An image decoding method is utilized to obtain high quality images without error
accumulation. Such an image decoding method comprises 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.
The motion compensation includes calculating intensity values at points where no
pixels actually exist in the reference image by interpolation performed according
to information specifying a positive rounding method or a negative rounding method
when the current frame is a P frame, and using a predetermined rounding method
which is a positive rounding method or a negative rounding method when the current
frame is a B frame.
