Adaptive post-filtering for reducing blocking and ringing artifacts in low bit-rate video coding

In this paper, we present a method to reduce blocking and ringing artifacts in low bit-rate block-based video coding. For each block, its DC value and DC values of the surrounding eight neighbor blocks are exploited to predict low frequency AC coefficients. Those predicted AC coefficients allow to infer spatial characteristics of a block before quantization stage in the encoding system. They are used to classify each block into either of two categories, low-activity and high-activity block. In the following post-processing stage, two kinds of low pass filters are adaptively applied according to the classified result on each block. It allows for strong low pass filtering in low-activity regions where the blocking artifacts are most noticeable, whereas it allows for weak low pass filtering in high-activity regions to reduce ringing noise as well as blocking artifacts without introducing undesired blur. In the former case, the blocking artifacts are reduced by one dimensional (1-D) horizontal and vertical low pass filters, and selective use of horizontal/vertical direction is adopted depending on the absolute values of the predicted AC coefficients. In the latter case, deblocking and deringing is conducted by a single filter, which makes the architecture simple. TMN8 decoder for H.263+ is used to test the proposed method. The experimental results show that the proposed algorithm is efficient and effective in reducing ringing artifacts as well as blocking artifacts from the low bit-rate block-based video coding.