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Interpolative picture coding refers to sending coded information about a few picture elements separated in space and interpolating all the rest of the picture elements. In this paper we consider sending coded information about picture elements separated by as large a distance as possible along a scan line. We study the effects of a few twodimensional interpolation strategies and evaluate the usefulness of several different error criteria required to judge the faithfulness of the interpolated signal. The error criteria are motivated by our knowledge of pictorial information processing in the human visual system. Based on the picture quality and entropy of the coded output as the criterion for judging the coding schemes, we find that error measures in which the interpolation error is filtered adaptively and compared to a varying threshold perform the best. The filter is adapted based on the spatial activity of the signal: high-bandwidth filter for low activity areas and low-bandwidth filter for high activity areas. The variation in threshold is based on the spatial masking of the interpolation error and has a high value in high activity areas and a low value in low activity areas. Our computer simulations indicate that, for head-and-head-and-shoulders-type pictures, it is possble, without affecting the picture quality, to reduce the entropy of the coded output by as much as 40 percent over that obtainable from previous element differential pulse code modulation (DPCM) system.
Date of Publication: May 1977