1. A method of identifying the direction of characters in an image block, comprising: performing optical character recognition processing on the image block by assuming various directions as assumed character directions, respectively, to obtain sub image blocks, recognized characters corresponding to the sub image blocks and correctness measures thereof in each of the assumed character directions; in sub image blocks in the assumed character directions with a 180° mutual relation therebetween, searching for a minimum matching pair of the sub image blocks, wherein the minimum matching pair is two sets of sub image blocks in the assumed character directions with a 180° mutual relation therebetween, which have corresponding positions, identical sizes and a minimum number of sub image blocks; when there is one sub image block in each of the two assumed character directions in a minimum matching pair and recognized characters corresponding to the two sub image blocks belonging to the minimum matching pair are the same rotation invariant character or belong to the same rotation invariant character pair, adjusting the correctness measures corresponding to the two sub image blocks to the same numerical value; calculating an accumulative correctness measure in each of the assumed character directions based on the adjusted sub image blocks; and identifying the direction of the characters in the image block according to the accumulative correctness measures.