1. A transcoding method, comprising the steps of: receiving a first bitstream of compressed data having a first group of pictures (GOP) structure; obtaining first and second motion information from the first bit stream; weighting the first and second motion information; utilizing the first motion information to extrapolate third motion information for a second bitstream of compressed data if the first motion information outweighs the second motion information, the second bitstream having a second group of pictures (GOP) structure different from the first group of pictures (GOP) structure; and outputting the second bitstream.