General splitting and merging of 2-D DCT in the DCT domain

An efficient method for splitting an N×N 2-D DCT block into four (N/2)×(N/2) or two N×(N/2) (or (N/2)×N) 2-D DCT blocks is presented and vice versa. The computational complexity of the proposed methods is lower than the direct approach and the same as the most efficient converting approach existed in the literature. Besides, the proposed DCT splitter/merger is suitable for implementation by specific multimedia instruction set available nowadays. When N=8, our method can be applied to realize the transcoding between the latest video coding standards AVC/H.264 and the older ones, such as MPEG-1, MPEG-2 and MPEG-4 part 2.