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Overlapping Block Transform For Image Coding Preserving Equal Number Of Samples And Coefficients

An approach to transform coding of images is presented, which uses overlapping blocks without increasing the number of coefficients. This is achieved by subsampling in the frequency domain, thus causing spatial domain aliasing in each block. This spatial domain aliasing is however cancelled out by an overlapping addition of the backward transform output of neighboring blocks. Conditions are stated, under which this aliasing cancellation property holds even when a spatial window function is incorporated into the transform. By the design of the window function, block boundary artefacts typical for conventional transform coding can be completely avoided. The application of 2-dimensional overlapping block transform (2D-OBT) to picture coding is examined, the nature and visibility of quantization errors in the case of lossy coding are analyzed. The incorporation of 2D-OBT in a low bitrate hybrid image sequence coding environment is described. Simulation results are presented, showing the achievable picture quality at 64 kBit/s using 2D-OBT as compared to DCT.