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Development of a Transcoding Algorithm from MPEG to H.263

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

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Development of a Transcoding Algorithm from MPEG to H.263

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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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