1. An image processing method comprising: reading a first document to obtain a first image; extracting a plurality of first feature vectors from the first image; storing the first feature vectors and a first identifier assigned to classify the first document; successively processing with a processor a plurality of subsequently-read documents, the processing of each subsequently-read document comprising: reading the subsequent document to obtain a subsequent image; extracting a plurality of subsequent feature vectors from the subsequent image; determining a number of matched feature vectors by determining how many of the subsequent feature vectors match one or more of the first feature vectors; based on the determined number of matched feature vectors, assigning the first identifier or a new identifier to the subsequent document; when the new identifier is assigned, storing the subsequent feature vectors and the new identifier; and classifying the subsequent document based on its assigned identifier.