Optimized Algorithm For HEVC Intra-prediction Based On Fast Three-step Selection

HEVC(High Efficiency Video Coding) is a new generation of video coding standard after H.264/AVC. It is established by joint collaboration video coding group JCT-VC composed of VCEG organization and MPEG organization. HEVC retains the traditional hybrid video coding framework based on block-level motion estimation and compensation, meanwhile some new techniques have also been adopted in both Intra-prediction, Inter-prediction, orthogonal transform and quantization, and the entropy coding. These all make HEVC over perform H.264/AVC in all cases, while its coding efficiency is significantly improved.HEVC coding performance is so excellent at the expense of the greater coding complexity. The key technologies of high calculation complexity such as Quad-tree recursive block division structure, rate-distortion optimization decisions, 35 kinds of intra prediction direction have become the bottleneck of its practical application. In order to reduce the coding complexity as well as improve the coding efficiency of HEVC, its Intra-coding processing is deeply studied in this paper. The intra prediction take a very important role in the intra-frame coding. Hence, the Intra-prediction technology is discussed from two aspects to analyze its complexity in this paper, one is the quad-tree traversal structure, and the other is the intra-prediction mode selection, which complete a full analysis of the complexity of intra prediction. It shows us that within a LCU, the existence of intra coding may have 1024 kinds of ways to divide, and intra prediction has as many as 35 prediction direction, and for each PU, rate-distortion must be done to each of the 35 directions to decide the best mode. As a result, an optimized algorithm for HEVC intra-prediction based on fast three-step selection is proposed in this paper. Aiming at the huge amount of calculation as a result of the intra prediction directions, the relationship between RMD processing and RDO processing is made clear, which two are the basic steps required in the process of intra prediction. And then an analysis of these two processing is made in detail, to find the optimization point could be implemented for both of them as follows:(1) Optimized algorithm based on three-step selection for the RMD(Rough Mode Decision) processing. In this proposed algorithm, the RMD processing is divided into determine the direction first, and then search it in a small range. This makes it able to select the best prediction mode traversing as little as 15 modes instead of 35 modes, thereby reducing a half of the required number of SATD cost calculation mode.(2) Optimized algorithm based on video features for the RDO processing. Taking the advantage of the correlation of MPMs and the first candidate mode in the modelist got in Step1, this algorithm puts forward the last step to select the best prediction mode, that is, reduce the number of modes involved in the rate-distortion cost calculation adaptively according to the different size of the PU.The HEVC Standard Test sequences have been carried on the test to verify the algorithm for HEVC intra-prediction based on fast three-step selection proposed in this paper. Compared to HM10.0, the experiment result shows that can proposed algorithm could save about 40% coding time, while the bit-rate only increases about 0.2% and PSNR decreases about 0.1d B on average at different parameter configurations. In summary, the optimized algorithm proposed in this paper is able to improve the coding efficiency effectively, which is positively significant for the application of HEVC.