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7 May 2003 A DCT-domain filtering algorithm for MPEG-4 encoding
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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 get rid of, or to help, 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 DCT filter are cleaner, with less ringing and, above all, fewer blocking artifacts than the ones without any processing. Visual quality is therefore better for a constant bit consumption. Depending on cases, post-processing is either useless or improved by our process.
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Gwladys Hermant and Francoise Groliere "A DCT-domain filtering algorithm for MPEG-4 encoding", Proc. SPIE 5022, Image and Video Communications and Processing 2003, (7 May 2003);

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