6. H.261 Video Coding Standard

Transcription

1 6. H.261 Video Coding Standard ITU-T (formerly CCITT) H-Series of Recommendations 1. H Frame structure for a 64 to 1920 kbits/s channel in audiovisual teleservices 2. H Frame synchronous control and indication signals for audiovisual systems 3. H System for establishing communication between audiovisual terminals using digital channels up to 2 Mbits/s 4. H Video codec for audiovisual services at p 64 kbits/s 5. H Narrow-band visual telephone systems and terminal equipment 106

2 H Video codec for audiovisual services at p 64 kbits/s This recommendation describes the video coding and decoding methods for the moving picture component of audiovisual services at the rates of p x 64 kbits/s, where p is in the range of 1 to 30. Coding Control Video signal Source Coder Video Multiplex Coder Tranmission Buffer Tranmission Coder Coded bit stream (a) Video coder Video signal Source Decoder Video Multiplex Decoder Receiving Buffer Tranmission Decoder Coded bit stream (b) Video decoder Figure 6.1 Outline block diagram of the video codec 107

3 Common Input Format (CIF) Number of active lines Luminance (Y) Chrominance (Cb, Cr) Number of active pixels per line Luminance (Y) Chrominance (Cb, Cr) CIF QCIF CIF is a non-interlaced format at Hz. Conversions of other video inputs and outputs to and from CIF are not defined. All codecs must be able to operate using QCIF. 108

4 Conversion between CIF and QCIF CIF QCIF 2:1 X 1 X 2 X 3 X 4 X 5 X 6 X 7 P 1 P 2 P3 P 4 P1 1 2 / = (X + X ) 2, = (X + X ) 2, = (X + X ) 2 P2 3 4 / P3 5 6 / QCIF CIF 1:2 P 1 P2 P 3 P 4 X 1 X 2 X 3 X 4 X 5 X 6 X 7 X P = 0.75P1 2, X P P2 X P =, = 0.75P2 3, X P P3 =. 109

5 Significant Pixel Area CIF QCIF Y Y C r, Cb C r, Cb 1 Aspect ratio 4 : 3 110

6 Position of luminance and chrominance pixels Luminance sample Chrominance l Figure 6.2 4:1:1 picture format. The number of pixels per line is compatible with sampling the active portions of the luminance and chrominance signals from 525- or 625- sources at 6.75 and MHz, respectively. 111

7 Bit rate versus frame rate Frame rate Subsampling factor No. of active pixels/second CIF Mbits/s No. of active pixels/second QCIF Mbits/s 30 Hz 1 : 1 4,561, ,140, Hz 1 : 2 2,280, , Hz 1 : 3 1,520, , Hz 1 : 4 1,140, ,

8 H.261 encoder CC h t q Video in T Q VLC v Q -1 T -1 ME + F P m f T Q P F CC ME VLC Transform Quantiser Picture memory Loop filter Coding control Motion estimation Variable length coder h t v m f q Flag for INTRA/INTER Flag for transmitted or not Coded bit stream Motion vector Loop filter on/off Quantization parameter Figure 6.3 H.261 video encoder 113

9 H.261 decoder Side information Buffe Demultiplexin Variable length decoder Inverse transform + Side information Motion compensation Figure 6.4 H.261 decoder. 114

10 Macroblock (MB) structure Consists of 4 luminance (Y) blocks and two chrominance (Cb & Cr)blocks, each of 8x8 pixels Cb Cr Y Group of blocks (GOB) structure Arrangement of Mbs in a GOB 115

11 QCIF CIF Arrangement of GOBs in a picture Format No. of GOB in a frame No. of of MB in a GOB Total no. of MB in a frame CIF QCIF Relationship between number of MBs and picture format 116

12 Macroblock address (MBA) A variable length codeword indicating the position of a macroblock within a GOB. For the first transmitted MB in a GOB, MBA is the absolute address. For subsequent MBs, MBA is the difference between the absolute addresses of the MB and the last transmitted MB. VLC table for macroblock addressing MBA Code MBA Code MBA Code Macroblocks are not transmitted when they contain no information for that part of the picture. An extra codeword is available in the table for bit stuffing immediately after a GOB header or a coded macroblock (MBA stuffing). This codeword should be discarded by the decoder. 117

13 Discrete cosine transform F( u, v) f ( x, y) 1 4 = C( u) C( v) = 7 7 uπ (2x + 1) vπ (2y + 1) f ( x, y)cos cos 16 x= 0y= 0 16 uv, = 01,,..., 7 uπ (2x + 1) vπ (2y + 1) C( u) C( v) F( u, v)cos cos 16 u= 0v= , u, v = 0 C ( u), C( v) = 2, otherwise 1 x, y = 01,,..., 7 The arithmetic procedure for computing the transforms are not defined, but the inverse should meet the error tolerance specified in Annex 1. To cater for dynamic range of the DCT coefficients, 12 bits are needed. The output of inverse DCT ranges from -256 to +255 after clipping to be represented by 9 bits. 118

14 Quantization Variable thresholding (Optional) A variable threshold is applied independently of the quantization strategy to increase the number of zero coefficients. Its value depends on the length of the string of zeroes. It is assumed that the coefficients have been zig-zag scanned. ξ = g ξ max = g + g 2 Yes Coef < ξ No Yes ξ < ξ max No ξ = g ξ = ξ + 1 ξ = ξ max Coef = 0 Figure 6.5 Flowchart for variable thresholding. Example: For g = 32: Coefficients Threshold ξ New coefficients Quantised value

15 Quantization strategy The uniform quantizer is defined by a step g and controlled by the buffer state (RM8). The quantizer threshold has a value T = g. The decision levels are not defined. q dec ( n) = T + ( n 1) g, n = 1, 2,... q dec (0) = 0 Taking into account the negative values the expression becomes: q dec ( n) = n n { T + ( n 1) g}, n = 1, 2, 3,... q rec ( n) = q dec ( n) + q dec 2 ( n + n / n ) for n = 1, 2, 3,... q rec ( n) = 0 where q dec = the decision level q rec = the reconstruction level g = the quantizer stepsize T = threshold Example: A transform coefficient c with: g c < 2g is quantized to the value of 1.5g. 120

16 Quantizer characteristic q rec T+1.5g T+0.5g (T+2g) (T+g) T T T+g T+2g qdec (T+0.5g) (T+1.5g) Figure 6.6 H.261 quantizer characteristic. The INTRA dc coefficient is uniformly quantized with a step size of 8 with no dead zone and are represented with 8 bits. All other coefficients are quantized with 31 uniform quantizers with an even step size ranging from 2 to 62 and a central dead zone around zero. 121

17 Reconstruction For all coefficients other than the INTRA dc one, the reconstruction levels (REC) are in the range 2048 to 2047 and are given by clipping the results of the following formulae: For QUANT = odd, REC = QUANT*(2*LEVEL+1) ; LEVEL > 0 REC = QUANT*(2*LEVEL 1) ; LEVEL < 0 For QUANT = even, REC = QUANT*(2*LEVEL+1) 1 ; LEVEL > 0 REC = QUANT*(2*LEVEL 1) + 1 ; LEVEL < 0 REC = 0; LEVEL = 0 Note: QUANT ranges from 1 to 31 and is transmittted by either GQUANT or MQUANT. 122

18 Coding of transform coefficients (TCOEFF) Figure 6.7 Zig-zag scanning The quantized transform coefficients are sequentially transmitted according to the zig-zag scanning path. EVENTs are coded consisting of a RUN of zeroes preceding a LEVEL of non-zero magnitude. Example: (0,3) (1,2) (7,1) EOB

19 VLC tables for TCOEFF Events are coded with Huffman s algorithm. That is the more probable events are coded with shorter codewords and vice versa. However, events with very low probabilities are coded using fixed length codewords. Run Leve Code Run Leve Code l l EOB 10 ESCAPE

20 Macroblock types (MTYPE) Decision tree (RM8, without loop filter) Coded Inter Coded + Q No MC Not coded Input block Intra Coded Coded + Q Coded MC Inter Coded + Q Not coded Intra (does not exist) VLC table for MTYPE Prediction MQUANT MVD CBP TCOEFF VLC Intra x 0001 Intra x x Inter x x 1 Inter x x x Inter + MC x Inter + MC x x x Inter + MC x x x x Inter + MC + FIL x 001 Inter + MC + FIL x x x 01 Inter + MC + FIL x x x x

21 Coded block pattern (CBP) CBP is presented if indicated by MTYPE. It signifies at least one coefficient is transmitted in one of the blocks in a MB. Pattern number = 32*P1 + 16*P2 + 8*P3 + 4*P4 + 2*P5 + P6 Pn = 1 if any coefficient is present for block n, else 0. (See MB structure) VLC table for CBP CBP Code CBP Code CBP Code

22 Motion compensation Motion compensation (MC) is optional in the encoder. The decoder will accept one motion vector per MB and use it for all the blocks in the MB. Both horizontal and vertical components of motion vector are ±15 pixels. Motion vector of chrominance blocks is half of the luminance s. MC/No MC decision dbd x y = MC off MC on bd 256 Figure 6.8 MC/No MC decision. dbd denotes the displaced block difference and bd the block difference. Note that MC off includes the solid line. 127

23 Motion vector data (MVD) MVD is included for all MC macroblocks. It is obtained from the MB motion vector by subtracting the motion vector of the preceding MB. The vector of the preceding MB is taken as zero for: 1. Evaluating MVD for MB 1, 12 and Evaluating MVD for MB in which MBA does not represent a difference of MTYPE of the previous MB was not MC. It consists of a variable length codeword for the horizontal component followed by one for the vertical component. MVD Code MVD Code -16 & & & & & & & & & & & & & & & & & & & & & & & & & & & & &

24 Loop filter A low pass spatial filter is employed after MC to: Reduce the high frequency artifacts introduced by MC. Reduce the quantization noise in the feedback loop. Filter impulse response h ( m, n) = The filter is applied on a block of 8x8 pixels. It is switched on/off for all 6 blocks in a MB according to the MTYPE. Filtering inside the block boundaries

25 Intra mode decision For better performance at scene cuts, areas of fast motion and uncovered backgrounds. Better error resilience and very simple implementation of forced update. The criterion for choice of mode and transmitting a block are not subject to recommendation and may be varied dynamically as part of the coding control strategy. Intra/inter decision (RM8) VAROR inter 64 intra 64 VAR Figure 6.9 Intra/inter decision. VAROR = current block variance, VAR = block difference energy. Inter mode includes the solid line. 130

26 Video multiplexing The video multiplex is arranged in a hierarchical structure with 4 layers. Picture Group of blocks (GOB) Macroblock (MB) Block Picture layer PSC TR PTYPE PEI PSPARE GOB LAYER Picture Start Code (PSC) Temporal Reference (TR) 20 bits 5 bits Type Information (PTYPE) 6 bits Bit 1: Split screen indicator. "0" off, "1" on. 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 to 6: Spare. Extra Insertion Information (PEI) Spare Information (PSPARE) 1 bit 0/8/16/... bits 131

27 Group of blocks layer GBSC GN GQUANT GEI GSPARE MB LAYER GOB Start Code (GBSC) Group Number (GN) Quantizer Information (GQUANT) Extra Insertion Information (GEI) Spare Information (GSPARE) 16 bits 4 bits 5 bits 1 bit 0/8/16/... bits 132

28 Macroblock layer MVD MBA MTYPE MQUANT MVD CBP BLOCK LAYER CBP MBA STUFFING Macroblock Address (MBA) Type Information (MTYPE) Quantizer Information (MQUANT) Motion Vector Data (MVD) Coded Block Pattern (CBP) Variable length Variable length 5 bits Variable length Variable length Block layer TCOEFF EOB DCT Coefficients (TCOEFF) End of Block (EOB) Variable length 2 bits 133

29 Forward Error Correction for coded video signal The transmitted bitstream contains a BCH (511,493) Forward Error Correction Code. The generator polynomial is g( x) = ( x + x + 1)( x + x + x + x + 1 ) Use of this by the decoder is optional. Multipoint consideration 1. Freeze picture request Causes the decoder to freeze its displayed picture until a freeze picture release signal is received or a timeout period of six seconds has expired. 2. Fast update request Causes the encoder to encode its next picture in INTRA mode with coding parameters such as to avoid buffer overflow. The transmission of the above two signals is by external means. 3. Freeze picture release Allows a decoder to exit from its freeze picture mode and display decoded pictures in the normal manner. The signal is transmitted by bit 3 of PTYPE. 134