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H.264 to VC 1 Transcoding

H.264 to VC 1 Transcoding. Vidhya Vijayakumar Multimedia Processing Lab MSEE, University of Texas @ Arlington vidhya.vijayakumar@mavs.uta.edu Guided by Dr. KR Rao. What is…. H.264 The new industry standard

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H.264 to VC 1 Transcoding

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  1. H.264 to VC 1 Transcoding Vidhya Vijayakumar Multimedia Processing Lab MSEE, University of Texas @ Arlington vidhya.vijayakumar@mavs.uta.edu Guided by Dr. KR Rao

  2. What is… • H.264 • The new industry standard • Massive quality, Minimal files • Scalable from 3G to HD and Beyond • VC 1 • Informal name of the SMPTE 421M video codec • Standard initially developed by Microsoft • Supported standard for HD DVDs, Blu-ray Discs, and Windows Media Video

  3. What is… • Transcoding • Converting a previously compressed video signal into another one with different format • Change in bit rate, frame rate, frame size, or even compression standard • 2 Ways • Decode fully and encode in target standard • Change the bit stream format from one standard to another without its undergoing the complete decoding and encoding process. • Limitations • Compression artifacts are cumulative

  4. Why Trancode H.264 to VC-1? • The two high definition DVD formats HD-DVD and Blue ray have mandated MPEG-2, H.264 and VC-1 as video compression formats • As H.264 based and VC-1 based content and products become available, transcoding in both directions will become widely used capabilities. • From an end user point of view, any VC-1 decoder can now become twice as powerful as it was earlier.

  5. Why VC1? • Requires less computational power and can be decoded at full 1080i/p resolution on today’s off-the-shelf PC • Advanced Profile delivers compression efficiencies far superior to MPEG-2 • Delivers HD content at bit rates as low as 6-8 Mbps • Better visual quality against H.264 and MPEG-2 demonstrated in independent tests

  6. More of VC1… • DCT-based video codec design • Coding tools for interlaced video sequences as well as progressive encoding • 8-bit, 4:2:0 format • Uses block based transform and motion compensation with quantization and entropy coding.

  7. Decoder – Simple & Main profile

  8. Decoder – Advanced Profile

  9. 8x8 blocks can be encoded using 1_8x8 2_8x4 2_4x8 4_4x4 Frame / Macroblock/Block signaling Block level for coarse and fine level specification Frame level for overhead reduction Only 8x8 used for I frames Block Transforms (Integer DCT)

  10. WMV 9 H.264 HP 8x8 Integer DCT Matrices

  11. Key features of the Transforms • The norms of vectors of the ratio 288:289:299 • The variation in the norm accounted for in the encoder itself • At the decoder, inverse transform (rows) -> rounding-> inverse transform (columns) ->rounding (to operate in the 16 bit range)

  12. Quantization • Same rule applied to all block sizes • Both types with (bit savings at low bit rates) and without dead zone available • Type used signaled at the frame level to the decoder • At the encoder side automatic switch from uniform quantization to dead zone quantization as Q – parameter increases • Other factors like noise and rate control can be used to control this switch

  13. Loop Filtering • Done to remove blocky artifacts and thus quality of current frame for future prediction • Operates on pixels on the border of blocks • The process involves a discontinuity measurement • Checks are computationally expensive so done only for one set of pixel per boundary

  14. Motion Estimation and Compensation • Max resolution of ¼ pixel (i.e. ¼, ½, ¾) allowed • 16x 16 motion vectors used by default but 8x8 allowed • Bicubic filter with 4 taps/ Bilinear filters with 2 taps to generate subpixel precision. • 4 combined modes 1.Mixed block size (16x16 and 8x8), ¼ p ,bicubic 2.16x16, ¼ p, bicubic 3.16x16, ½ p, bicubic 4.16x16, ½ p, bilinear • Bilinear filters for chroma components

  15. Advanced entropy coding • Simple VLC codes • Multiple code tables for encoding each particular alphabet out • A possible set of code tables is chosen (based on frame level quantization parameter) and signaled in the bitstream • Additional information e.g. motion vectors resolution coded using bitplane coding

  16. Interlacedcoding • Supports field and frame coding

  17. Advanced B frame coding • B frames:- employ bi-directional prediction • Fractional position definition with respect to the reference frames for better scaling of motion vectors • Intra coded B frames for scene changes • Allow inter field reference

  18. Overlap smoothing • The deblocking filter smoothens out the block as well as true edges and it may be disabled in less complex profiles • A lapped transform (input spans to pixels from other blocks as well) is used at the edges • Used in spatial domain as pre and post processing • Used only at low bit rates where blocking artifacts are higher • Signaled at macroblock level so can be turned off in smooth regions

  19. Low rate tools (<100 Kbps) • Code frames at multiple resolutions (both in X and Y direction) • A frame can be downscaled at the encoder and then upscaled at the decoder for LBR transmission • The downscaling factor needs to remain same from the start of 1 I frame to the start of next I frame. • The frame must be upscaled prior to display (upscaling out of scope of the standard).

  20. Fading compensation • Large amount of bits required for scenes having effects like fade-to-black ,fade-from-black • Not possible to predict motion using normally used techniques. • Effective fading detection (original reference image- current video image > threshold = fading) • If detected then encoder computes fading parameters which specify a pixel-wise first order linear transform for the reference image. • Also signaled to the decoder

  21. Profiles

  22. Comparison of H.264 and VC-1

  23. Comparison of H.264 and VC-1

  24. Comparison of H.264 and VC-1

  25. 8x8, 4x8, 8x4, 4x4adaptive block size • Frequency-independent dequantization scaling • VLC-based entropy coding • 4 tap bicubic filters for MC • Relatively-simple loop filter • Overlap intra filtering • Range reduction/expansion • Resolution red./exp. • 8x8 and 4x4adaptive block size • Frequency-dependent dequantization matrix • CABAC or VLC • Long filters for MC • Complex loop filter • Spatial intra prediction • Multi-picture arbitrary-order referencing • Intra PCM Block motion 16-bit integer transforms Bit-exact spec Fading prediction Loop filter VC-1 H.264 Graphically…

  26. Transcoding point of view Adaptive In High profile

  27. Stepping forward… • Algorithm to deduce the picture type in VC-1 from H.264 picture types • Algorithm to effectively handle transform size mismatch between H.264 and VC-1 • Algorithm to choose the best reference picture of H.264 to be used for MC in VC-1

  28. References • An efficient algorithm for VC-1 to H.264 video transcoding in progressive compression - Jae-Beom Leeand Hari Kalva • http://www.avsforum.com/avs-vb/showthread.php?p=9931723&&#post9931723 • http://www.microsoft.com/windows/windowsmedia/howto/articles/vc1techoverview.aspx • Windows Media Video 9: overview and applications Sridhar Srinivasan, Pohsiang (John) Hsu, TomHolcom b, Kunal Mukerjee, Shankar L. Regunathan, Bruce Lin, Jie Liang, Ming-Chieh Lee, Jordi Ribas-Corbera Windows Digital Media Division, Microsoft Corporation, Redmond, WA 98052, USA, available online at www.sciencedirect.com

  29. Thank You Vidhya Vijayakumar vidhya.vijayakumar@mavs.uta.edu

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