Optimal Bit Allocation for CTU Level Rate Control in HEVC

For High Efficiency Video Coding (HEVC), the R–
$\lambda $
scheme is the latest rate control (RC) scheme, which investigates the relationships among allocated bits, the slope of rate-distortion (R-D) curve
$\lambda $
, and quantization parameter. However, we argue that bit allocation in the existing R–
$\lambda $
scheme is not optimal. In this paper, we therefore propose an optimal bit allocation (OBA) scheme for coding tree unit level RC in HEVC. Specifically, to achieve the OBA, we first develop an optimization formulation with a novel R-D estimation, instead of the existing R–
$\lambda $
estimation. Unfortunately, it is intractable to obtain a closed-form solution to the optimization formulation. We thus propose a recursive Taylor expansion (RTE) method to iteratively solve the formulation. As a result, an approximate closed-form solution can be obtained, thus achieving OBA and bit reallocation. Both theoretical and numerical analyses show the fast convergence speed and little computational time of the proposed RTE method. Therefore, our OBA scheme can be achieved at little encoding complexity cost. Finally, the experimental results validate the effectiveness of our scheme in three aspects: R-D performance, RC accuracy, and robustness over dynamic scene changes.