$\lambda $ -Domain Optimal Bit Allocation Algorithm for High Efficiency Video Coding
Rate control typically involves two steps: bit allocation and bitrate control. The bit allocation step can be implemented in various fashions depending on how many levels of allocation are desired and whether or not an optimal rate–distortion (R-D) performance is pursued. The bitrate control step has a simple aim in achieving the target bitrate as precisely as possible. In our recent research, we have developed a <inline-formula> <tex-math notation="LaTeX">$ {\lambda }$ </tex-math></inline-formula>-domain rate control algorithm that is capable of controlling the bitrate precisely for High Efficiency Video Coding (HEVC). The initial research showed that the bitrate control in the <inline-formula> <tex-math notation="LaTeX">$ {\lambda }$ </tex-math></inline-formula>-domain can be more precise than the conventional schemes. However, the simple bit allocation scheme adopted in this initial research is unable to achieve an optimal R-D performance reflecting the inherent R-D characteristics governed by the video content. In order to achieve an optimal R-D performance, the bit allocation algorithms need to be developed taking into account the video content of a given sequence. The key issue in deriving the video-content-guided optimal bit allocation algorithm is to build a suitable R-D model to characterize the R-D behavior of the video content. In this paper, to complement the R-<inline-formula> <tex-math notation="LaTeX">$ {\lambda }$ </tex-math></inline-formula> model developed in our initial work, a D-<inline-formula> <tex-math notation="LaTeX">$ {\lambda }$ </tex-math></inline-formula> model is properly constructed to complete a comprehensive framework of <inline-formula> <tex-math notation="LaTeX">$ {\lambda }$ </tex-math></inline-formula>-domain R-D analysis. Based on this comprehensive <inline-formula> <tex-math notation="LaTeX">$ {\lambda }$ </tex-math></inline-formula>-domain R-D analysis framework, a suite of optimal bit allocation algorithms are developed. In particular, we design both picture-level and basic-unit-level bit allocation algorithms based on the fundamental R-D optimization theory to take full advantage of the content-guided principles. The proposed algorithms are implemented in HEVC reference software, and the experimental results demonstrate that they can achieve an obvious R-D performance improvement with a smaller bitrate control error. The proposed bit allocation algorithms have already been adopted by the Joint Collaborative Team on Video Coding and integrated into the HEVC reference software.