ends and another b starts la indicates the last picture of sequence a fb indicates the first picture of sequence b the change of delivery data from sequence a to sequence b is shown by a change in thickness of the buffer occupancy line with the chain linked line indicating pictures from sequence a at some time t x all the data for sequence a has been delivered and the vbv video buffering verifier the mpeg decoder model has an occupancy of b x bits from this time on all the data delivered to the buffer is for sequence b it will be noted though that some pictures from the end of sequence a are still in the buffer which pictures are all removed by time t l when the buffer has an occupancy of b l bits the encoder goes through a process known as targeting when it tries to achieve a certain occupancy in the vbv buffer during targeting the encoder assumes that the vbv buffer has a certain target occupancy when the first picture it has coded is put into the buffer this places an upper limit on the size of the first picture at the end of a coding run the encoder targets the vbv occupancy at the point in time just before the first picture for the next sequence would be removed from the buffer point b t in fig1 it targets this state by changing the size of the last or last few pictures as it codes in the example shown in.