FIG. 6 shows an example optical flow trajectory in the Bi-directional Optical flow (BIO) method. Here, τ0 and τ1 denote the distances to the reference frames. Distances τ0 and τ1 are calculated based on POC for Ref0 and Ref1: τ0=POC(current)?POC(Ref0), τ1=POC(Ref1)?POC(current). If both predictions come from the same time direction (either both from the past or both from the future) then the signs are different (e.g., τ0·τ1<0). In this case, BIO is applied if the prediction is not from the same time moment (e.g., τ0≠τ1). Both referenced regions have non-zero motion (e.g., MVx0, MVy0, MVx1, MVy1≠0) and the block motion vectors are proportional to the time distance (e.g., MVx0/MVx1=MVy0/MVy1=?τ0/τ1). In the JEM, BIO can be applied to all bi-predicted blocks when the two predictions are from different reference pictures. When Local Illumination Compensation (LIC) is enabled for a CU, BIO can be disabled.
In some cases, a FRUC flag can be signaled for a CU when its merge flag is true. When the FRUC flag is false, a merge index can be signaled and the regular merge mode is used. When the FRUC flag is true, an additional FRUC mode flag can be signaled to indicate which method (e.g., bilateral matching or template matching) is to be used to derive motion information for the block.
