Encoding and decoding method and apparatus using plus and/or minus rounding of images

An image decoding method comprising:

• receiving an encoded bitstream including information of P and B frames;
• and
• executing motion compensation by synthesizing a predicted image of a current frame using motion vector information included in the encoded bitstream and a reference image which is a previously decoded image,
• wherein said motion compensation includes calculating intensity values at points where no pixels actually exist in the reference image by interpolation,
• wherein said interpolation is done according to information specifying a positive rounding method or a negative rounding method included in a header section of the information of the current frame in the encoded bit stream, when the current frame is a P frame, and
• wherein said interpolation is done using a predetermined rounding method which is a positive rounding method or a negative rounding method, when the current frame is a B frame.