Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
2000-07-27
2004-06-08
Lee, Thomas D. (Department: 2624)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S003060, C382S237000, C382S260000
Reexamination Certificate
active
06747758
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of The Invention
The present invention generally relates to image compression techniques, and more particularly to a digital halftoning method in which a gray-scale image is compressed into a halftone image with a limited number of gray levels. The digital halftoning is an important technique that is applied to processed images or images being processed when visualizing such images by using an output device, such as a display or a printer.
Further, the present invention relates to a digital halftoning apparatus which carries out digital halftoning processing. The present invention relates to a computer readable medium storing program code instructions which cause a processor to execute digital halftoning processing.
2. Description of The Related Art
Digital halftoning used in imaging systems is known as a technique for generating the pixel patterns with a limited number of gray levels or colors. For example, two gray levels are used to generate a bilevel image, eight colors are used to generate a RGB color image, and
16
colors are used to generate a CMYK color image. As the approach for digital halftoning of this type, various conventional methods, such as the thresholding method, the dithering method or the error diffusion method, have been developed.
Recently, the ability of data processing and computation in microprocessors and digital signal processors has remarkably been increased, and attention is being given to model-based digital halftoning.
As disclosed in Japanese Laid-Open Patent Application No.5-91331, a few digital halftoning techniques are known. These techniques may be categorized into a first scheme for digital halftoning based on a human visual model, and a second scheme for digital halftoning based on a printer characteristic model. The human visual model used by the first scheme is to represent a digital halftone image with improvement of the quality of the image from the viewpoint of the human vision. The printer characteristic model used by the second scheme is to represent a digital halftone image with improvement of the quality of the image from the viewpoint of the printer output characteristics.
Generally, the human visual model depends upon the resolution of the digital image and upon the eye-to-paper distance between the human eye and the paper. The human visual model may be implemented by using a digital finite impulse response (FIR) filter.
Further, the printer characteristic model is defined with a dot radius of the printer, and it is provided by using a lookup table in which pixel values of the respective pixels corresponding to the digital image are locally arranged so as to be in conformity with the printer output characteristics. In particular, the least square model-based (LSMB) method, which is known as one of the model-based digital halftoning techniques, is characterized by the computation of an optimum halftone image so as to minimize the human visual errors between the original gray-scale image and the printout halftone image.
The advantage of the model-based digital halftoning is that the model parameters are adjusted based on the image and output device characteristics, and the adaptive control of an output image is provided. It is known that the quality of halftone images created by the model-based digital halftoning can be higher than the quality of images created by the screening or the error diffusion method. The model-based digital halftoning will improve the smoothness of the image texture and remarkably reduce the undesired effects, such as the so-called “worm”.
On the other hand, some approaches for improvement of the quality of images created by the screening or the error diffusion method have been proposed. In an improved error diffusion method, the smoothness of the image texture is increased by setting, at random, the filter weight coefficients, and the edge sharpening is attained by the modulation of the threshold values.
In the above-mentioned LSMB method, the computation of the optimum halftone image for minimizing the human visual errors requires a fair amount of computations on the microprocessor or the digital signal processor.
In the digital halftoning, the information contained in the original image will be partially lost because the number of gray levels or colors is reduced from that of the original image. However, it is desirable to retain, from the viewpoint of the human vision, the appearance of the original gray-scale image in the resulting halftone image. For this purpose, an improved image coding method that does not cause the loss of the human visual information in the resulting halftone image is demanded. Preferably, the improved image coding method is capable of not only retaining the image features (for example, the image smoothness) but also providing improvement of the image quality from the viewpoint of the human vision.
For example, when the image texture is complicated or there is an abrupt change in the pixel value in the neighborhood of a particular pixel in the original image, the edge sharpening should be provided in the halftone image. This measure is often taken for character or graphics regions of the original image. When the original image contains a smooth region where the pixel value scarcely changes, the moire or other synthetic patterns should be avoided from appearing in the halftone image.
However, in the above-mentioned model-based digital halftoning methods, it is difficult to provide the adaptive control of the quality of the output image even if only one of either the smoothness or the sharpness can be improved. Specifically, it is difficult for the above LSMB method to improve both the smoothness and the sharpness. The use of the FIR filter provides improvement of the smoothness or the sharpness of the entire output image. If the FIR filter is configured to improve the smoothness, the sharpness of the output image will be lost. In a case of a mixed image in which the characters and the graphics coexist, the characters of the output image will be difficult to read. On the other hand, if the FIR filter is configured to improve the sharpness, the moire or other synthetic patterns will appear in the smooth regions of the output image.
Similar problems arise with the other conventional methods. For example, in the improved error diffusion method, the smoothness of the image is increased by setting, at random, the filter weight coefficients, and the edge sharpening is attained by the modulation of the threshold values. However, the method of this type does not use a quantitative parameter model that directly affects the quality of the output image, and it is difficult to control both the smoothness and the sharpness of the output image by adjusting a small number of quantitative parameters.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an improved digital halftoning method and apparatus in which the above-mentioned problems are eliminated.
Another object of the present invention is to provide a model-based digital halftoning method that provides the adaptive control of the human visual quality of the output image by using a visual-quality control filter based on the local features of the original image, including the smoothness and the sharpness, without excessively increasing the amount of computations needed to achieve the digital halftoning.
Another object of the present invention is to provide a model-based digital halftoning apparatus that provides the adaptive control of the human visual quality of the output image by using a visual-quality control filter based on the local features of the original image, including the smoothness and the sharpness, without excessively increasing the amount of computations needed to achieve the digital halftoning.
Another object of the present invention is to provide a computer readable medium storing program code instructions for causing the computer to execute a model-based digital halftoning processing that provides the adaptive control of the human visual quality of the output
Lee Thomas D.
Ricoh & Company, Ltd.
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