Design and optimization of a differentially coded variable block size motion compensation system

While motion fields estimated by maximizing the temporal prediction quality tend to be noisy and demand a large number of bits to encode, finding the motion vectors (MVs) that allow efficient representations requires explicitly considering rate and distortion simultaneously. However, the application of rate-distortion optimization to current variable block size motion compensation (MC) systems is hampered by the fact that MV dependency introduced by the differential coding stage, e.g., the medium differential coding in the advanced prediction mode of the ITU-T standard H.263, makes the rate-distortion (R-D) optimization process extremely difficult. We propose two 1-D differential MV coding frameworks for variable block size MC systems. We show that for one of the proposed differential coding structures the optimal block sizes and the MVs can be jointly obtained by applying dynamic programming (DP) and tree-pruning techniques hierarchically without enumerating all combinations, while a near-optimal solution can be obtained for the other one by adopting a similar optimization procedure with little modification. By comparing the performance of our R-D encoding scheme with that of the H.263 test model TMN5, we find that our approaches achieve 30-60% bit-rate reductions in coding motion vectors, which results in greater than 1 dB gains within a MC hybrid coding environment for most head-and-shoulder videophone sequences under a low-bit-rate constraint