Mode activity based adaptive fast intra mode decision for HEVC/H.265

In the intra coding of HEVC/H.265, the computational complexity of intra mode decision is very high because up to 35 intra prediction modes are supported. Though a fast intra mode decision based on rough mode decision (RMD) is adopted in reference software HM12.0, only a fixed number of intra modes are selected for rate distortion optimization (RDO). In this paper, an adaptive fast intra mode decision for HEVC/H.265 is proposed. We define mode activity by analyzing the rough costs of all modes, and the computational complexity of mode activity is very low. Based on the analysis of relation between mode activity and rate distortion cost gain, we propose to adaptively reduce the number of candidate intra modes for RDO by utilizing the mode activity, so accelerated intra coding is realized. Compared with the RMD in reference code HM12.0 of HEVC/H.265, the proposed algorithm can reduce encoding time with only negligible loss of compression efficiency.