1. An image processing method 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 method comprising steps of: storing features associated with each of a plurality of the reference images in advance; generating, based on a pixel value of each pixel of the document image, a plurality of layer images having different pixel value ranges; performing a binarization process on each of the generated layer images to extract a feature point; calculating features of the document image based on the feature point extracted for each of the layer images; determining whether the calculated features corresponds to the features associated with the reference image; voting for, when the features are determined to be corresponding to each other, the reference image associated with the features for each corresponding features; determining, based on the number of votes obtained by the voting, whether the document image is similar to the reference image; 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; and generating a plurality of layer images for the region determined as being a pictorial component, wherein one or more of the steps are performed using one or more processors.