Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
1998-11-13
2001-07-24
Lee, Thomas D. (Department: 2724)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C382S251000
Reexamination Certificate
active
06266157
ABSTRACT:
The present invention generally relates to a digital halftone correction system and more particularly to an improved system for halftone correction which addresses the effects of printed dot overlap in halftoning and solves the problem of causality in the correction process.
Digital halftoning, also referred to as spatial dithering, is a process in which digital input signals to a digital printer are modified prior to printing a hard copy, such that a digitally printed version of a photographic image creates the illusion of the continuous tone scale of the photographic original. Most hard copy devices such as ink-jet printers and laser printers, whether write-black, write-white, or in color, operate in a binary mode, i.e. a printed dot is either present or absent on a two-dimensional printer medium at a specified location. Thus, due to the binary nature of such printers, a true continuous tone reproduction of a photographic image is not possible with digital printers. However, to approach the appearance of continuous tone, digital input signals to the printer are modified prior to printing. Thus, the printer is controlled to spatially distribute fewer or more printed dots in the neighborhood or vicinity of a designated dot, increasing or decreasing the distribution of printed dots about a designated area on the print.
Different types of printers, and even different printers among the same printer type, produce differently sized and shaped printed dots. Even a particular digital printer frequently generates printed dots having a size variation as a function of dot position on a page. It has become apparent that a halftone correction system must be tailored to the characteristics of a particular chosen digital printer.
Frequently, printed dots are of a size and shape such that dots printed adjacent to each other tend to overlap. Accordingly, a successful halftone correction system should include considerations related to dot overlap correction.
In a recent publication, titled “Measurement of Printer Parameters for Model-based Halftoning”, Pappas et al., Journal of Electronic Imaging, Vol. 2 (3), pages 193-204, July 1993, there are described various approaches toward halftone correction based on a dot overlap model of dots printed by a particular digital printer. To accomplish halftone correction, Pappas, et al. describes printing of a variety of test patterns by the same printer. The test patterns are intended to be used for characterization of printed dot overlap and are measured by a reflection densitometer (see particularly pages 198 and 199 of the Pappas, et al. publication) so as to obtain measured values of average reflectance of these various test patterns. The calculated printer model parameters, based on the measurement of test patterns, are then used to provide halftone correction or gray scale rendition of digital image data representative of an original image to be printed. See also, U.S. Pat. No. 5,649,073 to Knox.
Halftone correction can be accomplished for example by a known so-called modified error diffusion algorithm or by a known least-squares model algorithm. In the overlap correction approach described by Pappas et al., each printed dot is positioned within a superimposed or overlaid virtual Cartesian grid such that the center of each dot is coincident with the center of the spacing between adjacent grid lines. Accordingly, Pappas, et al. requires at least 32 total test patterns for the simplest shape of the scanning window, 512 possible test patterns for a 3×3 scanning virtual window, and a total of 33,554,432 possible test patterns for a 5×5 scanning virtual window matrix. Even when considering that dot overlapping can be symmetric about both the x and y directions of the grid, thereby reducing the number of possible patterns, the computational complexity and associated complicated optimization calculations become formidable in the overlap correction approach described by Pappas, et al.
Another publication, titled, “Measurement-based Evaluation of a Printer Dot Model for Halftone Algorithm Tone Correction”, by C. J. Rosenberg, Journal of Electronic Imaging, Vol. 2 (3), pages 205-212, July 1993, describes a tone scale correction approach for digital printers which produce potentially overlapping circular dots, each dot centered at the center of a grid opening of a superimposed grid. This dot-overlapping model assumes that all printed dots have a perfectly circular shape. Here, the reflectance of a number of constant gray scale test patches or test patterns is measured, and the reflectance values are inverted to obtain a correction curve. This measurement-based calibration of a printer (see FIG. 2 of the Rosenberg paper) is repeated for all digital gray levels anticipated to be printed by the printer. The tone response correction curves are then used in conjunction with one of several known halftoning algorithms to generate a calculated dot diameter that would provide a best fit to the measured data.
U.S. Pat. No. 5,469,267 to Wang et al. describes a process, where, prior to printing on a digital printer a halftone reproduction of a continuous one original image, digital image signals are corrected for the effects of printed dot overlap generated by a particular chosen digital printer. The dot overlap correction is based upon superimposing a virtual screen on the printer-generated dot patterns such that the printer dots are centered at the orthogonal intersections of the lines defining openings in the screen. This centering approach allows for determination of printed dot overlap by a 2×2 matrix, so that only seven test patterns are required for characterization of the printer and for dot overlap correction of halftone prints produced by the printer.
However, one problem exists for applying 2×2 correction to error diffusion due to the causality constraints. In error diffusion, the pixels are processed from top to bottom and from left to right. In determining the gray level of a pixel, four 2×2 matrices (upper left, upper right, bottom left, bottom right) are involved. In processing a pixel, only one of the 2×2 matrices (the upper left one) is available as all the others contain unprocessed pixels.
SUMMARY OF THE INVENTION
In accordance with the invention, and error diffusion method is used to quantize pixels, using 2×2 neighborhood correction.
In accordance with one aspect of the invention there is provided method of quantizing pixels from a first pixel depth to a second includes adding to an original value of each pixel to be quantized, an error value resulting from quantization of any previous pixels, to generate a modified pixel value; comparing each modified pixel with threshold varying in accordance with the gray difference that a printed mark would make to a neighborhood pixels and outputting second depth pixels responsive to said comparison; and determining a halftoning error as a function of the modified pixel values, the gray difference, and the binary signals, and distributing say error to other gray level pixels in an image.
A method of quantizing gray pixels to binary pixels for printing, including the steps of: storing halftone response characterizations in memory, representing an amount of gray difference that a mark on paper will provide for a selected printer; adding to an original value of each pixel to be quantized, an error value resulting from quantization of any previous pixels, to generate a modified pixel value; using said stored halftone response characterizations to control a thresholding process; thresholding the modified pixel values to binary signals; and determining a halftoning error as a function of the modified pixel values, the halftone characterizations, and the binary signals, and distributing the halftoning error to other gray level pixels in an image.
The invention uses 2×2 color correction in an error diffusion halftoning process. Local color correction and error diffusion can be integrated naturally and free of the causality problem. In error diffusion, quantization error
Fan Zhigang
Wang Shen-ge
Brinich Stephen
Costello Mark
Dudley Mark Z.
Lee Thomas D.
Xerox Corporation
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