Sheet processing apparatus and sheet processing method

A sheet processing apparatus which determines a category of a sheet, comprising:

• an image obtaining unit configured to obtain an image from the sheet;
• a feature-amount extraction unit configured to extract a feature amount from the image obtained by the image obtaining unit;
• a standard pattern storage unit configured to prestore a standard pattern group for each different soil level, the standard pattern group comprising a standard pattern for each category;
• a selection unit configured to select one standard pattern group from a plurality of standard pattern groups prestored in the standard-pattern storage unit;
• a similarity calculation unit configured to calculate a similarity, based on at least one standard pattern comprised in the one standard pattern group selected by the selection unit, and on the feature amount extracted by the feature-amount extraction unit;
• a comparison unit configured to compare the similarity calculated by the similarity calculation unit with a preset threshold;
• a determination unit configured to determine a category of the sheet when the similarity is equal to or greater than the threshold, as a result of comparison by the comparison unit;
• and
• a control unit configured to control the selection unit to select one other standard pattern group which has not been selected among the plurality of standard pattern groups stored in the standard-pattern storage unit, when the similarity is smaller than the threshold, as a result of comparison by the comparison unit.