Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
1998-09-01
2001-09-18
Lee, Thomas D. (Department: 2724)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S451000, C382S237000, C382S252000
Reexamination Certificate
active
06292268
ABSTRACT:
This application is based on application No. 9-239464 filed in Japan, the contents of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image processor and an image processing method, and more particularly, to an image processor and an image processing method for processing multilevel image data by an error diffusion method.
2. Description of the Related Art
Conventionally, for example, a technology of processing multilevel image data by use of the error diffusion method has been proposed as shown in Japanese Laid-open Patent Application No. H4-2271.
For example, image data representing the image density of each pixel with 8 bits, i.e. image data of 256 gradations are input as input data, and by comparing the image data with a plurality of threshold values, the number of gradations is reduced to that of image data representing the image density of each pixel with 4 bits, i.e. that of image data of 16 gradations. In this processing, the error diffusion method is used.
The error diffusion method is intended for, when the image is viewed as a whole, reducing the difference between the input image data and the output image data having undergone a conversion, i.e. an error caused by the conversion. Specifically, when a conversion is performed for a target pixel, an error or the difference between the pre-conversion value of the target pixel and the post-conversion value thereof is dispersed to pixels situated on the periphery of the target pixel to thereby restrain the error in the image as a whole.
However, the error diffusion method has the following problem:
According to the image data to be converted, streaks sometimes appear in the post-conversion image data. Specifically, the gradation level 16 of the input image data of 256 gradations completely corresponds to the gradation level 1 of the post-conversion image data of 16 gradations, so that no error is caused. On the contrary, for example, for the gradation level 8 of the 256-gradation image data, since it is situated between the gradation level 0 and the gradation level 1 of the 16-gradation image data, an error is caused irrespective of which level it is converted to. The error is dispersed to peripheral pixels. Consequently, image data having the gradation level 8 of the 256-gradation image data are alternately converted to the gradation level 0 and to the gradation level 1 of the 16-gradation image data, so that the image data are represented with the gradation level 0 and the gradation level 1 by dithering. Thus, in an image which changes with a gentle density gradient between gradation levels accurately represented without any errors and gradation levels represented by dithering, when the number of gradations is reduced by error diffusion, the boundaries appear as streaks.
OBJECTS AND SUMMARY
The present invention is made in view of the above-described circumstances, and an object thereof is to provide an improved image processor and image processing method.
Another object of the present invention is to restrain image quality degradation when the number of gradations of input multilevel image data is reduced in an image processor and an image processing method using the error diffusion method.
Still another object of the present invention is to prevent streaks from appearing on the image when the number of gradations of input multilevel image data is reduced in the image processor and the image processing method using the error diffusion method.
To achieve the above-mentioned and other objects, an image processor, which converts image data represented with m gradations into image data represented with n gradations by use of the error diffusion method, comprises: a comparator for comparing image data of a target pixel with a predetermined threshold value to perform gradation conversion; a subtracter for calculating an error between the image data having been converted by the comparator and image data before the conversion; an error memory for diffusing the error calculated by the subtracter to pixels situated on the periphery of the target pixel; a selecting unit for selecting an image data processing mode; and weight assigning means for adding a weight assigning value corresponding to the processing mode selected by the selecting unit to the pixel data of the pixel to be converted.
The above-mentioned m and n are both given integers and satisfy 2<n<m.
In the present invention, in the above-described image processor, the weight assigning value added by the weight assigning means is decided in accordance with the value of the above-mentioned n.
REFERENCES:
patent: 5243445 (1993-09-01), Koike
patent: 5898796 (1999-04-01), Kumashiro
patent: 4-002271 (1992-01-01), None
patent: 4-057476 (1992-02-01), None
patent: 4-094266 (1992-03-01), None
patent: 4-124974 (1992-04-01), None
Hirota Soh
Ishiguro Kazuhiro
Tada Kaoru
Burns Doane , Swecker, Mathis LLP
Lee Thomas D.
Minolta Co. , Ltd.
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