2. The image processing method according to claim 1 , further comprising: calculating an image similarity based on the determined number of matched feature vectors, wherein, when the number of stored identifiers does not reach a predetermined extent the assignment of the stored first identifier or the new identifier to the subsequently-read document is further based on the calculated image similarity.
3. The image processing method according to claim 2 , wherein, when the number of stored identifiers reaches the predetermined extent, the stored first identifier is assigned to the subsequently-read document or the subsequently-read document is classified as non-similar based on the calculated image similarity.