7. The image processing apparatus according to claim 6 , wherein the extraction section performs, for the region determined as being a text component by the area detection section, a binarization process on the document image to extract a feature point, while the extraction section extracts, for the region determined as being a pictorial component by the area detection section, a feature point for each of the layer images, and wherein the calculation section calculates features of the document image based on the feature point extracted for each component.
6. An image processing apparatus for extracting features of a document image composed of a plurality of pixels, and for determining similarity between the document image and a reference image based on the extracted features, the apparatus comprising: a storage section for storing features associated with each of a plurality of the reference images in advance; a generation section for generating, based on a pixel value of each pixel of the document image, a plurality of layer images having different pixel value ranges; an extraction section for performing a binarization process on each of the layer images, generated by the generation section, to extract a feature point; a calculation section for calculating features of the document image based on the feature point extracted by the extraction section for each of the layer images; a features determination section for determining whether the features, calculated by the calculation section, corresponds to the features associated with the reference image; a voting section for voting for, when the features are determined to be corresponding to each other by the features determination section, the reference image associated with the features for each corresponding features; a similarity determination section for determining, based on the number of votes obtained by the voting of the voting section, whether the document image is similar to the reference image; and an area detection section for determining, based on the pixel value of each pixel of the document image, whether at least a region of the document image is a text component or a pictorial component, wherein the generation section generates a plurality of layer images for the region determined as being a pictorial component by the area detection section.