The vertical motion compensation is illustrated in figure 5.1. 5.3 Special prediction mode 5.3.1. This mode has been included in particular for low delay applications. In the syntax of the Special prediction mode, for forward prediction, one field motion vector is transmitted, followed by a differential motion vector. A separate section defines precisely how field motion vectors are scaled.
Plain arrows represent the transmitted field motion vector.
Vertical arrows represent the transmitted differential motion vector. If the reference field and the predicted field are same parity, the field motion vector used is equal to the transmitted field motion vector. If the reference field and the predicted field are different parity, the field motion vector used is obtained by adding the differential motion vector to the scaled transmitted motion vector. The transmitted field motion vector is used for computing two scaled field motion vectors that serve in the Special prediction mode when reference field and predicted fields are opposite parity. The rules of prediction are same as for lumanance.