A fast algorithm for DCT-domain inverse motion compensation based on shared information in a macroblock

The ability to construct intracoded frame from motion-compensated interceded frames directly in the compressed domain is important for efficient video manipulation and composition. In the contest of motion-compensated discrete cosine transform (DCT)-based coding of video as in MPEG video, this problem of DCT-domain inverse motion compensation has been studied and, subsequently, improved faster algorithms were proposed. These schemes, however treat each 8 x 8 block as a fundamental unit, and do not take into account the fact that in MPEG, a macroblock consists of several such blocks. In this paper, we show how shared information within a macroblock, such as a motion vector and common blocks, can be exploited to yield substantial speedup in computation. Compared to previous brute-force approaches, our algorithms yield about 44% improvement. Our technique is independent of the underlying computational or processor model, and thus can be implemented on top of any optimized solution. We demonstrate an improvement by about 19%, and 13.5% in the worst case, on top of the optimized solutions presented in existing literature.