H.264為現在壓縮效果及畫面品質都很好的視訊標準，相較於以往MPEG-2和H.263，H.264在相同位元率下之影像品質獲得大幅改善，因此H.264在多媒體裝置及網路應用上扮演著越來越重要的角色，然而，在不同網路環境下，其傳輸頻寬不同，如何使視訊串流滿足各種頻寬要求是一大課題，而轉換編碼是其中一種可以克服此問題的技術，而本篇論文利用畫面跳張(frame-skipping)的方式將高位元率之H.264視訊串流轉換編碼成為較低位元率的H.264視訊串流，使其能適應不同的頻寬限制，而轉換編碼最直接的作法就是不利用原始資訊直接重新編碼，然而這樣運算複雜度過高，但若完全再利用(reuse)原始資訊將導致編碼效能嚴重下降，因此，為了降低複雜度並維持一定的效能，我們利用原始資訊分別針對區塊模式與移動向量提出了轉換編碼演算法來幫我們簡化移動估測，實驗結果也顯示，能在維持一定的畫面品質效能下，大幅降低轉換編碼的時間。

The emerging H.264/AVC achieves better coding performance compared with prior video coding standards, and applications of H.264/AVC become popular and important in networked multimedia services, such as teleconferencing, video on demand, and distance learning. However, because different networks might have different limitations of channel bandwidth, it is often needed to reduce the bit-rate of the coded video bit streams. Frame-skipping transcoding is one key technology to provide such interoperability between different systems and devices. The most straightforward method for frame-skipping transcoding is to decode the video stream and re-encode the reconstructed video sequences after frame-skipping. However, the computational complexity of the fully decoding and re-encoding process, referred to as complex cascaded pixel domain transcoding (CCPDT), is extremely high.
Reusing information existing in the original incoming video stream, referred to as simple cascaded pixel domain transcoding (SCPDT) can reduces computational complexity but lead to severe degradation in coding performance due to mode and motion vector mis-matched problems. Consequently, to reduce the computation cost and maintain high coding performance during transcoding, we propose an efficient inter/intra mode decision algorithm and an efficient motion vector composition method. The experimental results show that our proposed transcoding algorithms can maintain high coding performance and speed up transcoding process simultaneously for a reduced frame rate video transcoder.

[1] “Draft ITU-T recommendation and final draft international standard of Joint Video Specification (ITU-T Rec. H.264 | ISO/IEC 144496-10 AVC),” Joint Video Team of ISO/IEC and ITU-T, Nov. 2007.
[2] “Generic coding of moving pictures and associated audio information,” ISO/IEC 13818-2: Video (MPEG-2), May 1996.
[3] “Video coding for low bit rate communication, version 1,” ITU-T Recommendation H.263, 1995.
[4] “Coding of audio-visual objects - part 2: visual,” in ISO/IEC 14496-2 (MPEG-4 Visual Version 1), April 1999.
[5] T. Wiegand, G.J. Sullivan, G.. Bjontegaard and A. Luthra, “Overview of the H.264/AVC video coding standard,” IEEE Transactions on Circuits and Systems for Video Technology, vol.13, pp.560-576, July 2003.
[6] C. D. Wu and Y. Lin, “Efficient inter/intra mode decision for H.264/AVC inter frame transcoding,” in Proc. IEEE International Conference on Image Processing, 2009, pp. 3697-3700.
[7] J. Youn, M. T. Sun and C. W. Lin, “Motion vector refinement for high-performance transcoding,” IEEE Transactions on Multimedia, vol. 1, no. 1, pp. 30-40, March 1999.
[8] M. J. Chen, M. C. Chu and C. W. Pan, “Efficient motion estimation algorithm for reduced frame-rate video transcoder,” IEEE Transactions on Circuits and Systems for Video Technology, vol. 12, no. 4, pp. 269-275, April 2002.
[9] K. T. Fung, Y. L. Chan and W. C. Siu, “Low-complexity and high-quality frame-skipping transcoder for continuous presence multipoint video conference,” IEEE Transactions on Multimedia, vol. 6, no. 1, pp. 31-46, February 2004.
[10] K. T. Fung, Y. L. Chan and W. C. Siu, “New architecture for dynamic frame-skipping transcoder,” IEEE Transactions on Image Processing, vol. 11, no. 8, pp. 886-900, August 2004.
[11] W. J. Lee and W. J. Ho, “Adaptive frame-skipping for video transcoding,” in Proc. IEEE International Conference on Image Processing, September 2003, vol. 1, pp. I-165-8, 14-17.
[12] J. N. Hwang, T. D. Wu, and C. W. Lin, “Dynamic frame-skipping in video transcoding,” in Proc. IEEE Second Workshop on Multimedia Signal Processing, Dec. 1998, pp. 616-621, 7-9.
[13] J. Youn, and M. T. Sun, “Fast motion vector composition method for temporal transcoding,” in Proc. IEEE International Symposium on Circuits and Systems, June 1999, vol. 4, pp. 243-246.
[14] S. Yang, D. Kim, Y. Jeon, and J. Jeong, “An efficient motion re-estimation algorithm for frame-skipping video transcoding,” in Proc. IEEE International Conference on Image Processing, September 2005, vol. 3, pp. III - 668-71.
[15] J. D. Wu, Study of H.264 intra frame coding and transcoding, NCU, Taiwan, July 2008.
[16] W. C. Teng, Study of spatial downsizing transcoding for H.264, NCU, Taiwan, June 2009.
[17] H. H. Hsu, H.264/H.263/MPEG-2 to H.264 intra and inter frame transcoding, NCU, Taiwan, June 2009.
[18] J. Youn, and M. T. Sun, “Motion estimation for high performance transcoding,” in Proc. IEEE International Conference on Consumer Electronics, June 1998, pp. 136-137.
[19] B. Shen, I. E. Sethi and B. Vasudev, “Adaptive motion vector resampling for compressed video down-scaling,” in Proc. IEEE International Conference on Image Processing, Oct. 1997, vol. 1, pp. 771-774.
[20] F. Lonetti and F. Martelli, “Motion vector composition algorithm in H.264 transcoding,” in Proc. IEEE International Conference on Image Processing, June 2007, no. 4, pp. 401-401.
[21] F. Pan, Z. P. Lin, X. Lin, S. Rahardja, W. Juwono and F. Slamer, “Content adaptive frame skipping for low bit rate video coding,” in Proc. IEEE International Conference on Information, Communications and Signal, Dec 2003, vol. 1, pp. 230-234.
[22] Joint Video Team software JM12.2
http://iphome.hhi.de/suehring/tml/download/