1. ITU-T Recommendation H.261
H.261 VClab 2008
2017/4/17
ITU-T Recommendation H.261

2. H.261 VClab 2008
2017/4/17
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
H.261 VClab 2008
2017/4/17
H.261 Coder
Video in
DCT Q
Inverse
DCT
Loop
Filter
Motion
Compensation

4. H.261 VClab 2008
2017/4/17
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. Coding of Motion Vectors
H.261 VClab 2008
2017/4/17
MVD Code
… …
-7&
-6&
-5&
-4&
-3&
-2&
0 1
2&
3&
4&
5&
6&
7&
Coding of Motion Vectors
DPCM
Example: … … …

6. Scan Line Order, MB by MB Scan Line Order Search Range Frame n-1
H.261 VClab 2008
2017/4/17
Scan Line Order, MB by MB
Scan Line Order
Search Range
Frame n-1
Frame n
MV(1,0)
MV(0,0)
2017/4/17
H.261 VClab 2008

7. Motion Compensation & Motion Estimation
H.261 VClab 2008
2017/4/17
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. H.261 VClab 2008
2017/4/17
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
2017/4/17
H.261 VClab 2008

9. H.261 VClab 2008
2017/4/17
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) / 8 with inverse F = 8C
=2,4, …, 62

10. H.261 VClab 2008
2017/4/17
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. ×¼ ×½

11. H.261 Decoder Intra Inverse DCT Inter Loop Filter Motion Compensation
H.261 VClab 2008
2017/4/17
H.261 Decoder
Intra
Inverse
DCT
Inter
Loop
Filter
Motion
Compensation

12. H.261 Video Formats Luminance (Y) Chrominance(Cb, Cr) CIF 352 288 176
H.261 VClab 2008
2017/4/17
H.261 Video Formats
Video
Format
Luminance (Y)
Chrominance(Cb, Cr)
pixels/line
lines/frame
CIF
352
288
176
144
QCIF 88 72
Y pixel
Cb, Cr pixel
Block boundary

13. H.261 VClab 2008
2017/4/17
Arrangement of H.261
352
176
176
176 48 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 48
288
QCIF
CIF

14. Arrangements of H.261 1 2 3 Y1 Y2 Y3 Y4 U V 176 176 1 2 3 4 5 6 7 8 9
H.261 VClab 2008
2017/4/17
Arrangements of H.261
176
176 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 1 2 3
144 48
GOB (Group Of Block)
QCIF picture Y1 Y2 Y3 Y4 8 16 U 8 V 8 8 16
MB (Macro Block)

15. Positioning of luminance and chrominance samples
H.261 VClab 2008
2017/4/17
Positioning of luminance and chrominance samples
Y pixel
Cb, Cr pixel
Block boundary

16. Data Structure of Compressed Bitstream in H.261
H.261 VClab 2008
2017/4/17
Data Structure of Compressed Bitstream in H.261
Picture
Layer
Picture Header
GOB data …
GOB
Layer
GOB Header
MB data … MB
Layer
MB Header
Block data …
Block
Layer
TCOEFF …
EOB
Fixed Length Code
Variable Length Code

17. Structure of picture layer
H.261 VClab 2008
2017/4/17
Structure of picture layer
PSC TR
PTYPE
PEI
PSPARE …
GOB data
Picture start code (PSC) (20 bits)
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. Structure of picture layer
H.261 VClab 2008
2017/4/17
Structure of picture layer
PSC TR
PTYPE
PEI
PSPARE …
GOB data
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. GOB Layer Group of blocks start code (GBSC) (16 bits)
H.261 VClab 2008
2017/4/17
GOB Layer
GBSC GN
GQUANT
GEI
GSPARE …
MB data
Group of blocks start code (GBSC) (16 bits)
(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. GOB Layer Quantizer information (GQUANT) (5 bits)
H.261 VClab 2008
2017/4/17
GOB Layer
GBSC GN
GQUANT
GEI
GSPARE …
MB data
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. MB Layer Macroblock address(MBA) (Variable length: TABLE 1)
H.261 VClab 2008
2017/4/17
MB Layer
MBA
MTYPE
MQUANT
MVD
CBP
Block data
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. MB Layer Quantizer (MQUANT) (5 bits)
H.261 VClab 2008
2017/4/17
MB Layer
MBA
MTYPE
MQUANT
MVD
CBP
Block data
Quantizer (MQUANT) (5 bits)
MQUANT is present only if so indicated by MTYPE (1, 3, 6, 9).

23. MB Layer Motion vector data (MVD) (Variable length: TABLE 3) MBA MTYPE
H.261 VClab 2008
2017/4/17
MB Layer
MBA
MTYPE
MQUANT
MVD
CBP
Block data
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. MB Layer Coded block pattern (CBP) (Variable length: TABLE 4) 1 2 3 4
H.261 VClab 2008
2017/4/17
MB Layer
MBA
MTYPE
MQUANT
MVD
CBP
Block data
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. 1 2 3 4 Cb 5 Cr 6 Y

25. Block Layer Transform coefficients (TCOEFF) (Variable length: TABLE 5)
H.261 VClab 2008
2017/4/17
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:
H.261 VClab 2008
2017/4/17
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. Structure of H.261 Bitstream
H.261 VClab 2008
2017/4/17
Structure of H.261 Bitstream
PSC TR
PTYPE
PEI
PSPARE …
GOB data
GBSC GN
GQUANT
GEI
GSPARE
MB data
MBA
MTYPE
MQUANT
MVD
CBP
Block data

28. Coding of H.261 Bitstream Picture Layer PSC TR PTYPE PEI PSPARE
H.261 VClab 2008
2017/4/17
Coding of H.261 Bitstream
Picture Layer
PSC TR
PTYPE
PEI
PSPARE
GOB Layer
GOB Layer
GBSC GN
GQUANT
GEI
GSPARE
MB Layer

29. Coding of H.261 Bitstream MBA MTYPE MQUANT MB Layer MVD CBP
H.261 VClab 2008
2017/4/17
Coding of H.261 Bitstream
MBA
MTYPE
MQUANT
MB Layer
MVD
CBP
Block Layer
MBA stuffing
TCOEFF
EOB
Fixed length
Variable length

30. H.263 H.263 = (H.261) + (MPEG-like features) Compared to H.261
H.261 VClab 2008
2017/4/17
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 Sub-CIF QCIF CIF 4CIF 16CIF Pels/line 128 176 352
H.261 VClab 2008
2017/4/17
H.263 Video Formats
Sub-CIF
QCIF
CIF
4CIF
16CIF
Pels/line
128
176
352
704
1408
Lines 96
144
288
576
1152

32. Four Negotiable Options
H.261 VClab 2008
2017/4/17
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.261 VClab 2008
2017/4/17
H 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
H.261 VClab 2008
2017/4/17
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
H.261 VClab 2008
2017/4/17
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
H.261 VClab 2008
2017/4/17
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