Fast PU Intra Mode Decision in Intra HEVC Coding

As an upgrade for H.264/AVC, high efficiency video coding (HEVC) achieves 50% bitrate reduction under the equivalent visual quality. However, high computational complexity increases dramatically for adopting up to 35 intra prediction modes. To deal with this issue, we explore spatial-temporal correlation between PUs to narrow rough mode decision (RMD) candidate list and rate distortion optimization (RDO) candidate list respectively. A fast PU intra mode decision scheme is proposed for HEVC fast intra encoding. For RMD, the proposed scheme early determines the impossible range of the PU optimal mode in line with spatial statistical analysis theory. It would narrow RMD candidate list and speed up RMD process. First, 4 subsets (Sb1, Sb2, Sb3 and Sb4) are defined in mode set with 33 directional prediction modes as shown in Figure 1. And then, on the basis of spatial statistical analysis theory, the impossible range of the parent PU optimal mode would be judged by its known optimal mode. Last, low probability subsets of current PU are removed, and candidate subsets are determined for current PU. Where ModeParBest and ModeCurBest are the parent PU optimal mode and the current PU optimal mode respectively. For RDO, the proposed scheme tries to combine temporal correlation with intra coding, which adds co-located optimal mode of the previous frame into the RDO list of the current PU. At the same time, the modes in RDO list are decreased focusing on most time-consuming PUs (4 4 PUs, 8 8 PUs) from 8 candidate modes to 3 candidate modes. Experimental results demonstrate that the proposed scheme yields average 29% encoding time reduction with average 1.19% BDBR gain and 0.06dB BDPSNR loss compared with HM16.9. Further, the proposed method implements fast CU encoding without additional computation during the encoding process.