A DCT-domain filtering algorithm for MPEG-4 encoding

One of the well-known weaknesses of block-based video compression schemes such as MPEG-4, at low bit-rates, is blocking artifacts. They can be well removed by post-processing solutions offering an efficient deblocking filter, unfortunately at the cost of a significant increase of the decoding process complexity. In this paper, we propose a new method to reduce blocking artifacts (thus increasing the visual quality of sequences or the compression rate) at the encoding stage to help, or to get rid of the post-processing step. We consider a filtering operation in the DCT domain, applied on blocks before quantization to leave the decoding process unmodified. The solution is adaptive and focuses on regions which would be 'a priori' blocky regarding some texture features and encoding parameters. It is based on the mix of quantization step reduction at the frame level, with DCT coefficients low-pass filtering at the block level, on top of any bit-rate regulation module. Tests showed the efficiency of the method: the sequences encoded with our filter are cleaner, with less ringing and, above all, fewer blocking artifacts than the ones without any processing. Visual quality is therefore better at a given bit-rate. Depending on cases, post-processing is either useless or facilitated by our process.