Patent US RE41776

A method and apparatus to remove a block effect and a ring effect appearing in a compression-coded image is disclosed. The present invention is especially applicable to an image compression-coded at a low bit rate. In particular, the present invention includes a variety of masks for the removal of the block/ring effect. Thus, one mask is select for a pixel to be filtered. Moreover, candidate pixels to be averaged with the pixel to be filtered is selected from the pixels of the selected mask to better maintain the details of the image, and a weight is applied to improve the calculation speed of the averaging operation. More particularly, the masks according to the present invention have longer tabs toward adjacent blocks at boundaries of the given block to remove the block and ring effect. In one embodiment, the filtering unit is configured to select at least four pixels of successive pixels according to a position of a pixel to be filtered. The four pixels include the pixel to be filtered. At least one of the four pixels may be in a first block, and three of the four pixels may be in a second block adjacent to the first block. The filtering unit is configured to determine a difference between two of the successive pixels. The filtering unit is configured to apply an averaging operation to the selected at least four pixels.