Patent Application: US-57009204-A

Abstract:
today &# 39 ; s video codecs require the intelligent choice between many coding options . this choice can efficiently be done using lagrangian coder control . but lagrangian coder control only provides results given a particular lagrange parameter , which correspond to some unknown transmission rate . on the other hand , rate control algorithms provide coding results at a given bitrate but without the optimization performance of lagrangian coder control . the combination of rate control and lagrangian optimization for hybrid video coding is investigated . a new approach is suggested to incorporate these two known methods into the video coder control using macroblock mode decision and quantizer adaptation . the rate - distortion performance of the proposed approach is validated and analyzed via experimental results . it is shown that for most bit - rates the combined rate control and lagrangian optimization producing a constant number of bits per picture achieves similar rate distortion performance as the constant slope case only using lagrangian optimization .

Description:
in the following sections this invention is described by an operational coder control using the inventional encoding process for video pictures . the described operational coder control combines the advantages of both approaches , the rate - distortion efficiency of lagrangian bit - allocation technique as well as the rate - control property of [ 9 ]. in [ 5 ], it has been observed that the lagrangian motion estimation has a very minor impact on the rate distortion performance when being used for h . 263 baseline coding . this is because the bit - rate consumed by the motion vectors assigned to 16 × 16 blocks is very small and the impact of an unsuitable choice of λ for the motion estimation process is very small . therefore , in our new encoding strategy the determination of the macroblock quantization parameters qp is based on an initial estimation of the residual signal using 16 × 16 blocks only . for that , the lagrangian parameter λ is set employing the average quantization parameter qp of the last encoded picture of the same picture type . the quantization parameters qp ( and thus the corresponding lagrangian parameters λ ) are selected similar to the approach of [ 9 ] using the estimated prediction error signals and the remaining bit budget . based on these parameters , the motion vectors as well as the macroblock and block modes are chosen by minimizing the corresponding lagrangian cost functions . since the subject of our invention is the suitable combination of two approaches concerning the operational control of the macroblock layer , the whole operational control algorithm is briefly described to avoid a misunderstanding of the concept . the main contribution is a simple low - cost solution of the interdependence problem between the lagrangian bit - allocation technique and the rate control approach . this problem is solved by introducing a low - cost pre - analysis / pre - estimation step using only 16 × 16 blocks and a single reference picture . as a consequence , some algorithmic details of the rate control approach in [ 9 ] had to be adapted . in the following sections 1 and 2 the whole algorithm of the operational control of the macroblock layer is described . experimental results comparing the performance of the proposed algorithm with the constant slope approach only using lagrangian optimization are given in section 3 . the target number of bits for a picture r total is set by a global rate control algorithm . the bit budget r b for transmitting the macroblock - layer syntax elements of that picture is initialized as where r header represents the average number of bits needed for encoding the picture and / or slice header information of the given picture type . for predictive coded pictures , an initial motion estimation step for 16 × 16 blocks and the temporally closest reference picture is performed for all macroblocks i of the picture . the corresponding initial motion vectors { circumflex over ( m )} i are obtained by minimizing the lagrangian cost function m ^ i = arg ⁢ ⁢ min m ∈ m ⁢ { d dfd ⁡ ( i , m ) + λ motion · r mv ⁡ ( i , m ) } ( 2 ) d dfd ⁡ ( i , m ) = ∑ ( x , y ) ∈ b i ⁢ ⁢  s ⁡ ( x , y , t ) - s ′ ⁡ ( x - m x , y - m y , t - δ ⁢ ⁢ t )  . ( 3 ) s ( . . . , t ) and s ′( . . . , t − δt ) represent the luminance signals of the original picture and the decoded reference picture , respectively . r mv ( i , m ) specifies the number of bits needed to transmit all components of the motion vector [ m x , m y ] t , m is the motion vector search range , and b i represents the area of the i - th macroblock . for this initial estimation step , the lagrangian multiplier λ motion is set using the average quantization parameter qp of the last encoded picture of the same picture type : based on this initial estimation or on the original source data ( for intra pictures ), a variance measure σ i 2 is calculated for each macroblock according to this invention n b and n p are the number of blocks ( luminance and chrominance ) used for transform coding inside a macroblock and the number of samples inside such a block , respectively . d i , j represents the residual signal of the block j inside the macroblock i , its average is denoted by d i , j . for intra pictures , this residual signal corresponds to the original macroblock samples , for predictive coded pictures , it represents the prediction error signal . based on the variance measures , a weighting factor α i is assigned to each macroblock i according to ( see [ 9 ]) α i = { 2 · r b / n · ( 1 - σ i ) + σ i : r b / n & lt ; 0 . 5 1 : otherwise , ( 7 ) model parameters : k 1 = k n ( last picture of same type ) c 1 = c n ( last picture of same type ) j k = 0 for the first picture of a sequence , the model parameters k 1 and c 1 are set to some predefined values . the target quantization step size q i * for the i - th macroblock is set according ( see [ 9 ]) to q i * = { max ( q min , min ( q max , k i · σ i · s i α i · ( b i - n i · c i ) ) ) : b i & gt ; n i · c i q max : b i ≤ n i · c i ( 8 ) where q min and q max are the minimum and maximum quantization step size supported by the syntax . based on the target quantization step size , the target quantization parameter qp i * is set according ( see [ 9 ]) to qp i *= max ( qp i − 1 − δqp max , min ( qp i − 1 + δqp max , f q ( q i *))), ( 9 ) where qp i − 1 is the quantization parameter of the last macroblock and δqp max is the maximum allowed quantizer changing ( given by the syntax or user - defined ). the function f q ( . . . ) specifies the mapping of quantization step sizes onto quantization parameters ; it depends on the underlying syntax . the lagrangian multipliers used for motion estimation and mode decision of the macroblock i are set according to [ 5 ] based on the chosen target quantization parameter as follows : h . 263 , mpeg - 4 :( λ motion , i ) 2 = λ mod e , i = 0 . 85 · qp i * 2 ( 10 ) h . 264 / avc :( λ motion , i ) 2 = λ mod e , i = 0 . 85 · 2 ^(( qp * i − 12 )/ 3 ) ( 11 ) for all motion - compensated macroblock / block modes the associated motion vectors m i and reference indices r i ( h . 263 annex u and h . 264 / avc ) are obtained by minimizing the lagrangian functional ( cf . ( 2 )) [ m i , r i ] = arg ⁢ ⁢ min m ∈ m , r ∈ r ⁢ { d dfd ⁡ ( i , m , r ) + λ motion , i · r mv ⁡ ( i , m , r ) } ( 12 ) at this , r denotes the set of reference pictures stored in the decoded picture buffer , m specifies the motion vectors search range inside a reference picture , t r is the sampling time of a reference picture referred by the reference index r , s ( . . . , t ) and s ′( . . . , t r ) represent the luminance signals of the original picture and the decoded reference picture , respectively , and r mv ( i , m , r ) specifies the number of bits needed to transmit all components of the motion vector m =[ m x , m y ] t as well as the reference index r . the determination of the macroblock ( or block ) modes for given macroblock ( block ) follows basically the same approach . from a given set of possible macroblock / block modes s mode , the mode p i that minimizes the following lagrangian cost function is chosen the distortion measure represents the sum of squared differences between the original macroblock / block samples s and the reconstructed samples s ′ d rec ⁡ ( i , p | qp i * ) = ∑ ( x , y ) ∈ b ⁢ ⁢ ( s ⁡ ( x , y ) - s ′ ⁡ ( x , y | p , qp i * ) ) 2 , ( 15 ) where b specifies the set of corresponding macroblock / block samples . r all ( i , p | qp i *) is the number of bits associated with choosing the mode p and the quantization parameter qp i *, it includes the bits for the macroblock header , the motion vectors and reference indices as well as the quantized transform coefficients of all luminance and chrominance blocks . the quantization parameter qp i used for transmitting the macroblock syntax elements depends on the chosen macroblock mode and its associated parameters as quantized transform coefficients . if the syntax allows a quantizer changing for the chosen macroblock parameters a quantization parameter of qp i = qp i * is chosen , otherwise the quantization parameter from the last macroblock is taken : qp i = qp i − 1 . after the encoding of a macroblock is finished , the model parameters of the operational coder control are updated . in a first step , the so - called macroblock parameters k mb and c mb are calculated according to this invention k mb = q i ** ·( r all , i − r mv ( { circumflex over ( m )} i ))/( σ i 2 ( 16 ) c mb = r mv ( { circumflex over ( m )} i ) ( 17 ) where q i ** denotes the quantization step size that corresponds to the target quantization parameter qp i *: q i **= f q − 1 ( qp i *). r all is the number of bits used for encoding the considered macroblock including all syntax elements , and r mv ({ circumflex over ( m )} i ) is the number of bits associated with the motion vector { circumflex over ( m )} i , which has been estimated in the initialization step ( sec . 1 ). the average model parameters of the currently encoded picture , k f and c f , are set according to ( see [ 9 ]) based on these parameters the model parameters used for encoding the following macroblock are updated as follows ( see [ 9 ]): remaining complexity measure : s i + 1 = s i − α i · σ i remaining macroblocks : n i + 1 = n i − 1 remaining bit budget : b i + 1 = b i − r all , i model parameters : k i + 1 = k f · i / n + k 1 ·( n − i )/ n ( 21 ) c i + 1 = c f · i / n + c 1 ·( n − i )/ n ( 22 ) the efficiency of our new encoding strategy is demonstrated for the h . 264 / avc video coding standard by comparing it to the encoding strategy only using lagrangian optimization ( for a fixed value of the quantization parameter for the whole sequence ). both encoders use only one intra picture at the beginning of the sequence , all following pictures are coded as predictive coded p - pictures . in both cases five reference pictures are used . the motion estimation is done by a logarithmic integer - pixel search over the range of [− 32 . . . 32 ]×[− 32 . . . 32 ] samples and a subsequent half - and quarter - pixel refinement . the entropy coding is done using context - adaptive binary arithmetic coding ( cabac ). for our new encoding strategy the following simple global rate control technique was used . given the number n of pictures to be encoded , the target average bit - rate r in kbit / sec , and the picture rate f in hz , the target number of bits b 1 * for the first intra picture i = 1 is determined by for all remaining p - pictures i & gt ; 1 , the target bit budget is set to b i * = 1 n - i + 1 · ( 1000 · n · r f - ∑ k = 1 i - 1 ⁢ ⁢ b k ) where b k denotes the number of bits actually consumed by the k - th picture . in the fig1 and 2 , the rate - distortion performance of both encoders is compared for two test sequences with different characteristics . the curves show the average psnr of the luminance component versus average bit - rate measured of the complete bit - stream . it can be seen , that our proposed encoding strategy provides virtually the same rate - distortion efficiency as the rate - distortion optimized encoder without rate control [ 8 ] while the target bit - rate is accurately hit . the obtained average bit - rates for our proposed encoder are shown in table 1 together with the target bit - rates . itu - t and iso / iec jtc1 , “ generic coding of moving pictures and associated audio information — part 2 : video ,” itu - t recommendation h . 262 — iso / iec 13818 - 2 ( mpeg - 2 ), november 1994 . itu - t , “ video coding for low bitrate communication ,” itu - t recommendation h . 263 ; 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