Shading correction method and apparatus

A shading correction method in which a standard surface having an imaginary matrix of pixels arrayed in a main-scanning direction to form scanning lines of pixels and arrayed in a sub-scanning direction to form scanning rows of pixels is preliminarily scanned prior to image reading of an objective original image to eliminate shading distortion appearing in an image scanning system, comprising:

• (a) partitioning said standard surface into a plurality of scanning blocks composed of equal scanning lines;
• (b) scanning the pixels on said standard surface in the main-scanning direction to obtain image signals by line;
• (c) repeating the scanning of the pixels in each scanning lines in the main-scanning direction until all the scanning lines in each scanning block is completely scanned;
• (d) effecting a first arithmetic processing to determine sets of block median values of the scanning blocks from said image signals of pixels in the scanning lines of the respective scanning blocks;
• (e) effecting a second arithmetic processing to determine one set of global median values from said block median values of said scanning blocks;
• and
• (f) subjecting objective image data obtained by reading said original image to shading correction based on said global median values.

A shading correction device comprising:

• an image scanning system for scanning a standard surface having a plurality of scanning lines and partitioned into a plurality of scanning blocks to read out image data from the standard surface by line;
• image memory means for storing said image data read out from said image scanning system by line, while moving said image scanning system relative to the standard surface;
• a first processing circuit for determining sets of block median values from said image data from said image memory means;
• block memory means for storing said block median values;
• a second processing circuit for determining a set of global median values from said block median values;
• and
• a shading correction circuit for making shading correction based on said global median values.