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Development of Efficient MPEG to H.263 Transcoding Algorithm for Improved Video Quality

This paper discusses the development of a transcoding algorithm that converts MPEG video streams to H.263 format. The motivation behind this work is to lower the bitrate for applications such as wireless transmission while maintaining picture quality. The algorithm leverages decoded information to minimize computational overhead and focuses on key transcoding issues, including managing motion vectors and macroblocks. The results show significant computational savings while preserving video quality, laying the groundwork for future improvements in real-time transcoding applications.

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Development of Efficient MPEG to H.263 Transcoding Algorithm for Improved Video Quality

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  1. Nick Feamster Development of a Transcoding Algorithm from MPEG to H.263

  2. Motivation: Transcode MPEG - High bitrate - DVD, Digital Television - 1.5 Mbps, 30 fps Development of a Transcoding Algorithm from MPEG to H.263 H.263 - Low bitrate - Wireless, etc. - 500 kbps, 10fps

  3. Transcoding Issues H.263 - I, P Frames - Unrestricted Motion Vectors on boundaries - Fewer I macroblocks per frame MPEG - I, P, B Frames - Restricted Motion Vectors on boundaries - More I macroblocks per frame vs. January 26, 1999

  4. Computation vs. Quality Goal: A fast transcoding algorithm which preserves picture quality. Method: Make use of available decoded information to avoid unnecessary computation. January 26, 1999

  5. MPEG vs. H.263 January 26, 1999

  6. MPEG vs. H.263 Conclusion: 1.5 Mbps MPEG Source minimizes loss due to recoding. January 26, 1999

  7. H.263 Motion Estimation MPEG Decode Drop B Frames H.263 Encode Initial Method • 9-10 Seconds per frame • Too much computation for real time transcoding January 26, 1999

  8. Alternative: Reuse Information H.263 Motion Estimation MPEG Decode Drop B Frames H.263 Encode Motion Vectors What is the best way to do this? January 26, 1999

  9. B I I B B B P P B B MPEG: P P I P H.263: Issue: MPEG I Frames Problem: Each MPEG GOP has an I frame with no motion vectors. Solution: Copy motion vectors from preceding MPEG P frame. MV’s from P frame Yes Drop B frames I frame? MPEG Decode H.263 Encode MV’s from current frame No January 26, 1999

  10. Issue: Small MV Errors Problem: Quantization losses result in motion estimation discrepancies Solution: Half pel search to refine motion vectors January 26, 1999

  11. Issue: MPEG Intra Macroblocks • Often more useful to code an H.263 macroblock as an intra block, even if the block is coded inter in MPEG Solution: Use the minimum error threshold specified by the H.263 encoder to correct modes. January 26, 1999

  12. Resulting Method MV’s from P frame Yes I frame? MPEG Decode Drop B frames Mode Correct Half Pel Correct H.263 Encode No MV’s from current frame January 26, 1999

  13. Other Considerations • Boundary Error • 0.5 dB loss, worst case (sequence dependent) • Much of this loss can be recovered with mode correction and half pel search • At a lower spatial resolution, boundary motion vectors take on increasing importance. January 26, 1999

  14. H.263 Boundary Motion Vectors January 26, 1999

  15. Other Considerations • MPEG I Frames • Tried weighted average of neighboring P frame MV’s based on overlap, not as effective as simple replacement. • Replacement works well on sequences with correlated motion vectors • High quality P frame from MPEG I frame ® Less degradation of subsequent P frame quality January 26, 1999

  16. Results Sequence Method 1 (sec) Method 2 (sec) Carousel (30 IP) 323 26 Bus (30 IP) 254 26 Girl (18 IP) 106 15 Bus (10 IP) 80 9 Carousel (10 IP) 90 8 Football (10 IP) 92 8 ~ 10x Efficiency Improvement January 26, 1999

  17. Results Carousel Bus Girl January 26, 1999

  18. Conclusions • Reusing motion vector information from decoded MPEG stream saves 85-90% of computation time with an average of 1.4 dB loss. • Much of the loss can be attributed to motion vector errors resulting from requantization of the original frame pixel values. Frame A’ MV(A) MPEG MPEG H.263 H.263 MV(A’) Method 1 Method 2 January 26, 1999

  19. Future Work • Algorithmic research (motion vector search for I frames, etc.) • Real-time implementation • Allow MPEG interlace as transcoder input January 26, 1999

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