Enhancement of compressed images by optimal shift-invariant wavelet packet basis

A novel postprocessing method based on the optimal shift-invariant wavelet packet (SIWP) representation and wavelet shrinkage is proposed to enhance compressed images. At the encoder, the optimal (in the mean square error sense) SIWP basis is searched using a fast optimization algorithm and the location of the best basis in the entire SIWP library is transmitted as overhead information to the decoder. The selected basis is jointly optimal in terms of both the time-frequency tiling and the relative time-domain offset (or shift) between a signal and its wavelet packet representation. After the decoder reconstructs the compressed image, the postprocessor performs wavelet shrinkage using the optimal basis. Due to its powerful adaptability, the method is shown to achieve a better trade-off between enhancement performance and decoder complexity than both the orthonormal wavelet transform and the undecimated wavelet transform-based methods. (C) 1998 Academic Press.