Subblock-Based Combined Inter and Intra Prediction Beyond VVC

Combined Inter and Intra Prediction (CIIP) is a coding tool that blends an inter prediction and an intra prediction to generate a hybrid prediction, which is rather efficient when neither intra nor inter prediction alone can well capture the content characteristic of a coding block. Due to the considerable coding performance, CIIP has been adopted in versatile video coding (VVC), and has been enhanced with further extensions in the recent enhanced compression model (ECM). However, CIIP is still less effective to handle the scenarios with complex motions since only translational motion model is leveraged. To tackle such issue, this paper investigates a method of subblock-based CIIP (subblock-CIIP), where the motion compensation signal of affine or subblock-based TMVP (SbTMVP) mode can be combined with an intra prediction to produce the CIIP prediction. Simulation results on ECM-11.0 indicate subblock-CIIP can achieve average 0.24% coding gain on sequences with abundant affine motions. The proposed subblock-CIIP method has been adopted in ECM-12.0.