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ITU-T Recommendation H.261

ITU-T Recommendation H.261. H.261. The Recommendation H.261 describes the video coding and decoding methods for the moving picture component of audiovisual services ( videophone , videoconference , etc.) at the rates of p × 64 kbit/s , where p is in the range 1 to 30. H.261 Coder.

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ITU-T Recommendation H.261

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  1. ITU-T Recommendation H.261

  2. H.261 • The Recommendation H.261 describes the video coding and decoding methods for the moving picture component of audiovisual services (videophone, videoconference, etc.) at the rates of p×64 kbit/s, where p is in the range 1 to 30.

  3. H.261 Coder Video in DCT Q Inverse DCT Loop Filter Motion Compensation

  4. Motion Estimation • For each 1616 macroblock (MB), ME searches the best match in the referenced frame, and returns a motion vector MV = (X,Y). • Both X and Y have integer value not exceeding ±15. • Only the difference (residuals) between the MB and the best match is DCT encoded

  5. MVD Code … … -7&25 0000 0111 -6&26 0000 1001 -5&27 0000 1011 -4&28 0000 111 -3&29 0001 1 -2&30 0011 -1 011 0 1 1 010 2&-30 0010 3&-29 0001 0 4&-28 0000 110 5&-27 0000 1010 6&-26 0000 1000 7&-25 0000 0110 … … Coding of Motion Vectors • DPCM • Example: • 14 -13 12 … • -1 -27 25 … 0110000101000000111 …

  6. MV(1,0) MV(0,0) Scan Line Order, MB by MB • Scan Line Order • Search Range Frame n-1 Frame n H.261 VClab 2008

  7. Motion Compensation & Motion Estimation • MC is optional for each MB • Only one MV for each MB • The ME compares a 16x16 macroblock in the luminance block (Y) throughout a small search area of the previously transmitted frame • Both horizontal and vertical components of these motion vectors have integer values not exceeding ±15. • The MV is used for all 4 Y blocks. The MV for both Cb and Cr is derived by halving the component values of the MB MV. • The displacement with the smallest absolute macroblock difference, determined by the sum of the absolute values of the pel-to-pel difference throughout the block, is considered the MV for the particular MB

  8. Compare the difference between two blocks (one is in the current frame, and the other is in the reference frame) p = 1, sum of absolute difference p = 2, mean square error | |p - Candidate block Current block H.261 VClab 2008

  9. Quantization • Within a MB, the same quantizer is used for all coefficient excepts the INTRA dc one • # of quantizers is 1 for INTRA dc coefficient and 31 for all other coefficients • MB quantization factor, Q, sometimes termed MQUANT: • C(u,v) = F(u,v) / 2Q if Q is odd • C(u,v) = (F(u,v)±1)/2Q if Q is even (F>0 +, F<0-) • Quantization for INTRA dc term: • C = (F+4) / 8with inverseF = 8C =2,4, …, 62

  10. ×¼ ×½ ×¼ Loop Filter (FIL) • The filter is separable into one-dimensional horizontal and vertical functions. • The function is non-recursive with coefficients of ¼, ½, ¼ except at block edges. • The function has coefficients of 0, 1, 0 at block edges. • The filter is switched on/off for all 6 blocks in a MB according to MTYPE.

  11. H.261 Decoder Intra Inverse DCT Inter Loop Filter Motion Compensation

  12. H.261 Video Formats Y pixel Cb, Cr pixel Block boundary

  13. Arrangement of H.261 352 176 176 176 48 48 288 QCIF CIF

  14. Arrangements of H.261 176 176 144 48 GOB (Group Of Block) QCIF picture 8 16 8 8 8 16 MB (Macro Block)

  15. Positioning of luminance and chrominance samples Y pixel Cb, Cr pixel Block boundary

  16. Data Structure of Compressed Bitstream in H.261 Picture Layer GOB Layer MB Layer Block Layer Fixed Length Code Variable Length Code

  17. PSC TR PTYPE PEI … PSPARE PEI … GOB data Structure of picture layer • Picture start code (PSC) (20 bits) 0000 0000 0000 0001 0000 • Temporal reference (TR) (5 bits) It is formed by incrementing its value in the previously transmitted picture header by one plus the number of non-transmitted pictures since that last transmitted one. (Only the five LSBs used)

  18. PSC TR PTYPE PEI … PSPARE PEI … GOB data Structure of picture layer • Type information (PTYPE) (6 bits) Bit 1 Split screen indicator Bit 2 Document camera indicator, “0” off, “1” on; Bit 3 Freeze picture release, “0” off, “1” on; Bit 4 Source format, “0” QCIF, “1” CIF; Bit 5 Optional still image model HI_RES, “0” on, “1” off Bit 6 Spare where Bit 1 is MSB • Extra insertion information (PEI) (1 bit) “1” signals the presence of the following optional data field.

  19. GBSC GN GQUANT GEI … GSPARE GEI … MB data GOB Layer • Group of blocks start code (GBSC) (16 bits) • 0000 0000 0000 0001 (if “0000” followed, then it is treated as a PSC) • Group number (GN) (4 bits) • GN indicates the position of the group of blocks. 13, 14 and 15 are reserved for future use. 0 (0000) is used in the PSC.

  20. GBSC GN GQUANT GEI … GSPARE GEI … MB data GOB Layer • Quantizer information (GQUANT) (5 bits) • The quantizer to be used in the GOB until overridden by any subsequent MQUENT. • Extra insertion information (GEI) (1 bit) • “1” signals the presence of the following optional data field. • Spare information (GSPARE) (0/8/16… bits) • If GEI = “1”, then the following 8-bits data is GSPARE.

  21. MBA MTYPE MQUANT MVD CBP Block data MB Layer • Macroblock address(MBA) (Variable length: TABLE 1) • MBA indicates the position of a MB within a GOB. It is the difference between the absolute addresses of the MB and the last transmitted MB. • Type information (MTYPE) (Variable length: TABLE 2)

  22. MBA MTYPE MQUANT MVD CBP Block data MB Layer • Quantizer (MQUANT) (5 bits) • MQUANT is present only if so indicated by MTYPE (1, 3, 6, 9).

  23. MBA MTYPE MQUANT MVD CBP Block data MB Layer • Motion vector data (MVD) (Variable length: TABLE 3) • MVD is obtained from the MV (for the MB) by subtracting the vector of the preceding MB. The vector of the preceding MB is regarded as zero in the following three situations: • 1) evaluating MVD for MB 1, 12, 23. • 2)evaluating MVD for MBs in which MBA does not represent a difference of 1 • 3) MTYPE of the previous MB was not MC. • Only one of the pair will yield a MV falling within the permitted range.

  24. MBA MTYPE MQUANT MVD CBP Block data MB Layer • Coded block pattern (CBP) (Variable length: TABLE 4) • CBP is present if indicated by MTYPE (2, 3, 5, 6, 8, 9). The codeword gives a pattern number signifying those blocks in the MB for which at least one transform coefficient is transmitted. • CBP = 32P1 + 16P2 + 8P3 + 4P4 + 2P5 + P6 where Pn = 1 if any coefficient is present for block n, else 0. Cb Cr Y

  25. Block Layer • Transform coefficients (TCOEFF) (Variable length: TABLE 5) • TCOEFF is always present for all six blocks in a MB when MTYPE indicates INTRA. In other cases MTYPE and CBP signal which blocks have coefficient data transmitted for them. • The most commonly occurring combination of successive zeros (RUN) and the following value (LEVEL) are encoded with variable length codes in TABLE 5. Other combinations of (RUN, LEVEL) are encoded with a 20-bit word consisting of 6 bits ESCAPE, 6 bits RUN and 8 bits LEVEL. (Run, Level)

  26. Block Layer • There are two code tables in TABLE 5: • 1) Being used for the first transmitted LEVEL in INTER, INTER+MC, and INTER+MC+FIL blocks. (EOB is not included). • 2) Being used for all other LEVELs (EOB is included) except the first one in INTRA blocks which is fixed length coded with 8 bits. • Coefficients after the last non-zero one are not transmitted. EOB is always the last item in blocks for which coefficients are transmitted.

  27. GBSC PSC MBA GN TR MTYPE PTYPE GQUANT MQUANT PEI GEI … PSPARE … MVD GSPARE PEI CBP GEI … Block data GOB data … MB data … … Structure of H.261 Bitstream

  28. GOB Layer Coding of H.261 Bitstream Picture Layer PSC TR PTYPE PEI PSPARE GOB Layer GBSC GN GQUANT GEI GSPARE MB Layer

  29. MB Layer MVD MBA MTYPE MQUANT MVD CBP Block Layer CBP MBA stuffing Coding of H.261 Bitstream TCOEFF EOB Fixed length Variable length

  30. H.263 • H.263 = (H.261) + (MPEG-like features) • Compared to H.261 • More allowable picture formats • Half-pixel motion estimation, no loop filter • Different VLC tables at macroblock and block levels • Four negotiable options • 3~4 dB better PSNR than H.261 at <64 kbps

  31. H.263 Video Formats

  32. Four Negotiable Options • Unrestricted Motion Vector: motion vectors can point outside the picture, -31.5 to 31.5 instead of –16 to 15.5 • Advanced Prediction Mode: 8  8 motion vectors, overlapped block motion compensation, and motion vectors can point outside the picture • Syntax-based Arithmetic Coding (about 5% decreasing in bit-rate) • PB-frame

  33. H.263+ 12 Optional Modes • Annex D: New Unrestricted Motion Vector (mv range up to +/- 256) • Annex I: Advanced Intra Coding • Annex J: Deblocking Filter • Annex M: Improved PB-Frame • Annex O: Temporal, Spatial, and SNR Scalability • Annex P: Reference Picture Resampling • Annex Q: Reduced Resolution Update

  34. H.263+ Optional Modes • Annex S: Alternative Inter VLC • Annex I: Modified Quantization Error Resilience • Annex K: Slice Structured • Annex R: Independent Segment Decoding • Annex N: Reference Picture Selection

  35. Codec Implementation Issues • Fast algorithm for motion estimation • Fast algorithm for DCT/IDCT • Huffman table implementation • Program design • Program diagram • Memory assess (frame stores) • Register assignment • Program redundancy

  36. Supplemental Enhancement Information • Enhanced features • Picture freeze and release • Tagging information • Snapshot • Video segment start/end • Progressive refinement start/end • Chroma key • Can be discarded by decoders that do not understand

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