Four motion vector coding and decoding in bi-directionally predicted interlaced pictures

For interlaced B-fields or interlaced B-frames, an encoder/decoder uses 4MV coding. For example, 4MV is used in one-direction prediction modes (forward or backward modes), but not in other available prediction modes (e.g., direct, interpolated). Using 4MV allows more accurate motion compensation for interlaced B-fields and interlaced B-frames; limiting 4MV to forward and backward modes reduces coding overhead and avoids decoding complexity associated with combining 4MV with modes such as direct and interpolated.