Image analysis – Image transformation or preprocessing – Changing the image coordinates
Reexamination Certificate
1998-12-23
2002-08-20
Boudreau, Leo (Department: 2621)
Image analysis
Image transformation or preprocessing
Changing the image coordinates
C382S299000, C382S297000, C382S295000, C382S209000, C358S400000, C358S001900, C358S001140
Reexamination Certificate
active
06438273
ABSTRACT:
This invention relates generally to a method and apparatus for improving the appearance of printed documents, and more particularly to the efficient use of template rotation within a template matching process for the enhancement of digital images.
BACKGROUND OF THE INVENTION
Information systems for handling numerous document and data formats are moving towards becoming open systems where different devices are tied to one another to provide solutions to customers' needs. A key factor in such open systems is enabling an electronic document to be printed so that the customer does not perceive any difference between versions printed on different output devices. In order to achieve complete device-independence, efficient methods of accurately altering image resolution and enhancement are required to take advantage of the technology. Hence, raster conversion technology, where a bitmap created for a first output device is altered so as to be printable on a second output device, has become an important aspect of the open system technology.
Another important aspect is the preservation of the investment in technologies, while improving printers and print appearances. A common example for this case is the storage of fonts, forms, etc. in the form of bitmaps at a specified resolution, say 300×300 spots per inch (spi). With improving print engines, output resolutions of 600×600 spi, 300×1200 spi and the like are possible. However, existing applications using, for example, 300×300 spi fonts have to be handled by these new output devices at a quality level that exceeds that of the 300×300 spi output devices.
The present invention is a method and apparatus for efficiently processing digital image data to produce enhanced output images. Generally speaking, the invention employs resolution enhancement filters in the form of templates in look-up tables or logic operations. The invention may be used in design and implementation processes of template matching applications available in the art. Such applications include, but are not limited to resolution enhancement, appearance tuning, appearance matching, conversion to gray-scale (e.g., halftone de-screening), image restoration, pattern detection and segmentation.
Enhancement filters operate on image bitmaps to improve document appearance by converting from an original resolution to an output resolution of the printing or display device, at an integer multiple in each direction of the input resolution. The resulting image signals may then be used to drive devices at the output resolution without negatively impacting spatially sensitive features within the input image. In one embodiment, the present invention associates individual templates with multiple-bit per pixel, statistically generated look-up tables to reduce the number of look up tables that is required to improve the appearance of reproduced document images. The invention includes a technique for rotating templates about an axis, and using the original and flipped templates with a single multiple bit per pixel look up table to generate output signals that accurately represent the original image. The generated pixel image signals may then be used to drive output devices with higher fast-scan resolution or with output devices capable of printing multiple-bit per pixel output.
The present invention may be utilized to control a scanning beam where the beam varies in intensity and duration according to the pulses used to control it. For example, a laser beam may be used in a printer for selectively exposing areas on a photoreceptor. The latent electrostatic image formed on the photoreceptor by the beam exposure attracts developing toner, in proportion to the latent image charge level, to develop the image. As another example, a cathode ray tube uses an electron beam to scan a phosphorous screen. The electron beam may be varied in intensity and duration to accurately display information on the phosphor screen. In both examples, a pulse forming circuit responsive to the multiple-bit per pixel image signal may be used to generate video pulses to control the intensity and operation time of the respective beams.
In the preferred embodiment, the present invention will be used to efficiently implement template matching filters. These template matching filters will be designed by any of a number of methods, including but not limited to calculations that are based on geometry, statistics and expert knowledge of specific pattern shapes. One important aspect of the present invention allows multiple template filters that are intended for different portions of a pixel enhancement to be reduced to a single (or reduced number of) filter which may then be applied using efficient symmetry determination as disclosed in the present invention. For instance, in converting an image from a resolution of 300 spi to 600 spi, a single template matching filter could be used at various symmetries and each symmetry would generate one pixel of the four high resolution image pixels that is substituted for each input pixel.
In another aspect of the present invention, filtering may be applied to achieve a pixel that is not necessarily spatially divided into higher resolution sample. For example, such a filter may be used for activating or deactivating binary pixels in operations for enhancement operations such as thickening, thinning, or restoration. Yet another aspect of the present invention is use in converting binary pixels to gray-scale, as in the case of halftone de-screening, as well as in some forms of restoration and anti-aliasing. The present invention would efficiently implement multiple symmetries of a pattern as a single pattern within the template look-up table.
Still another aspect of the present invention may be used to enhance the design of template matching filters. The invention allows for more accurate statistical design by pooling data acquired that has been acquired for multiple symmetries of a pattern into the statistics of a single (or reduced number) of representative patterns. Those skilled in the art will recognize that the invention may be adapted for use with other filter design methods. For example, one member of a symmetry may be designed and the other templates may be derived from that designed pattern.
Previously, various methods and apparatus have been used to improve the quality of a reproduced image by using a template matching scheme in conjunction with a look up table. The following disclosures may be relevant to aspects of the invention:
U.S. Pat. No. 4,437,122 to Walsh et al. teaches an improved method of converting low resolution images into images of higher resolution for printing so as to simultaneously increase density and smooth character edges. In a CRT display or hardcopy output apparatus, the invention is accomplished by converting an original pixel into a higher resolution 3×3 enhanced representation. The status of each of the nine elements in the enhanced representation is determined as a result of an examination of the neighboring pixels of the original pixel.
U.S. Pat. No. 4,847,641 and U.S. Pat. No. 5,005,139 to Tung disclose print enhancement circuitry for a laser beam printer. The bit map of a region of the image to be output is compared to a number of patterns or templates. When a match is detected, a section of the bitmap that was matched is replaced with a unique bitmap section designed to compensate for errors. The replacement bitmap section may include predetermined shifting of some dot positions to compensate for the error in the original bitmap section.
U.S. Pat. No. 4,933,689 to Yoknis describes a method for enhancing a displayed image in a laser exposed dot matrix format to produce softened edge contours. Using three pulses, a central pulse plus leading and trailing enhancement pulses that are separated therefrom. The purpose of the leading and trailing pulses is to create a blurred or grayed region at the leading and trailing edges of each associated character.
U.S. Pat. No. 5,237,646 to Bunce discloses a method for enha
Kingsley Jeffrey D.
Loce Robert P.
Boudreau Leo
Eipert William
Kassa Yosef
Xerox Corporation
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