Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
1998-09-23
2001-05-15
Rogers, Scott (Department: 2624)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S451000, C358S451000, C382S199000, C382S261000, C382S275000, C382S224000
Reexamination Certificate
active
06233060
ABSTRACT:
TECHNICAL FIELD
The present invention is related generally to image processing and is related more particularly to processing scanned representations of screened images such as screened halftone images to generate high-quality renditions with little or no visible moiré.
BACKGROUND ART
Moiré is an artifact that results whenever two geometrically-regular patterns are superimposed. This artifact often manifests itself as a ripple-like pattern in scanned representations of screened images because the geometrically-regular structure of the screened image is superimposed with the geometrically-regular pattern of scanning.
The nature of the moiré that results from scanning depends on a number of factors including: (1) the resolution of the geometrically-regular structure of the screened image, referred to herein as the screen frequency, (2) the resolution of the geometrically-regular pattern for the scanning process, referred to herein as the scan frequency, (3) the relative angle between these two patterns, referred to herein as the screen angle, and (4) the characteristics of the process used to render the scanned representation. Moiré can be reduced by decreasing the screen frequency, increasing the scan frequency, and/or carefully selecting the screen angle, but these simple techniques are generally unsatisfactory.
A tradeoff between screen frequency and scan frequency is generally unable to achieve a satisfactory reduction in moiré because, on the one hand, a decrease in screen frequency reduces the resolution of the image and, on the other hand, an increase in scan frequency greatly increases the amount of resources required to process and store the scanned representation. In some applications such as high-quality printed publications, screen frequencies in excess of 200 lines per inch (about 79 lines per centimeter) are used, making it essentially impossible to reduce moiré by merely increasing the scan frequency.
A careful selection of screen angle is generally unable to achieve a satisfactory reduction in moiré because the angle is difficult to select precisely in many applications. Precise selection is difficult because the relative orientation of the screened image with respect to the orientation of the scanning apparatus is difficult to control precisely. The problem is even greater in color applications where the screen angle for three or more colors must be very carefully selected.
A variety of techniques exist that can reduce moiré by reducing the regularity of the pattern in either or both the image screen and the scan. These techniques usually are not satisfactory because it is often impossible to control how the screen image is produced and irregular scanning patterns are difficult to implement.
A variety of other techniques attempt to reduce moiré by using so called stochastic screens to introduce a degree of randomness in either a scanned representation or a printed replica of a scanned representation. These techniques are not attractive because considerable processing resources are required to perform the quasi-stochastic processes needed to implement the stochastic screens. In addition, a considerable amount of random-access memory (RAM) is required for these processes to operate on the scanned representations.
A variety of other techniques reduce moiré by applying a low-pass filter to the scanned representation. If a filter is applied uniformly to the representation, however, the degree of filtering required to achieve an acceptable reduction in moiré usually results in an unacceptable reduction in resolution. For this reason, a number of adaptive filtering techniques have been developed. According to one technique, different filters are applied to a scanned representation and weighted combinations of the filter outputs are formed according to whether edges or other high-spatial-frequency components are present in the image. This techniques are sometimes well suited for certain specialized applications like enhancing X-ray images for examination by trained medical personnel but are not as well suited for high-quality reproduction of arbitrary images.
According to another technique, the scanned representation is filtered by a spatially-variant filter that is adapted according to the presence and orientation of edges in the image. This technique is not attractive because considerable processing resources are required to implement the spatially-variant filters. In addition, considerable memory is required for these filters to operate on the scanned representations.
All known techniques for reducing moiré are deficient because they cannot achieve high-quality reproduction of all types of screened images using essentially any screen and scan frequency and/or screen angle for display by essentially any display technique. Furthermore, as mentioned above, implementation of many of these techniques require an amount of processing and memory resources that cannot be provided inexpensively. These known techniques also do not work well for images that include periodic structures of multiple screen frequencies.
DISCLOSURE OF INVENTION
It is an object of the present invention to provide for a method and apparatus to process scanned representations of screened images, such as screened halftone images, in a manner that is independent of screen frequency, scan frequency and screen angle, to generate high-quality renditions that are substantially free of visible moiré. The advantages of the present invention may be realized in low-cost implementations that require only modest processing and memory resources.
According to teachings of the present invention, a screened image is reproduced with reduced moiré by analyzing a plurality of representations of the screened image to detect edges, where each representation has a respective resolution; generating a control signal for each region of the screened image, where the control signal is generated for a respective region according to which of the representations have edges detected in that respective region; and generating an output signal reproducing the screened image with reduced moiré by filtering the screened image, where a respective region is filtered by an amount that varies according the control signal for the respective region.
One embodiment of an apparatus that may be used to carry out the present invention comprises a plurality of filters, each filter having a size, a filter output, and a filter input receiving a first input signal conveying a screened image; a plurality of input terminals, each input terminal receiving a plurality of second input signals representing the image in respective resolutions; a plurality of edge detectors, each of the edge detectors having a detector input coupled to a respective input terminal and having a detector output; a controller having a control output and a plurality of control inputs, each control input coupled to the detector output of a respective edge detector; a selector having a selector-control input, a plurality of selector inputs and a selector output, the selector-control input coupled to the control output of the controller and a respective selector input coupled to the filter output of a respective filter; and an output terminal coupled to the selector output to convey an output signal reproducing the image in a form having reduced moiré.
The various features of the present invention and its preferred embodiments may be better understood by referring to the following discussion and the accompanying drawings in which like reference numerals refer to like elements in the several figures. The contents of the following discussion and the drawings are set forth as examples only and should not be understood to represent limitations upon the scope of the present invention.
REFERENCES:
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patent: 4782399 (1988-11-01), Sato
patent: 5001576 (1991-03-01), Tanaka et al.
patent: 5022091 (1991-06-01), Carlson
patent: 5231677 (1993-07-01), Mita et al.
patent: 5379130 (1995-01-01), Wang et al.
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patent: 5
Bhattacharjya Anoop
Lin Tsung-Nan
Shu Joseph
Rogers Scott
Seiko Epson Corporation
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