Lagrange multiplier adaptation at CTU-level for VVC low-delay hierarchical coding

Independent rate-distortion optimization (RDO) is tractable to implement in video coding, but it cannot achieve global optimal rate-distortion performance. In this paper, we convert the problem of dependent RDO into the form of independent RDO by adjusting the Lagrange multiplier of a coding unit, and present a Lagrange multiplier adaptation method for versatile video coding (VVC). Firstly, the current frame is preencoded by an integer-pixel inter-picture prediction with the size of 32×32 to obtain the coding distortion and motion compensation predicted (MCP) error. Secondly, a distortion effect factor is estimated with the coding distortion and MCP error according to the location of the current frame in group of picture (GOP). Finally, a Lagrangian factor is introduced based on the distortion effect factor to adjust the Lagrange multiplier at coding tree unit (CTU) level. Experimental results show that, on the platform of VVC test model (VTM17.0), the proposed algorithm obtains 3.03% and 2.34% coding gains in terms of Bjøntegaard delta bitrate (BDBR), respectively, under the configurations of low-delay B and P slices.