Fast algorithms for DCT-domain image downsampling and for inverse motion compensation

Straightforward techniques for spatial domain processing of compressed video via decompression and recompression are computationally expensive. We describe an alternative approach wherein the compressed stream is processed in the compressed, discrete cosine transform (DCT) domain without explicit decompression and spatial domain processing, so that the output compressed stream, corresponding to the output image, conforms to the standard syntax of 8×8 blocks. We propose computation schemes for downsampling and for inverse motion compensation that are applicable to any DCT-based compression method. Worst case estimates of computation savings vary between 37% and 50% depending on the task. For typically sparse DCT blocks, the reduction in computations is more dramatic. A by-product of the proposed approach is improvement in arithmetic precision