1. An apparatus, comprising: a buffer comprising two ordered sets of reference pictures and an associated index value for each of the reference pictures, wherein each ordered set of reference pictures comprises a reference picture temporally before or after a B picture, each index value corresponding to the ordered position of the associated reference picture in the two ordered sets; and a processor configured to compute, responsive to a bi-predictive coding mode, a motion-compensated prediction based on selection of one or more of the reference pictures, the selection via the respective index value, the computation further based on values of motion vectors corresponding to one or more spatially neighbouring blocks that refer to the reference pictures of the same ordered set as referred to by the motion vector to be predicted, the computation independent of a temporal direction of the reference picture referred to by the current motion vector corresponding to a current block relative to a temporal direction and a temporal distance of the reference pictures corresponding to the motion vectors corresponding to the spatially neighbouring blocks, wherein the processor is further configured to set the current motion vector equal to the motion vector that uses the same ordered set in the block immediately above if the current motion vector applies to a top half of a 16×8 partitioned macroblock and a block immediately above the current block in the picture contains a motion vector that uses the same ordered set and index value as the current motion vector.