Dainippon Screen Mfg. Co., Ltd.

Masami Aragaki

Makoto Hirosawa

An image is conceptually divided into pixel blocks each having a plurality of pixels. RGB image data representing the image are converted into luminance data and color difference data in the NTSC form. Standard deviation values of these data are calculated in each pixel block and are compared with threshold values, so that the pixel blocks are classified into two types. The image data in the pixel blocks of respective types are compressed at different compression rates to obtain image data compressed.

Method of an apparatus for compressing images data

Image data (f.sub.ij) depicting an image in a pixel block are converted by 2-D Discrete Cosine Transformation into transformation coefficients (F.sub.mn or F.sub.mn *). Only a part of the coefficients are employed to form compressed image data (D.sub.t). The selection of the part of the coefficients is executed using code tables (CT). Several groups of code tables are prepared, and one of those groups is selected in the data compression operation in response to a standard deviation (.sigma.) of the image data in the pixel block. When the compressed image data is restored, identification data specifying the selected group in the previous data compression is stored in the restored image data. Then, in the next data compression of the restored image data, the selected group is again selected as a function of the identification data.

Method of and apparatus for compressing image data

A gradation converting circuit for converting digital input data to output data, in accordance with a predetermined input/output relationship. The input and output data is provided as data words having a predetermined number of bits 'm'. Since the resolution provided in the output by the 'm' bits is insufficient for accurately expressing the aforementioned input/output relationship and to satisfy a demand to keep the number of bits at 'm', an improved output resolution is obtained by varying the value outputted for any given input data over time such that the average of the outputs is closer to a desired output demanded by the input/output relationship. This is accomplished either by storing, for each input data, a plurality of outputs whose average is equal to the desired predetermined, input/output relationship, or, by outputting for each digital input data a so-called 'decimal' value which is indicative of how often a fixed predetermined output value should be increased by one such that the average of the output should approximate the desired output value.

Gradation converting circuit employing lookup table

An image file consists of a file header, reduced-and-compressed image data, and compressed original image data. The reduced-and-compressed image data is obtained by compressing reduced image data expressing a reduced image of an original image in serial order of bit planes. The reduced image is then reproduced and displayed on an image display for an image search, in place of the original image itself, whereby efficiency in an image search is improved.

Method of storing and transmitting image data as an image file suitable

Interblock distortion often appears on a boundary of pixel blocks in a reproduced image produced from compressed image data. The interblock distortion is eliminated by correcting a density distribution in the reproduced image. First, an average density (A.sub.ij) of a selected pixel block (B.sub.ij) is computed. Second, a standard deviation (.sigma..sub.ij) of a density distribution in the selected deviation is computed. If the standard deviation is equal to zero, correction value distributions (.DELTA.f.sub.m, .DELTA.f.sub.n) along a main scanning direction and a subscanning direction are obtained. Finally, corrected density distribution (f.sub.mn) is obtained as a sum of the average density and the correction value distributions.

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Kyoto, Japan

Masami Aragaki