A transform domain approach to spatial domain image scaling

Straightforward techniques for spatial domain scaling of compressed video via decompression and re-compression are computationally expensive. We describe an alternative approach wherein the compressed stream is processed in the compressed, DCT domain without explicit decompression and spatial domain scaling, so that the output compressed stream corresponds to a down-sampled image and it conforms the standard syntax of 8×8 blocks. We propose computation schemes for scaling factors of 2, 3 and 4 that are applicable to any DCT-based compression method. Worst-case estimates of computation savings very between 37% and 50% depending on the scaling factor. For typically sparse DCT blocks, the reduction in computations can be as much as 80%. A byproduct of the proposed approach is improvement in arithmetic precision