A new predictive coding is used to increase the temporal frame rate and
coding efficiency without introducing excessive delay. Currently the
motion vector for the blocks in the bi-directionally predicted frame is
derived from the motion vector of the corresponding block in the forward
predicted frame using a linear motion model. This however is not
effective when the motion in the image sequence is not linear. The
efficiency of this method can be further improved if a non-linear motion
model is used. In this model a delta motion vector is added to or
subtracted from the derived forward and backward motion vector,
respectively. The encoder performs an additional search to determine if
there is a need for the delta motion vector. The presence of this delta
motion vector in the transmitted bitstream is signalled to the decoder
which then takes the appropriate action to make use of the delta motion
vector to derive the effective forward and backward motion vectors for
the bi-directionally predicted block.