Improved POCS-based de-blocking algorithm for block-transform compressed images

This paper presents a postprocessing technique based on the theory of projections onto convex sets (POCS) to reduce the blocking artifacts in low bit rate BDCT-coded images. In the block discrete cosine transform (BDCT), the image is divided into a grid of non-overlapped 8 × 8 blocks, and then each block is coded separately. Thus, a block, which is shifted one pixel diagonally from the original grid of BDCT, will include the boundary of the original 8 × 8 block. If blocking artifacts are introduced along the block boundary, the frequency characteristics of this shifted block will be different from those of the original block. Therefore, a comparison of frequency characteristics of these two overlapping blocks can detect the undesired high-frequency components, mainly caused by the blocking artifacts. By eliminating these undesired high-frequency components adaptively considering the local statistical properties of the image, robust smoothing projection operator can be obtained. Computer simulation results indicate that the proposed scheme provides better visual performance than conventional postprocessing algorithms.