in one embodiment , the method includes obtaining reference picture information , and determining a first reference picture of the bi - predictive image block . the first reference picture has a co - located image block with respect to the bi - predictive image block . a second reference picture is determined based on a motion vector of the co - located image block . at least one motion vector of the bi - predictive image block is obtained according to the motion vector of the co - located image block and the reference picture type of at least one of the first reference picture and the second reference picture . the reference picture information indicates reference picture types of reference pictures , and the reference picture type being based on temporal distance between a current picture including the bi - predictive image block and the reference picture . the bi - predictive image block is decoded by motion - estimating the bi - predictive image block using the motion vector of the bi - predictive image block .