1. A method for a moving picture coding system to decode a bi-predictive image block by using a first reference picture and a second reference picture, comprising: obtaining, by the moving picture coding system, a type of the first reference picture; determining, by the moving picture coding system, motion vectors of the bi-predictive image block based on the type of the first reference picture, the type being one of a long-term type and a short-term type, the type characterizing a temporal distance of the first reference picture with respect to the bi-predictive block, the determining step determining the motion vectors of the bi-predictive image block according to a first set of expressions if the first reference picture is of the short-term type and the determining step determining the motion vectors of the bi-predictive image block according to a second set of expressions if the first reference picture is of the long-term type, the second set of expressions being different than the first set of expressions; and decoding, by the moving picture coding system, the bi-predictive image block based on the determined motion vectors.