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CABAC Based Bit Estimation for Fast H.264 RD Optimization Decision

Li Liu Xinhua Zhuang

CCNC 2009

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Outline

Introduction Based on CAVLC

[1] Mohammed Golam Sarwer and Lai-Man Po, “Fast Bit Rate Estimation for Mode Decision of H.264/AVC,” Circuits and Systems for Video Technology, IEEE Transactions on,Vol.17, no. 10, pp 1402-1407, Nov. 2007.

Based on CABAC Experimental Results Related paper

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Introduction

Rate distortion optimization(RDO) is used to decide the best coding mode of a macroblock in H.264/AVC. Efficient to improve coding performance. Considerable complexity increase of the encoder.

Lagrange cost function:

Fast bit estimation is adopted to avoid entropy coding computation during the mode decision.

)( RRRSSDJ resmotionheadermRD

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Introduction

DCT & Q Entropy coding

DCT’& Q’

RD cost calculation

Residue data

Rate Distortion

RDO computation

)( RRRSSDJ resmotionheadermRD

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Based on CAVLC

Estimate the number of bits for each of five different types of symbols of CAVLC separately.

CAVLC encoding of a block of transform coefficients proceeds as follow: Encode the number of coefficients and trailing

ones. Encode the sigh of each TrailingOne. Encode the levels of the remaining nonzero

coefficients. Encode the total number of zeros before the last

coefficient. Encode each run of zeros.

4x4 Block

Reordered Block: 0,3,0,1,-1,-1,0,1,0,0,0,0,0,0,0,0 TotalCoeff = 5; TotalZeros=3; T1s =3 (max value)

Encoding

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3

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Transmitted bitstream 000010001110010111101101 (24 bits)

CAVLC exampleStep1: Encode the number of coefficients and trailing ones.Step2: Encode the sigh of each TrailingOne.Step3: Encode the levels of the remaining nonzero coefficients.Step4: Encode the total number of zeros before the last coefficient.Step5: Encode each run of zeros.

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Based on CAVLC

Encode the number of coefficients and trailing ones. Four VLC tables are used for encoding coefficient

token. Bit consumption to encode the coefficient token is

increased with number of coefficients.

Bit consumption to encode the coefficient token is decreased with number of trailing .

x-axis: number of nonzero coefficientsy-axis: true rate of Coeff_token

1

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Based on CAVLC

The number of bits required to encode the coefficient token is :

is the total number of nonzero coefficients. is the number of trailing . are weighting constants. Experiments show that at and

better RD performance was found.

wTwTwR occoeff 321

1

T c

T o

www 321,,

121 ww 03 w

TTR occoeff

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Encode the sigh of each TrailingOne

For each , a single bit encodes the sigh(0+, 1-).

The number of bits to encode the trailing ones is :

T o

TR otrail 1

Encode the levels of the remaining nonzero coefficients From the observation of level-VLC tables, it is shown

that bit requirement is increased with magnitude of nonzero coefficients.

The number of bits to encode the level information is :

is the absolute values of all levels of quantized transform residual block.

is the sum of absolute values of all levels of quantized transform residual block.

is a positive constant. Better results were found with .

10SATwR llevel 4

TC

kkl LSAT

1

||

||Lk

SAT l

w4

14w

x-axis: SATy-axis: true rate of level

SATR llevel

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Encode the total number of zeros before the last coefficient.

From the observation of total zero VLC tables, it is shown that bit consumption to encode the total zero is increased with number of total zero.

The number of bits for total zero is :

is a positive constant. is the total number of zeros before the last

nonzero coefficients. Better results were found with .

TwR zzero 5

w5

T z

15w

TR zzero

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Encode each run of zeros

After DCT, the high-frequency coefficient usually has small energy. By quantization, more zeros are found at the high-frequency position of quantized transform block, so the value of run for high-frequency nonzero coefficients is larger.

From the observation of run VLC tables, it is shown that more bits are required for large value of run, so bit consumption is higher to encode the run of high-frequency nonzero coefficient.

4x4 Block

Reordered Block: 0,3,0,1,-1,-1,0,1,0,0,0,0,0,0,0,0

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Encode each run of zeros

The rate for run of each nonzero coefficients is :

is the frequency of th nonzero coefficient reordered block.

Example: A string of coefficients [0,3,0,1,-1,-1,-

1,0,1,0,0…] frequency of (3) is 1 frequency of (1) is 7 Better results were found with .

150,run 6R(k)

kfw k

fk k

3.06w

150,3.0runR(k)

kfk

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Total estimated bits

From previous analysis, the total estimated bits needed to encode a 4x4 residual block are:

T

runestc

kres kzeroleveltrailcoeff RRRRRR

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)()(

T

estc

res kklzc fSATTTR

1

3.0)(

x-axis: Estimation error of a symbol is the absolute difference between actual bit rate and estimated bit rate of that symbol.

Perfectly match

Most of estimation error

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Experimental Results

JM 8.3 CAVLC is enabled Frame rate is 30 Motion vector search range is 32 Number of frame is 100 Percentage of difference coding time

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Experimental Results

All frames are I frames Foreman sequence

[9] C. H. Tseng, H. M.Wang, and J. F. Yang, “Enhanced intra 4x4 mode decision for H.264/AVC coders,” IEEE Trans. Circuits Syst. Video Technol., vol. 16, no. 8, pp. 1027–1032, Aug. 2006.

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Experimental Results All frames are I frames: 47% time reduction IPP + fast motion estimation algorithm[15]

34% time reduction IBPBP + fast motion estimation algorithm[15]

32% time reduction

[15] Z. Chen, P. Zhou, and Y. He, “Fast integer pel and fractional pel motionestimation for JVT,” Joint Video Team (JVT) Docs, JVT-F017, Dec. 2002.

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Experimental Results

All I frames

IPP

IBPBP

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Experimental Results

Experiments with full search motion estimation IPP + number of frames is set to 50

17% time reduction

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Experimental Results

Comparison with other method[9] Only intra 4x4 modes are used

[9] C. H. Tseng, H. M.Wang, and J. F. Yang, “Enhanced intra 4x4 mode decision for H.264/AVC coders,” IEEE Trans. Circuits Syst. Video Technol., vol. 16, no. 8, pp. 1027–1032, Aug. 2006.

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Based on CABAC

The CABAC encoding process of a symbol consists of three elementary steps: Binarization Context modeling Binary arithmetic coding

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Flow diagram of the CABAC

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Coding of residual data with CABAC

For each block: coded_block_flag

0: The coded block flag is insignificant, no further information is transmitted.

1: a significance map specifying the positions of significant. The absolute value of the coefficient level as well as the sign is encoded for each significant transform coefficient.

Number of bits: 1 bit for each block.

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Coding of residual data with CABAC For each coefficient: significant_coeff_flag

1: current scanning position contains a significant coefficient.(nonzero)

0: current scanning position contains a insignificant coefficient.(zero)

last_significant_flag 1: current significant coefficient is the last one inside the

block. 0: current significant coefficient is not the last one inside

the block. Number of bits:

is the total number of zero coefficients before the last

nonzero coefficient. is the total number of nonzero coefficients. and are weighted coefficients.

NCNCR ssmzztMapSignifican

N s

C z C sm

N z

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Coding of residual data with CABAC

For each significant coefficient: coeff_abs_level_minus1

The absolute value of the significant coefficient level minus one.

coeff_sign_flag The sigh of the significant coefficient level. Number of bits:

NCR ssisign

26Coding of residual data with CABAC

Estimate total number of bits to encode all symbol coeff_abs_level_minus1

is the absolute value of the i-th nonzero coefficient.

is estimated number of bits to encode .

stands for average number of bits encoding absolute level value k calculated from previous frames.

Total number of bits:

coefftotal

iilevelabscoeff LR Est

_

1__ )(

Li

)(LiEst

Li

otherwiseTkif

kavgk

kEst

)(

)(

)(kavg

otherwiseQP

T15

85

RNCR levelabscoeffssilevel __

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Coding of residual data with CABAC

The total number of bits estimated to code a coefficient block using CABAC entropy coding as:

where 1__ RNCNCR levelabscoeffsszzestimate

15.2__

RNNR levelabscoeffszestimate

CCC smsis

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Experimental Results

JM 10.1 Saving ratio of total encoding time

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Experimental Results

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Experimental Results

30% RDO time saving 0.02 dB less 0.22% bit rate reducing

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Related paper

Jongmin Hahm and Chong-Min Kyung, “Efficient CABAC Rate Estimation for H.264/AVC Mode Decision, IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS FOR VIDEO TECHNOLOGY, VOL. 20, NO. 2, FEBRUARY 2010 Arithmetic coding is replaced by a table lookup .