Fast HEVC Intra Coding by Predicting the Rate-distortion Cost for a Low-complexity Encoder

Abstract

High Efficiency Video Coding (HEVC) intra coding achieves significant improvements in coding efficiency compared with H.264/AVC intra coding by adopting 35 spatial intra prediction modes and a quadtree-based block partitioning structure. However, the encoding complexity is extremely high from performing rate-distortion optimization (RDO) on each mode and depth level. This paper proposes fast intra mode decision algorithms for real-time HEVC encoding. For a fast intra prediction mode decision, we estimate the lower bound on the rate-distortion (RD) cost of a prediction unit with non–most-probable modes (non-MPMs). If the RD cost of the MPM is lower than the lower bound, the MPM is selected as the best mode. For a fast intra coding unit (CU) size decision, we predict the RD cost of four split sub-CUs from the RD of the current CU using the relationship between the RDs of the current CU and its split sub-CUs. In the HEVC Test Model software, the proposed algorithm saves 48.56% of the encoding time, on average, while the RD loss is only 0.59%. In x265, the proposed algorithm improves the encoding speed to 71.95 fps, on average, from 49.46 fps. It achieves 1.8 times faster speed with a bit increase of only 0.44%, compared to x265.