Overlapped motion compensation for subband coding of video sequences

Block motion compensation (MC) is widely used in hybrid coding schemes. The presence of motion discontinuities inside the blocks introduces inefficiencies that reduce the performances of MC methods. Moreover the displaced frame difference (DFD) image shows block discontinuities. This fact is the main difficulty in using block-based MC in subband coding systems. This limitation does not appear in transform-based algorithms, where the boundaries of the transform support coincide with the MC block boundaries. Overlapped window motion estimation and compensation have been proposed to overcome these limitations leading to a reduction of the DFD energy and of the blocking artifacts. Overlapped MC is eqivalent to a linear combination of predictors based on neighboring block motion vectors. In this paper we describe a motion compensation scheme with new overlapped window modes. We report the improvements achieved in terms of DFD energy reduction and block boundaries reduction. Then a complete coding system based on subband decomposition is defined to evaluate the performances of the extended MC scheme in terms of bit rate reduction, SNR, and perceived visual quality. Simulation results have shown noticeable gains of the overlapped motion compensation over the MPEG2 MC scheme used for subband coding increasing only the complexity of the MC part.