Minimization of aliasing artifacts during partial subband reconstruction with Wiener filters

Separable subband coding is an increasingly popular method of image compression. This technique divides an image into spatial frequency bands by convolving it with one-dimensional low- and high-pass filters and then decimating the results. Reconstruction is performed by interpolating the subbands using a mathematically related set of filters. Since the analysis filters are not ideal, their spectral overlap results in aliasing from the subsampling step. However, certain restrictions can be made on the form of the analysis and synthesis filters which will cancel the aliasing when the images are reconstructed. Filter banks so designed are said to have the perfect reconstruction property. This property is lost when not all the subbands are used in the reconstruction process. Sometimes, lack of channel bandwidth or decoder processing power makes it necessary to discard some of the higher frequency bands and reconstruct only the lower frequencies. In this case, the aliasing is no longer cancelled, such that the resulting picture is not just a low-passed version of the original and its appearance is noticeably degraded. In order to improve the picture quality, a technique is proposed which minimizes the amount of aliasing during the reconstruction process when the high subbands are not present. This method models the spectrum of the image and employs an optimal Wiener filter instead of the normal subband low-pass synthesis filter. Although no additional computation is required beyond that for a normal reconstruction, experiments show that the images produced are improved in both a visual and a signal-to-noise ratio sense.