Dual-path block size decision for fast motion search in H.264/AVC

We propose a fast motion search method for H.264/AVC with dual-path block size decision. H.264/AVC employs variable block sizes for motion compensation to reduce coding bits of inter-frame prediction error, which requires considerable amount of computation time when motion estimation is performed for every block size. Our algorithm contains two strategies to reduce computation time for motion search; block size mode reduction and search range reduction. According to these strategies, our algorithm consists of two stages. At the first stage, RDCost-based block size mode reduction is conducted. Rate-distortion function (RDCost) for skip mode is calculated at first, which determines the smallest block size for motion search. The second stage is a fast variable block size motion estimation which contains two paths, 16x16-first and 8x8-first. The 16x16-first path is invoked when the minimum block size determined at the first stage is larger than 8x8. In the 8x8-first path, search range for blocks larger than 8x8 is reduced according to distance between motion vectors for 8x8 blocks. From our experiment using JM 8.5, it is confirmed that our algorithm can reduce about 89.3% of computation time as compared to JM, with only negligible PSNR degradation.