Gradient based fast mode decision algorithm for intra prediction in HEVC

High Efficiency Video Coding (HEVC) is an ongoing video compression standard and a successor to H.264/AVC. It aims to provide significantly improved compression performance as compared to all existing video coding standards. In the intra prediction of HEVC, this is being achieved by providing up to 35 intra modes with larger coding unit. The optimal mode is selected by a rough mode decision (RMD) process from all of the available modes first, and then through the rate-distortion optimization (RDO) process for the final decision. Because the fact that every coding unit with different sizes is traversed in both procedures makes it very time-consuming, a gradient based fast mode decision algorithm is proposed in this paper to reduce the computational complexity of HEVC. Prior to intra prediction, gradient directions are calculated and a gradient-mode histogram is generated for each coding unit. Based on the distribution of the histogram, only a small part of the candidate modes are chosen for the RMD and the RDO processes. As compared to the default encoding scheme in HEVC test model HM 4.0, experimental results show that the fast intra mode decision scheme provides almost 20% time savings in all intra low complexity cases on average with negligible loss of coding efficiency.