Low-complexity MPEG-2 to H.264 transcoding

Jan Lievens; Dieter Van de Walle; Jan De Cock; Joeri Barbarien; Rik Van de Walle; Peter Schelkens

doi:10.1117/12.731786

24 September 2007 Low-complexity MPEG-2 to H.264 transcoding

Jan Lievens, Dieter Van de Walle, Jan De Cock, Joeri Barbarien, Rik Van de Walle, Peter Schelkens

Proceedings Volume 6696, Applications of Digital Image Processing XXX; 669604 (2007) https://doi.org/10.1117/12.731786
Event: Optical Engineering + Applications, 2007, San Diego, California, United States

Abstract

In this paper, two systems for low-complexity MPEG-2 to H.264 transcoding are presented. Both approaches reuse the MPEG-2 motion information in order to avoid computationally expensive H.264 motion estimation. In the first approach, inter- and intra-coded macroblocks are treated separately. Since H.264 applies intra-prediction, while MPEG-2 does not, intra-blocks are completely decoded and re-encoded. For inter-coded macroblocks, the MPEG-2 macroblock types and motion vectors are first converted to their H.264 equivalents. Thereafter, the quantized DCT coefficients of the prediction residuals are dequantized and translated to equivalent H.264 IT coefficients using a single-step DCT-to-IT transform. The H.264 quantization of the IT coefficients is steered by a rate-control algorithm enforcing a constant bit-rate. While this system is computationally very efficient, it suffers from encoder-decoder drift due to its open-loop structure. The second transcoding solution eliminates encoder-decoder drift by performing full MPEG-2 decoding followed by rate-controlled H.264 encoding using the motion information present in the MPEG-2 source material. This closed-loop solution additionally allows dyadic resolution scaling by performing downscaling after the MPEG-2 decoding and appropriate MPEG-2 to H.264 macroblock type and motion vector conversion. Experimental results show that, in terms of PSNR, the closed-loop transcoder significantly outperforms the open-loop solution. The latter introduces drift, mainly as a result of the difference in sub-pixel interpolation between H.264 and MPEG-2. Complexity-wise, the closed-loop transcoder requires on average 30 % more processing time than the openloop system. The closed-loop transcoder is shown to deliver compression performance comparable to standard MPEG-2 encoding.

Citation Download Citation

Jan Lievens, Dieter Van de Walle, Jan De Cock, Joeri Barbarien, Rik Van de Walle, and Peter Schelkens "Low-complexity MPEG-2 to H.264 transcoding", Proc. SPIE 6696, Applications of Digital Image Processing XXX, 669604 (24 September 2007); https://doi.org/10.1117/12.731786

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