Counter based adaptation for CAVLC in HEVC

Compared to the CAVLC in H.264/AVC, CAVLC in HEVC has been improved much. One mechanism that contributes to the improvement is adaptive code word swapping based on previous coded symbol in the early design of HEVC. However, such a instantly swapping strategy often makes a wrong estimation of the real statistics and may lead to inefficient coding that may even worse than without adaptation. In this paper, we analyze potential loss caused by instantly swapping and propose a counter-based adaptation scheme to further improve the entropy coding efficiency. We discuss the bit-depth of those counters that achieves a trade-off between the adaptation capability and a good estimation of the statistics. The proposed scheme introduces a sum counter for each code table and a counter for each code. The sum counter determines when to shift all the counters to prevent overflow and make sure symbols far away influence less. The experimental results show that the counter based algorithm brings about 0.7% bits saving compared with the instantly swapping adaptation algorithm. Compared with the CAVLC without adaptation, the performance gain of the counter adaptation algorithm is about 4.0% bits saving. A variant of the proposed counter-based adaptation has been adopted by HEVC design.