Implementation And Optimization Of The Hierarchical B Structure In H.265/HEVC

Rapid development of network technologies has stimulated an increment of high definition (HD) and ultra high definition (UHD) applications, which leads to great challenge of the video transmission on the internet. In order to solve this problem, a new video coding standard H.265/HEVC has been standardized by JCT-VC (Joint Collaborative Team on Video Coding) working group, which is also the most efficient video coding standard.Among the three coding structures utilized by the H.265/HEVC encoder, the random access structure achieves much higher compression ratio compared with the all-intra and low-delay structures, as it uses the hierarchical B structure which has been shown to achieve high coding efficiency. Besides, its coded video stream can also adapt to different network environments and terminal needs. Therefore, it is of great significance to implement hierarchical B structures into the transmission of HD/UHD video stream on networks. However, the corresponding coding complexity of the hierarchical B structure is very high, which leads to much more latency and power consumption during the video transmission process. Thus, research on reducing the complexity of hierarchical B structure is crucial for the practical use of the H.265/HEVC encoder.In this paper, the algorithm of hierarchical B frame coding in H.265/HEVC is firstly analyzed, based on which all the function modules in the encoder are implemented, including reference picture management, inter prediction, entropy coding, deblocking filter, etc. These implementations make it possible for H.265/HEVC encoders to support B frames, especially for hierarchical B coding structures, rather than only support only I and P frames as previous H.265/HEVC encoders did. Experiment results indicate that compared with low-delay structure, the hierarchical B structure can increase the PSNR by 0.424 dB with the same coding bitrate.Compared with the low-delay structure, the mode selection process of hierarchical B structure makes a great contribution to the increase of its complexity. In this paper, the mode selection process of hierarchical B structure is analyzed and implemented firstly. Then, based on the temporal correlation of the current frame and its reference frames, a fast algorithm of inter frame mode selection is proposed for reducing the coding complexity of the hierarchical B structures. Finally, the proposed algorithm is implemented into the software. Experiment results show that the proposed algorithm achieves an average of 34.72% coding time reduction in the inter mode selection process with negligible loss of quality.