A spline-based adaptive filter for the removal of blocking artifacts in image sequences coded at very low bitrate

In this paper, a method for the reduction of blocking effect in image sequences coded at very low bitrate is presented. The proposed operator is capable of adapting its behaviour to the local characteristics of the images. In areas containing details, the degradation introduced by block-based coding is often masked by the texture itself and a satisfactory visual effect can be achieved by simply leaving the area unchanged. In uniform areas, instead, the coarse quantization normally used at very low bitrate results in visible steps in the gray level. A local action along the border is not sufficient for the removal of the artifact (specially when the frames are later viewed in a sequence). In this case, a spline interpolation is performed on the gray levels of pixels located on a wider area across the border. The extension of the filtered zone inside the blocks is evaluated by means of an index of activity so as to maximize the smoothing effect, while preserving the details inside the blocks.