1. A method of inverse discrete wavelet transforming subband data in segments, wherein a plurality of different computational procedures for performing the inverse DWT may be used, and wherein a current state is maintained between a current and previous segments of subband data, the method comprising the steps of: testing said current state and a subset of the current segment of subband data to determine if the current segment can be inverse transformed with a reduced said computational procedure; if the test is positive performing the inverse DWT using said reduced computational procedure; otherwise performing the inverse DWT of said segment using another said computational procedure.