A GOP-Level Variable Bit Rate Control Algorithm for H.265 Video Coding Standard

A rate control algorithm at the group of picture (GOP) level is proposed in this paper for variable bit rate applications of the H.265/HEVC video coding standard with buffer constraint. Due to structural changes in the HEVC compared to the previous standards, new rate control algorithms are needed to be designed. In the proposed algorithm, quantization parameter (QP) of each GOP is obtained by modifying QP of previous GOP according to target bit rate and buffer status. Buffer status and target bit rate are input variables selected to expand a two dimensional lookup table. Output of the lookup table is provided in a way to allow short-term variations in bit rate, in order to reach better and more uniform visual quality of reconstructed video. In addition, a QP cascading technique is used for calculating QP of frames in each GOP that operates like a bit allocation scheme and causes suitable trade-off between quality and compression rate. Unlike conventional methods, proposed scheme uses a lookup table instead of using a rate-distortion model that significantly reduces the computational complexity. Several video sequences with completely different contents were used for experiments. Some short video sequences are concatenated to attain long video sequences which are closer to variable bit rate applications. Lookup table based (LUT) proposed algorithm is implemented on HM reference software and compared with λ-domain rate control algorithm (λ-RC) and constant QP (CQP) case that defined as anchor. In almost the same average bit rate (CQP: 1527.97, LUT: 1520.92, λ-RC: 1529.41), average QP (28.09, 28.18, 29.91) and average peak signal to noise ratio (PSNR) (37.88, 37.87, 37.76) of LUT is closer to CQP than that of λ-RC. Average values of QP standard deviation (1.13, 2.28, 4.27) and PSNR standard deviation (1.37, 2.11, 2.15) of LUT is smaller than λ-RC and closer to CQP. From rate control point of view, minimum buffering delay on average for all video sequences resulted by LUT is the same with that of λ-RC which is one of the best rate controllers proposed for the HEVC (0.94, 0.36, 0.35). Consequently, experimental results show that not only bit rate is perfectly controlled according to buffer constraints, but also the quality of reconstructed video is well maintained.