Macroblock type selection for compressed domain down-sampling of MPEG video

Digital video databases in compressed form are becoming widely available. In applications such as video browsing, and picture in picture, for a lower bitrate there is a need to down-sample the video before transmission. The conventional approach to downscale a compressed video sequence is to decompress it at the video server, perform the down-sampling in the pixel domain and recompress it again for efficient delivery. This process is computationally expensive due to the motion estimation process needed during the recompression phase. In the alternative compressed domain based approach the downscaled motion vectors are computed directly from the original ones. We propose a method to improve the process of downscaling in the compressed domain by better selecting the motion prediction type of P and B frame macroblocks. Simulations suggest that the proposed scheme improves the compressed domain down-sampling quality by up to 2.2 dB without any substantial increase in computation.