Local bandwidth constrained fast inverse motion compensation for DCT-domain video transcoding
Discrete cosine transform (DCT) based digital video coding standards, such as MPEG and H.26x, are becoming more widely adopted for multimedia applications. Since the standards differ in their format and syntax, video transcoding, where a compressed video bit-stream is converted from one format to another, is of interest for purposes such as channel bandwidth adaptation and video composition. DCT-domain video transcoding is generally more efficient than spatial-domain transcoding. However, since the data is organized block by block in the DCT domain, inverse motion compensation becomes the bottleneck for DCT-domain methods. We propose a novel local bandwidth constrained fast inverse motion compensation algorithm operating in the DCT domain. Relative to Chang's algorithm, we achieve computational improvement of 25%-55% without visual degradation. A by-product of our algorithm is a reduction of blocking artifacts in very low bit-rate compressed video sequences. The proposed algorithm can be combined with other reported fast methods for more computational savings. We also present a look-up-table (LUT) based implementation method by modeling the statistical distribution of the DCT coefficients in natural images and video sequences. By this method, we obtain a further 31%-48% improvement in computation. The memory requirement of the LUT is about 800 kB, which is reasonable. Moreover, the LUT can be shared by multiple DCT-domain video processing applications running on the same computer or video server.