Image processing apparatus, image forming apparatus and image reading apparatus including the same, and image processing method

An image processing apparatus comprising:

• a first feature point calculating section for specifying a consecutive component in which pixels are connected based on document image data, and extracting a feature point in the specified consecutive component;
• a reverse process section for obtaining image data that a pixel value of the document image data is reversed;
• a second feature point calculating section for specifying a consecutive component in which pixels are connected based on the image data that the document image data is reversed, and extracting a feature point in the specified consecutive component;
• a feature point adding section for adding the feature point calculated by the first feature point calculating section and the feature point calculated by the second feature point calculating section;
• a features calculating section for obtaining features of the document image data based on the feature point added in the feature point adding section;
• a storage section for storing in advance the calculated features and the document image in association with each other;
• and
• a similarity determining section for determining, based on the features obtained by the features calculating section and features of a document image which has been stored in advance, whether or not the document image and the stored document image are similar to each other.