A near-optimal filtering scheme for low bit-rate block coders using variable projection

Recent work on JPEG compression for low bit-rate coding has shown that employing a JPEG coder within a sampling scheme where the image is downsampled prior to coding (and upsampled after the decoding stage) results in superior performance compared to standard JPEG coding. In this paper, we explore the use of optimal decimation and interpolation filters in this coding scheme. We shall first consider only optimization of the interpolation filter, by formulating the problem as leastsquares minimization. We then turn to consider the joint optimization over both the decimation and the interpolation filters, which is formulated as a separable least-squares problem and solved using the Variable Projection method. The experimental results presented clearly exhibit a significant improvement over existing approaches.