Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
1999-02-05
2002-02-12
Lee, Thomas D. (Department: 2724)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S534000, C358S536000, C358S451000, C358S296000, C358S451000
Reexamination Certificate
active
06346993
ABSTRACT:
This application includes an Appendix containing computer code that performs halftoning of images in accordance with this invention.
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to techniques for printing polychromatic continuous tone images. More particularly, this invention is directed to systems and methods for halftoning images for high quality color printing using line screening for color image separation layers and hexagonal dot screening for the black image separation layer.
2. Description of Related Art
Conventional halftoning adds a two-dimensional, spatially periodic, dot screen or line screen structure to the images to be halftoned. Typically, the same screen, or at least a number of essentially identical screens, are used to halftone each of the color image separation layers of a polychromatic, i.e., color, image. However, the halftone screens are oriented at different angles for printing the respective halftone color image separation layers.
Xerographic images are ordinarily printed for reflective mode viewing. Accordingly, xerographic color images typically are composed by printing in superimposed registration a number of subtractive primary color image separation layers usually together with a neutral image separation layer that characterizes the luminescence of the source image. The subtractive primary colors are usually cyan (C), magenta (M), and yellow (Y), while the neutral is usually black (“K”). Additionally, other image separation layers for even further characterizing the source image can be used. Printing in superimposed registration is performed by overlaying the image separation layers produced by the halftone screens at different angles.
Digital halftoning has evolved as a method of rendering the illusion of continuous tone, or “contone”, images using devices that are capable of producing only binary picture elements. However, digital halftoning can suffer from misregistration between the various color image separation layers used in color images, for example, cyan, magenta, yellow and black (CMYK). This misregistration can be caused by misalignment among the various halftone screens and also by misalignment between the halftone screens and an image forming apparatus grid structure, i.e., the output grid structure, used to generate electronic image data from an image on an image forming member. It can also include errors in rotation in the screen angle. The misregistration can cause moiré patterns, which are detrimental to the accurate rendering of the color image.
Moirè patterns are “beating”, i.e., periodially mismatching, patterns of interference that degrade resulting rendered images. When the overlaid halftone screens provide different component colors, as during rendering of a multicolor image, the moirè pattern can result in a color shift or variation in tone.
Substantial effort and expense have been invested in minimizing the moirè patterns caused by halftoning techniques for producing binary renderings of contone images. Misregistration, improper screen angle and improper screen frequency can increase the halftone screens' susceptibility to moirè patterns. Additionally, because the moirè patterns can be caused by halftone screens beating with the output grid structure, the moirè pattern may be caused by a difference between the halftone screen pitch frequencies and the re-sampling rate frequency within the image forming apparatus. Even minor variations in dot position caused by systematic errors such as quantization round-off errors can produce moirè patterns resulting from beat frequencies between the periodic screens.
The perceived quality of the resulting color image is strongly dependent on the precision with which the color image separation layers are spatially registered with each other and the precision with which the halftone screens are oriented in relationship to a scan grid used by the image forming apparatus.
Additionally, conventional halftoning methods adjust or warp the image data produced by an image data generator, such as a grayscale image generator, or binary image generator, to minimize moirè, such as those disclosed in U.S. Pat. No. 5,732,162 to Douglas M. Curry, incorporated herein by reference in its entirety and U.S. Pat. No. 4,537,470 to Schoppmeyer.
However, merely warping the image data to minimize the moirè patterns results in offsets within the image data which have no corresponding adjustment or warp in the halftone screens used to render the color image separation layers. Therefore, moirè pattern minimization is conventionally improved by also warping halftone screens in a halftone screen system to correspond to the warping of the image data, as disclosed in U.S. Pat. No. 5,485,289 to Curry, incorporated herein by reference in its entirety. The '289 patent provides a detailed discussion of warping both image data and halftone screens.
Another conventional method for minimizing moirè maximizes the screen angles between the halftone screens. This is done because increasing the screen angles reduces the prominence of moirè because interference between the image separation layers is more frequent but to a lesser degree.
Initially, maximizing the angular displacement between the screens to minimize moiré might suggest that line screens should be used for halftoning color images, because a line screen is rotationally symmetric only every 180°, while a dot screen is rotationally symmetric every 90°.
Thus, for example, when printing a four color (CMYK) halftoned image with a line screen, an average allowable separation-to-separation screen displacement angle is 45°. This means that the respective image separation layers may be printed with a line screen oriented at, for example, 0°, 45°, 90° and 135° relative to each other.
Moiré can degrade the image when the color image separation layers that are screened at relative orientations of 0° and/or 90° overlap the color image separation layers that are screened at relative orientations of 45° and/or 135°. Alternatively, if a dot screen is used for halftoning the four color image separation layers of the image, the average allowable screen displacement angle is 22.5°. As a result, there is more opportunity for moiré when the screen orientations are closely spaced.
Just as errors in frequency and angle can cause moiré, so can imperfections in the scanner, or set of scanners, which provide the scan structure for the printer. If the respective scan structure for all four color layers do not exactly overlap, the halftones can be mis-registered so as to be another source of moiré.
The halftone marks, especially dots, must periodically be placed so that the centers of the dots do not exactly line up with the scan structure of the marking device to make halftones at exactly the right frequency or angle or to compensate for non-overlapping scan structures to avoid the pitfalls of moiré. This offset of the halftone marks from the scan structure is referred to as being out of phase. Obviously, not all frequencies, angles or registration corrections may be printed by the marking device without periodically being forced to place marks out of phase.
Increasing the quantization of the marking device can provide more in-phase places to place marks. Typically, a laser marking device, such as a polygon scanner, favors the fast scan direction with high quantization over the process or scan pitch direction, which generally has a coarser quantization. Therefore, halftone dots, which go out of phase in two dimensions when rotated at some arbitrary angle or warped to compensate for some arbitrary error, are more difficult to place in their proper positions because the process direction limits their placement precision. A line screen, on the other hand, can be made to be more or less perpendicular to the fast scan direction, which is the highest quantization direction, so that the line screen mark can be placed with higher precision in that direction.
SUMMARY OF THE INVENTION
Despite the greater average screen displacement angle provided by using line screen
Lee Thomas D.
Oliff & Berridg,e PLC
Xerox Corporation
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