Gradient-Based Feature Extraction for Early Termination and Fast Intra Prediction Mode Decision in HEVC

Abstract

High-efficiency video coding is the most recent video compression standard. HEVC is designed to decrease the bit rate of video transmission without affecting the video quality. The intra prediction of HEVC features about 35 directional modes. The planar mode and DC modes are included into it. The decision about the most appropriate intra mode within the coding unit of high-efficiency video encoder is a vital component in video coding. The intra mode decision has been a crucial, computationally complex processing step and has a share of 85% in the overall video coding complexity. The optimal mode is selected by rough mode decision (RMD) process from all 35 modes and final decision of partitioning is taken through the rate distortion optimization (RDO) process. The brute force RD cost calculation process consumes a large portion of HEVC encoding complexity. This paper presents analysis of the spread of 35 directional modes over the video frame and the correlation between the homogeneous or non-homogeneous characteristics of video content and the spread of directional modes over the video frame. The proposed method is based on the sum of average gradient evaluated for each of the 35 directional modes which help to reduce the number of candidate modes for rough mode decision and RD cost calculation. The performance of the proposed algorithm is evaluated on three distinct classes of video sequences. The offline classification accuracy of the proposed scheme is measured to be 90%. The exhaustive analysis of mode decision carried out in the proposed method will be subsequently useful for training machine learning algorithm for early decision about coding unit depth and fast prediction of the appropriate intra mode. The early depth decision and reduction in the number of candidate coding units to be passed through iterative RD cost computation will drastically reduce the computation complexity and increase the encoding time of high-efficiency video encoder.