a computer - readable medium having stored thereon an image decoding program which , when executed by a computer , performs image decoding , wherein synthesizing a prediction image is performable using a positive rounding method and a negative rounding method for interpolating intensity values of pixels ; wherein the interpolation of intensity values of pixels is performed using a rounding method specified by the rounding method information ; wherein the rounding method information is included in coded information of the currently decoded image ; wherein the rounding method information specifies one of two values ; wherein one of the two values specifies a positive rounding method , and another one of the two values specifies a negative rounding method ; and wherein the rounding method information consists of one bit .