A fast scheme for altering resolution in the compressed domain
Given a video frame or image in terms of its 8/spl times/8 block-DCT coefficients we wish to obtain a downsized (lower resolution) or upsized (higher resolution) version of this frame also in terms of 8/spl times/8 block -DCT coefficients. We propose an algorithm for achieving this directly in the compressed domain which is computationally much faster, produces visually sharper images and gives significant improvements in PSNR (typically 4 dB better compared to other compressed domain methods based on bilinear interpolation). The downsampling and upsampling schemes combined together preserve all the low-frequency DCT coefficients of the original signal. This implies tremendous savings for coding the difference between the original (unsampled image) and its prediction (the upsampled image). This is desirable for many applications based on scalable encoding of video.