of moire problems can be found in “Spectral Analysis and Minimization of Moiré Patterns in Color Separation,” written by Issac Amidror, Roger D. Hersch, and Victor Ostromoukhov, and published in the Journal of Electronic Imaging, Vol. 3(3), July 1994.
In addition, halftone images may be rendered by using dither matrices designed by the stochastic halftone process feature dots with maximum dot dispersion. This approach is also known as the void-and-cluster method. A general discussion of the void-and-cluster method can be found in “The Void-and-Cluster Method for Dither Array Generation,” written by Robert Ulichney, and published in SPIE/IS&T Symposium on Electronic Imaging Science and Technology, San Jose, Calif., 1993.
Unfortunately, this maximum dot dispersion approach is undesirable since it generally causes the halftone image to be unstable (e.g., noisy), reproduce poorly, and often have an unacceptable level of dot gain. The halftone pattern reproduces poorly due to a steep tone curve that causes calibration problems. An unacceptable level of dot gain refers to printed dots that are bigger than the printing grid, thereby undesirably causing adjacent dots to overlap.
Based on the foregoing, it is desirable to provide a method for generating stochastic clustered-dot dither matrices for use in halftone printing. In particular, it is desirable to provide a method for generating stochastic clustered-dot dither matrices by utilizing an additional cluster cost function to encourage clustering of the dots in the halftone image.
SUMMARY OF THE INVENTION
The present invention provides a method for generating stochastic clustered-dot dither matrices for use in halftone printing. The stochastic clustered-dot dither matrices produced by the present invention may be utilized to generate halftone images that feature clustered dots.
According to the present invention, a design technique for generating dither matrices for halftone printing is provided. This technique generates dither matrices by using a total cost function that incorporates a modified human visual system model cost function and a cluster cost function. A human visual system model cost function is received and modified so that the cluster sizes of the primary colors are the same (e.g., by modifying the human visual system model cost function to be color-blind). A cluster cost function for generating clustered dots is added to the human visual system model cost function to arrive at the total cost function. The total cost function includes a clustering factor for specifying the amount of clustering effect in the halftone image. Preferably, the clustering factor is gray-level dependent and may be implemented with a look-up table that specifies a dispersion distance between dots for each specific gray level.
In an alternative embodiment, the cluster cost function may include a structure suppression term for suppressing the formation of long horizontal structures or long vertical structures. The structure suppression term includes a symmetry factor for, controlling the symmetry properties related to the cluster cost function. Preferably, the symmetry factor is gray-level dependent and may be implemented with a look-up table that specifies a symmetry factor for each specific gray level.
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Thomas M. Holladay, “An Optimum Algorithm for Halftone Generation for Displays and Hard Copies,” published in the Proceedings of the SID, vol. 21/2, 1980.
Issac Amidror, Roger D. Hersch, and Victor Ostromoukhov, “Spectral Analysis and Minimization of Moiré Patterns in Color Separation,” published in the Journal of Electronic Imaging, vol. 3(3), Jul. 1994.
Robert Ulichney, “The Void-and-Cluster Method for Dither Array Generation,” published in SPIE/IS&T Symposium on Electronic Imaging Science and Technology, San Jose, Calif., 1993.
Fredrik Nilsson, “Digital Halftoning using Pre-Computed Maps,” Journal of Imaging Science and Technology, vol. 43, No. 2, Mar./Apr. 1999, pp. 153-159.
Allebach Jan P.
Lin Qian
Hewlett--Packard Development Company, L.P.
Lee Thomas D.
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