Fast motion vector refinement for MPEG-1 to MPEG-4 transcoding with spatial down-sampling in DCT domain

In this paper, we introduce an architecture of a video transcoder for converting MPEG-1 video to MPEG-4 simple profile with lower spatial resolution in the DCT-domain. For the DCT-domain transcoding to lower spatial resolution pictures, DCT-domain down-sampling is applied and a base motion vector (BMV) for the down-sampled MPEG-4 macroblock is to be calculated from the input motion vectors operating on the higher spatial resolution image. Quality can be significantly improved by refining the BMV. Starting with the BMV, the motion vector refinement (MVR) scheme searches for a refinement motion vector. We propose a fast MVR scheme for video down-sampling in the DCT-domain, based on minimizing the number of required check points and a computationally efficient method for extracting a motion compensated DCT (MC-DCT) block. We also show an efficient scheme for selecting a coding mode for the macroblock in the lower resolution video from those of the corresponding higher resolution video.