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Efficient Reference Frame Selector for H.264

Efficient Reference Frame Selector for H.264. Tien-Ying Kuo, Hsin-Ju Lu. IEEE CSVT 2008. Outline. Introduction H.264 Inter-Coding Proposed Method Experimental Results Conclusions. Introduction Background Knowledge (1/4). H.264 features for inter-frame coding

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Efficient Reference Frame Selector for H.264

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  1. Efficient Reference Frame Selector for H.264 Tien-Ying Kuo, Hsin-Ju Lu IEEE CSVT 2008

  2. Outline Introduction H.264 Inter-Coding Proposed Method Experimental Results Conclusions

  3. IntroductionBackground Knowledge (1/4) H.264 features for inter-frame coding Variable block size motion compensation Subpixel motion estimation Multiple reference frame motion compensation N - 5 Current frame N N - 2 N - 1

  4. IntroductionBackground Knowledge (2/4) Why do Multiple reference frame help predictions ? Uncovered background Non-integer pixel displacement Lighting change Camera shaking Noises in the source signal Noise effect 4

  5. IntroductionBackground Knowledge (3/4) Sampling/noise effect of MobileCalendar (N-1) Ref block (N-2) Ref block Original block (N-2) Residual block (N-1) Residual block MSE = 32.4 MSE = 203.8 5

  6. IntroductionBackground Knowledge (4/4) Drawback of MRF-ME High computational complexity In order to reduce complexity Reduce search points Continuous tracking technique Early stop criteria 6

  7. IntroductionMain Purpose (1/1) A simple and effective method of selecting proper reference frames in MRF-ME. It enables working with any existing ME algorithms. Experimental results demonstrate the effectiveness of proposed algorithm. 7

  8. H.264 Inter-CodingOverview (1/3) Variable block size Macroblock partition : 16*16, 16*8, 8*16, 8*8 Submacroblock partition : 8*8, 8*4, 4*8, 4*4 16*16 16*8 8*8 8*16 8*8 8*4 4*8 4*4 8

  9. H.264 Inter-CodingOverview (2/3) Reference parameter REF Signal to which frame referred Coded only once for each submacroblock partition All subblock smaller than 8*8 in the same subpartition must refer to the same frame 9

  10. H.264 Inter-CodingOverview (3/3) Rate-distortion cost of each possible partition REF : Reference parameter m( REF ) : The motion vector in the reference frame p ( REF ) : The predicted motion vector from the neighbors s : Original video signal c : Coded video signal R : Rate function of motion vectors SA(T)D : SAD or SATD 10

  11. Reference frame selector Treating 8*8 block as a minimal unit Making the selection using a mode 8*8 motion search Proposed MethodMultiple Reference Frame Selection (1/9) 11

  12. Flow chart Proposed MethodMultiple Reference Frame Selection (2/9) Input a MB and perform four 8*8 block motion search on reference frame t-1 Variance of four MVs of 8*8 blocks ≦TH? No Yes Enter 2nd stage First Stage Perform motion estimation on reference frame t-1 Output results 12

  13. Flow chart Proposed MethodMultiple Reference Frame Selection (3/9) i = 2 Perform four 8*8 block motion search of the MB on the reference frame t-i No Is i ≧ N? i + 1 Yes Is ref-frame t-k reffered by one of the four 8*8 blocks? Yes No Second Stage Drop the non-referred ref frame t-k Drop the non-referred ref frame t-k No Is k ≧ N? Yes Perform motion estimation on each valid reference frame 13 Output results

  14. First stage Disjoint 16*16 macroblock into four 8*8 blocks Motion search on ref-frame t-1 and check threshold Proposed MethodMultiple Reference Frame Selection (4/9) 4 MVs and corresponding minimal R-D cost Reference frame t - 1 Current Frame t 14

  15. Motion vectors R-D cost Check x and y components of V t-1 Less than a small threshold ? Encoder terminate early by designating only the previous frame as reference frame Proposed MethodMultiple Reference Frame Selection (5/9) Early stop criteria 15

  16. The design of threshold depends upon the computational capacity of the encoder Larger threshold, lower complexity, but worse coding efficiency Proposed MethodMultiple Reference Frame Selection (6/9) Determine upper bound of the coding efficiency and the worst case of complexity 16

  17. Second stage Motion search of mode 8*8 should be tested on all of the remaining reference frames Proposed MethodMultiple Reference Frame Selection (7/9) N : The maximal number of reference frames 17

  18. For a given block index i, we let k be set as The frame selector sets the ref-frame n-k as the valid, qualified reference frame Proposed MethodMultiple Reference Frame Selection (8/9) Block i has the best motion vector with the lowest cost, by referring to reference frame n-k 18

  19. Drop unqualified frames Proposed MethodMultiple Reference Frame Selection (9/9) N = 5 Frame t-4 Frame t-3 Frame t-2 Frame t-5 Current frame t 19

  20. Verifying the effectiveness of frame selector Measuring the hit rate by comparing retained frames with the actual frames used via exhaustive search method (hit rate = 88% ~ 95%) Proposed Method Analysis hit rate and Frame usage(1/6) 20

  21. Even with the high hit rate, we expect that frame selector can drop as many frames as possible. The false alarm ranges from 13%~32% Proposed Method Analysis hit rate and Frame usage(2/6) 21

  22. Reference frame usage of the proposed frame selector for various video sequence Proposed Method Analysis hit rate and Frame usage(3/6) 22

  23. Why using mode 8*8 but not other modes in frame selection? Hit rate False alarm Motion estimation time spending Proposed Method Analysis hit rate and Frame usage(4/6) 23

  24. Analysis of hit rate and false alarm using different modes on various sequence Proposed Method Analysis hit rate and Frame usage(5/6) 24

  25. The motion estimation time spending on a MB Judging from the hit rate, falsealarm and complexity 8*8 is the best choice Proposed Method Analysis hit rate and Frame usage(6/6) 25

  26. Reference software : JM 9.2 Intel Pentium 4 3.0 GHz with 512MB RAM Encoder parameters Experimental ResultsOverview (1/2) 26

  27. Using Fast Full Search (FFS) and Fast Motion Estimation (FME) in JM 9.2 Comparing the results with a frame selection method of Li.[1] Experimental ResultsOverview (2/2) [1] X. Li, E. Q. Li, and Y. K. Chen, “Fast multi-frame motion estimation algorithm with adaptive search strategies in H.264 ,”in Proc. IEEE Int. Conf. Acoust., Speech, Signal Process., May 2004, vol.3, pp.369–372. 27

  28. R-D curve comparison (Foreman) Experimental Results 28

  29. R-D curve comparison (Mobile) Experimental Results 29

  30. Using BDPSNR and BDBR to measure the performance difference between the methods Calculates average PSNR and bitrate distance between two RD curves of two method, respectively. Experimental Results 30

  31. Discussing the computational complexity Experimental Results 31

  32. Speed up of SAD and SATD calculations Experimental Results 32

  33. Number of MBs references in each reference frame (Mobile) Experimental Results 33

  34. Number of MBs references in each reference frame (Foreman) Experimental Results 34

  35. An efficient reference frame selector is proposed for the h.264 encoder to deal with the complexity issue pertaining to MRFME. The experimental results demonstrate that the proposed algorithm can reduce significantly the complexity of ME at the encoder end, while keeping almost the same R-D performance as FFS. Conclusions 35

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