The motion derivation for sub-CU A starts by identifying its two spatial neighbors. The first neighbor is the N×N block above sub-CU A 501 (block c 513). If this block c (513) is not available or is intra coded the other N×N blocks above sub-CU A (501) are checked (from left to right, starting at block c 513). The second neighbor is a block to the left of the sub-CU A 501 (block b 512). If block b (512) is not available or is intra coded other blocks to the left of sub-CU A 501 are checked (from top to bottom, staring at block b 512). The motion information obtained from the neighboring blocks for each list is scaled to the first reference frame for a given list. Next, temporal motion vector predictor (TMVP) of sub-block A 501 is derived by following the same procedure of TMVP derivation as specified in HEVC. The motion information of the collocated block at block D 704 is fetched and scaled accordingly. Finally, after retrieving and scaling the motion information, all available motion vectors are averaged separately for each reference list. The averaged motion vector is assigned as the motion vector of the current sub-CU.

The bi-directional optical flow (BIO) method is a sample-wise motion refinement performed on top of block-wise motion compensation for bi-prediction. In some implementations, the sample-level motion refinement does not use signaling.

Let I^(k) be the luma value from reference k (k=0, 1) after block motion compensation, and ?I^(k)/?x, ?I^(k)/?y are horizontal and vertical components of the I^(k) gradient, respectively. Assuming the optical flow is valid, the motion vector field (v_x, v_y) is given by: