HIGH-PRECISION MOTION VECTOR REFINEMENT FOR BI-DIRECTIONAL OPTICAL FLOW

Bi-directional Optical Flow (BDOF), is a very effective tool, developed based on the optical flow concept, which assumes that the motion of an object is smooth and is in a straight line in a short period of time. The reference SW which is being developed for beyond VVC activity, i.e. ECM, has 2 BDOFs: One as Motion Vector (MV) refinement tool for each 8x8 subblock, and another one as sample adjustment BDOF similar to VVC one, but deriving the parameters for each sample. In this paper, a High-Precision BDOF is introduced which consists of the following parts: Firstly, a more accurate BDOF formula is used to derive the parameters for BDOF MV refinement. Next, a position-dependent weight is used to increase the focus of the parameter derivation. Finally, higher granularity as small as 4x4 is used for each subblock. For BDOF sample adjustment, another position-dependent weighted sum is used. The proposed method is implemented on top of ECM-7.0. Experimental results show that the proposed method provides up to -0.92% bitrate saving for some sequences compared to the latest ECM SW under random access configuration. The method explained in this paper is adopted into ECM-9.0.