Digital video is becoming widely available only in the compressed form. In applications such as video browsing or picture-in-picture or transcoding for a lower bit rate, there is a need to downscale the video before transmission. In such instances, the conventional approach of generating a downscaled video bitstream at the video server is to first decompress the video, perform the downscaling operation in the pixel domain and then recompress it for efficient delivery. This process is computationally expensive due to the motion-estimation process needed during the recompression phase. We propose an alternative compressed domain based approach that computes the motion vectors for the downscaled (N/2/spl times/N/2) video sequence directly from the original motion vectors residing in the N/spl times/N video sequence. Furthermore, the downscaling of the DCT information is also done in the DCT domain. The proposed approach can lead to significant computational savings compared to the conventional spatial domain approach.
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