An Improvement to Subblock-based Temporal Motion Vector Prediction Beyond VVC

Temporal motion vector predictor (TMVP) is a well-known coding technology that has been included in recent video compression standards. The basic concept of TMVP is to utilize the motion continuity between temporal pictures. It obtains motion vector predictor from temporal collocated block by performing temporal motion vector scaling to align the reference pictures of the collocated block to that of to-be-coded block. To exploit the coding performance of TMVP, subblock-based temporal motion vector prediction (SbTMVP) is introduced in Versatile Video Coding (VVC) and is further improved in Enhanced Compression Model (ECM), the software platform for exploring compression capability beyond that of VVC. SbTMVP shares a similar concept to that of TMVP, but obtains motion vectors at the subblock level with a motion displacement which is derived from neighboring blocks. In both VVC and ECM design, the SbTMVP is treated as one of subblock-based merge candidates. In this paper, it is proposed to extend SbTMVP to advanced motion vector predictor (AMVP) for coding motion explicitly. To allow more flexibility, the motion displacement is directly signaled in the bitstream in the proposed mode. Experimental results show that on top of ECM, the proposed mode provides {0.11% (Y), 0.04% (U), 0.05% (V)} and {0.36% (Y), 0.25% (U), 0.33% (V)} BD-rate reduction in random access and low delay B configurations, respectively. The proposed mode is currently being studied in exploration experiments due to its promising compression efficiency gain.