A fast algorithm for DCT-domain inverse motion compensation

One of the important tasks of a multiuser video network server is to composite compressed video streams from several sources into a single compressed video stream. A great deal of the computational load can be saved if this composition is performed directly in the compressed domain rather than using the brute-force approach of converting back to the uncompressed domain, compositing pixel-by-pixel in the spatial domain, and re-compressing the composite stream. We propose a fast algorithm that converts motion compensated compressed video into a sequence of DCT-domain blocks corresponding to the spatial domain blocks of the current frame alone, without prediction based on other frames, i.e., removing the inter-frame element of the compression-decompression. This step enables video compositing in the DCT compressed domain as well as several compositing operations, e.g., scaling, overlapping, translation, filtering, etc. The proposed algorithm saves about 47% of the computations compared to the brute-force approach even without assuming sparseness of the DCT blocks. For typical sparse DCT blocks, where only the top-left 4×4 quadrant is nonzero, the reduction in computational complexity is about 68%