medium having decoding program effecting : extracting motion vector information , quantized dct coefficients , and rounding method information from input information to be decoded ; dequantizing quantized dct coefficients to obtain dct coefficients ; performing inverse dct conversion on the dct coefficients to obtain an error image ; synthesizing a prediction image of a currently decoded image by performing motion compensation using motion vector information , rounding method information , and a previously decoded reference image ; and adding the prediction image to the error image ; wherein the motion compensation is performed with half - pixel accuracy and uses bilinear interpolation to calculate intensity values of chrominance or luminance at points where no pixels actually exist in the reference image , the bilinear interpolation being performable using a positive or negative rounding method ; wherein bilinear interpolation is performed using a rounding method specified by rounding method information ; and wherein the rounding method information is included , in the currently decoded image .