PURPOSE:To accurately discriminate a continuous image tone from a linear image at every block unit by dividing a digital image into picture element blocks calculating their mean value and a standard deviation value, setting up threshold corresponding to the standard deviation, and comparing the threshold with the means value. CONSTITUTION:When the mean value (m) of the picture element blocks is in the vicinity of the mean (Max+Min)/2 of the maximum value Max and the minimum value Min of image data, the threshold of the reference deviation is set up to large value Th1. As the mean value (m) reaches the Max or Min, the threshold is set up to a small TH2-TH5. Since the threshold for discriminat ing the picture tone is changed such as TH1-TH5 in accordance with the mean value (m) of the picture element density of the picture element blocks, picture element blocks 302, 304 are decided as picture element block including a linear image, i.e. an edge and a picture element block corresponding to a position 406 is accurately decided as a plane block.
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