In one embodiment, the method includes determining, first and second reference pictures; obtaining a type of the first reference picture; and determining motion vectors of the bi-predictive image block based on the type of the first reference picture. The type is one of a long-term type and a short-term type, and characterizes a temporal distance of the first reference picture with respect to the bi-predictive block. The motion vectors of the bi-predictive image block are determined according to a first set of expressions if the first reference picture is of the short-term type, and according to a second set of expressions if the first reference picture is of the long-term type. The second set of expressions is different than the first set of expressions. The method further includes decoding, by the moving picture coding system, the bi-predictive image block based on the determined motion vectors.