Toward the Achievable Rate-Distortion Bound of VVC Intra Coding: A Beam Search-Based Joint Optimization Scheme

In this paper, we present the first attempt at determining where the achievable rate-distortion (R-D) performance bound in versatile video coding (VVC) intra coding is when considering the mutual dependency in the rate-distortion optimization (RDO) process. In particular, the abundant search space of encoding parameters in VVC intra coding is practically explored with a beam search-based joint rate-distortion optimization (BSJRDO) scheme. As such, the partitioning, prediction and transform decisions are jointly optimized across different coding units (CUs) with a customized search subset instead of the full space. To make the beam search process implementation-friendly for VVC, the dependencies among the CUs are truncated at different depths. To facilitate finer computational scalability, the beam size is flexibly adjusted based on the characteristics of the CUs, such that the operational points that satisfy different complexity demands for diverse applications can be practically obtained. The proposed BSJRDO approach, which fully conforms to the VVC decoding syntax, can serve as both the way toward the optimal RDO bound and a practical performance-boosting solution. BSJRDO is further implemented on a VVC coding platform (VVC Test model (VTM) 12.0), and extensive experiments show that BSJRDO can achieve 1.30% and 3.22% bit rate savings compared to the VTM anchor under the common test condition and low-bit-rate coding scenarios, respectively. Moreover, the performance gain can also be flexibly customized with different computational overheads.