WITH the rapid development of visual communication

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1 IEEE TRASACTIOS O IMAGE PROCESSIG, VOL. 25, O. 7, JULY Low-Delay Rate Control for Consstent Qualty Usng Dstorton-Based Lagrange Multpler Maohu Wang, Member, IEEE, Kngggan,Fellow, IEEE, and Honglang L, Senor Member, IEEE Abstract Vdeo qualty fluctuaton plays a sgnfcant role n human vsual percepton, and hence, many rate control approaches have been wdely developed to mantan consstent qualty for vdeo communcaton. Ths paper presents a novel rate control framewor based on the Lagrange multpler n hgh-effcency vdeo codng. Wth the assumpton of constant qualty control, a new relatonshp between the dstorton and the Lagrange multpler s establshed. Based on the proposed dstorton model and buffer status, we obtan a computatonally feasble soluton to the problem of mnmzng the dstorton varaton across vdeo frames at the codng tree unt level. Extensve smulaton results show that our method outperforms the rate control used n HEVC Test Model HM) by provdng a more accurate rate regulaton, lower vdeo qualty fluctuaton, and stabler buffer fullness. The average pea sgnal-to-nose rato PSR) and PSR devaton mprovements are about 0.37 db and 57.14% n the low-delay P and B) vdeo communcaton, where the complexty overhead s 4.44%. Index Terms Rate control, dstorton model, Lagrange multpler, HEVC. I. ITRODUCTIO WITH the rapd development of vsual communcaton n recent years, effcent vdeo compresson technques have been consdered by the multmeda communty. To meet ths demand, ITU-T/SG16/Q6 VCEG) and ISO/IEC JTC1/SC29/WG11 MPEG) have establshed the Jont Collaboratve Team on Vdeo Codng JCTVC) to develop the latest vdeo codng standard Hgh Effcency Vdeo Codng HEVC) [1]. As HEVC nvolves many sophstcated codng features and technques, t has sgnfcantly mproved the compresson performance n comparson wth the prevous vdeo codng standards, such as MPEG-2, H.263, MPEG-4 and H.264/Advanced Vdeo Codng AVC) [2]. For example, HEVC supports dverse bloc szes, flexble quad-tree Manuscrpt receved December 28, 2014; revsed May 13, 2015, January 10, 2016, and February 22, 2016; accepted Aprl 5, Date of publcaton Aprl 11, 2016; date of ent verson May 11, Ths wor was supported n part by the Research Grants Councl, Hong Kong, under Grant CUHK415712, n part by the atonal atural Scence Foundaton of Chna under Grant , and n part by the Program for Scence and Technology Innovatve Research Team for Young Scholars n Schuan Provnce, Chna, under Grant 2014TD0006. The assocate edtor coordnatng the revew of ths manuscrpt and approvng t for publcaton was Prof. Rcardo L. De Queroz. M. Wang and K.. gan are wth the Department of Electronc Engneerng, The Chnese Unversty of Hong Kong, Hong Kong e-mal: wang.maohu@gmal.com; nngan@ee.cuh.edu.h). H. L s wth the School of Electronc Engneerng, Unversty of Electronc Scence and Technology of Chna, Chengdu , Chna e-mal: hll@uestc.edu.cn). Color versons of one or more of the fgures n ths paper are avalable onlne at Dgtal Object Identfer /TIP structure and effcent flters. However, t s worth notng that the above vdeo standards only specfy the syntax of decodng the btstream. Meanwhle, the compressed vdeo qualtes depend consderably on mplementng the rate control scheme on the encoder sde. Thus, computatonal rate control algorthms are wdely nvestgated after the nternatonal vdeo standards have been developed. ormally, varous vdeo applcatons, le vdeo broadcastng and vdeo survellance, are transmtted va a constant bt rate CBR) channel. To mantan a short-term constant output bt rate n the CBR channel, tradtonal codecs adopt a unform bandwdth allocaton scheme n a group of pctures GOP). However, the number of encodng bts changes from frame to frame owng to the tme-varyng vdeo complexty. In ths case, t s nfeasble to adjust encodng parameters to acheve the exact fxed bandwdth for each frame n a GOP. Consequently, an assocate encoder buffer s usually employed to regulate the output bts before transmttng. If the channel bandwdth s less than the output bt rate, the encoded bts wll accumulate n the encoder buffer. When the sze of the accumulated bts s too large, the encoder needs to sp some frames to allevate the buffer delay and avod the buffer overflow. Conversely, f the channel bandwdth s larger than the output bt rate, ths ndcates that some channel s wasted, and t may cause buffer underflow. Snce the buffer overflow and underflow result n undesrable effects on the vdeo qualty fluctuaton, t s essental to control the bt rate to mantan a consstent qualty over the entre vdeo sequence. In real-tme vdeo communcatons, rate control becomes more challengng as t needs to satsfy low-latency of transmttng vdeo data. In such a case, the encoder buffer must mantan a very small sze, and therefore, the encoder requres a more accurate bt allocaton and codng settngs to reduce the fluctuatons of buffer fullness as well as to avod the undesrable buffer overflow and underflow. The conventonal rate control schemes usually nvolve two steps: 1) the target bt s allocated to each basc unt accordng to ts relatve complexty and the buffer status, and 2) quantzaton parameter QP) s computed by rate-quantzaton R-Q) models, such as quadratc model [3] and ρ-doman model [4]. It should be ponted out that wth tradtonal R-Q models, QP can determne the bts for resdue nformaton.e., quantzed transformed coeffcent QTC)) but not for non-resdue nformaton.e., partton mode, moton and other header nformaton). Although the overhead bts can be predcted from the prevous frames, the accuracy of predcton s stll not well addressed n both H.264/AVC and HEVC IEEE. Personal use s permtted, but republcaton/redstrbuton requres IEEE permsson. See for more nformaton.

2 2944 IEEE TRASACTIOS O IMAGE PROCESSIG, VOL. 25, O. 7, JULY 2016 owng to tme-varyng vdeo complexty. On the other hand, smlar to H.264/AVC, the reference software of HEVC utlzes rate dstorton optmzaton RDO) to determne the encodng settngs so as to obtan the best rate-dstorton R-D) performance. To solve ths RDO problem, the Lagrangan method s used to acheve the optmal trade-off between rate and dstorton on the encoder sde. Addtonally, t can be seen that the Lagrange multpler affects the total bts, ncludng both the resdue and non-resdue bts. In the HEVC reference software, smulaton results ndcate that the Lagrange multpler s more senstve, smple and effcent n controllng the output target bts n comparson wth QP, and not surprsngly, an nterestng relatonshp between the target bts and Lagrange multpler λ also called Rate-lambda or R-λ model) was developed for HEVC n JCTVC-K0103 [5]. In the R-λ model, λ s frst determned by the target bts. Then, QP s computed by a logarthmc functon of λ n the HEVC reference software. Although the R-λ method has shown a sgnfcant mprovement n comparson wth conventonal methods, there are two man dffcultes. Inaccurate bt allocaton: The number of bts for a codng tree unt CTU) reles heavly on the frame budget, the prevous overhead bts and the weghts of CTUs n the ent frame, whch s explctly determned one-by-one n a raster-scannng order. In practce, the frame bandwdth can be easly consumed by the frst several CTUs due to naccurate estmaton of the model parameters. Hence, the buffer state greatly affects the target bts for the subsequent CTUs n a frame, and the raster-scannng bt allocaton scheme can cause naccurate bt rates and adverse effects on the overall qualty control. Inaccurate λ estmaton: As mentoned earler, snce there s the bt allocaton problem, λ adjustment s frequently appled for CTUs to acheve the frame budget. Specfcally, an extremely large or small) λ s usually computed accordng to the target bts and buffer fullness. To mantan λ n a reasonable range, many bound and crop operatons are employed on the encoder sde. For example, the HEVC reference software allows the maxmum λ varaton n terms of QP up to 10 between two successve frames whch results n a large qualty fluctuaton. Thus, naccurate λ greatly degrades the fnal rate control result. Although some mprovements [6], [7] n the R-λ models have been reported recently, the two above-mentoned lmtatons are stll not beng addressed satsfactorly. Ths paper proposes a new dstorton based Lagrange multpler method to mprove the HEVC rate control n low-delay communcaton. The major nnovatons of ths paper can be summarzed n three aspects. Frstly, wth the assumpton of consstent vdeo qualty codng, a new relatonshp between the dstorton and λ s establshed, whch can be used to control the vdeo qualty fluctuatons va λ. Secondly, a computatonally feasble soluton to the problem of mnmzng dstorton across the vdeo frames at the CTU level s obtaned based on the above dstorton model, whch can avod the raster-scannng bt allocaton. Thrdly, accordng to buffer fullness, the CTU level λ s adjusted such that the target bts satsfy the overall bandwdth n low-delay vdeo communcatons. The rest of the paper s organzed as follows. We ntroduce the related wor n Secton II. The new dstorton model and the assocated rate model are descrbed n Secton III. The proposed rate control algorthm s then mplemented n Secton IV. The smulaton results are presented n Secton V, and Secton VI contans the concludng remars. II. RELATED WORK I HEVC Recently, many rate control methods have been consdered n the HEVC vdeo codng system. In ths secton, we gve a bref revew of the dfferent rate models to facltate the study of the rate control algorthms. In HEVC, the rate control models can be roughly categorzed nto R-Q and R-λ models. A. R-Q Models Cho et al. proposed a pxel-wse unfed R-Q model URQ) [8] that s a drect extenson of the quadratc R-Q rate control method n the early HEVC reference software HM6.0). The quadratc R-Q model s frst developed for hybrd vdeo codng by Chang and Zhang [3] whch has been wdely studed n H.264/AVC [9]. Smlar to the quadratc R-Q method n H.264/AVC, n URQ, the target bt s allocated based on the mean absolute dfference MAD) - based complexty, and the quantzaton step s computed on the encoder sde. A detaled descrpton of URQ can be found n ther recent wor [10]. Although URQ shows a better performance to control the output bts than the anchor HM6.0, two major lmtatons of the quadratc model are not avoded. Frstly, the quadratc model only determnes the bts for the resdue nformaton but not for the non-resdue nformaton. Secondly, the quadratc model contans an nter-dependency relatonshp between the RDO process and the computaton of quantzaton step, whch results n a well-nown chcen and egg dlemma [11]. Addtonally, a general R-Q model s studed n HEVC, whch s based on the number of QTCs. He and Mtra [4] proposed the ρ-doman rate model for smple bloc structure based vdeo codng, where ρ s the percentage of zeros among QTCs. The ρ-doman model s based on the observaton that there s an approxmated lnear relatonshp between bts and ρ n a sngle transform bloc sze. Recently, Wang et al. developed a quadratc ρ-doman based Rate-GOP method [12] n HEVC. Wth Rate-GOP algorthm, QP can frst be determned by the pcture order count POC), and then the bts of non-zero QTCs are smulated by a quadratc functon of quantzaton step. Although smulaton results have shown ts superorty compared to tradtonal rate control methods, further nvestgaton s necessary to verfy the ρ-doman model n HEVC. Specfcally, n [13] and [14], experments show the facts that sgnal characterstcs wth respect to the codng unt depth levels are consderably dfferent. In addton, the varances of the transform coeffcents of ntra frames are sgnfcantly dfferent from that of nter frames. Thus, n the HEVC encoder, the ρ-doman methods should be modeled separately due to the depth of the codng unts and the types of

3 WAG et al.: LOW-DELAY RATE COTROL FOR COSISTET QUALITY USIG DISTORTIO-BASED LAGRAGE MULTIPLIER 2945 encoded frames. Unfortunately, ths s stll not well addressed n the Rate-GOP method. B. R-λ Models To avod the aforementoned problems, the Lagrange multpler based rate control methods have been studed n H.264/AVC and HEVC. Wth the RDO scheme on the encoder sde, larger λ s related to larger dstorton and fewer bt, and the converse holds for smaller λ. Furthermore, snce QP adjustments wll cause addtonal overhead bts, λ s used to adjust the rate and dstorton durng the actual encodng stage. Tang nto account ths observaton, Jang and Lng [15], [16] proposed to adjust λ adaptvely accordng to the rate cost. In [17], Wang and Yan establshed a new relatonshp between λ and MAD. Snce the MAD has been ntegrated nto the quadratc R-Q model n H.264/AVC, t ndcates that the relatonshp between rate and λ has been mplctly establshed. owadays, L et al. [5] see a detaled descrpton n [18]) developed a new relatonshp between rate and λ. Compared to the R-Q method, the R-λ model consders the overall bt rate, ncludng both the resdue and non-resdue bts. Addtonally, as the R-λ method outperforms the URQ method n terms of bt estmaton accuracy and vdeo qualty control, t has been recommended for HEVC by the JCTVC. However, t should be ponted out that the R-λ model only consders the target bt but gnores the characterstcs of vdeo data n frame level rate control. Thus, the frame-content complexty has been consdered to mprove the R-λ model. For example, Wang and Karczewcz [6] proposed to use summaton of absolute transformed dfferences SATD) to measure the complexty of an ntra frame. In [7], Wang and gan proposed a gradent based R-lambda GRL) model for ntra frame rate control. Smulaton results show that the SATD and gradent can be used to effectvely measure the frame-content complexty and enhance the performance of ntra-frame rate control. III. MODELIG RATE AD DISTORTIO I HIGH EFFICIECY VIDEO CODIG Rate-dstorton optmzaton has been wdely studed n bloc-based vdeo codng systems, such as n the reference software of H.264/AVC and HEVC. The R-D relatonshp ndcatesthat the hgherthe rate R s, the lower s dstorton D, and vce versa. Thus, the fundamental problem n rate control s to mnmze the dstorton subject to a gven rate constrant R max [19]. mn D, s.t. R R max, 1) Usually, ths constraned problem can be solved by the Lagrangan optmzaton method [20] n hybrd vdeo codng. The Lagrangan cost functon s J = D + λ R R max ), 2) where λ s the Lagrange multpler. Addtonally, t s noted that the R-D curve s convex n vdeo codng, and f we assume that both R and D are dfferentable everywhere, λ can be expressed by λ = D R. 3) As shown n equaton 2), R and D for a gven encodng bloc or frame depend on the scalng factor λ. Conventonally, λ s obtaned from the nput QP [21]. The relatonshp between R-Q and D-Q.e., dstorton-quantzaton) models has been studed and used for rate control n H.263, H.264/AVC and MPEG-4 codecs. However, t s nterestng to note that QP only determnes the resdue bts, but λ determnes the overall rate cost, ncludng both resdue and non-resdue bts. Furthermore, λ also affects the encodng mode selecton, such as parttons and motons. Thus, n ths secton, the essental goal s to model the effect of λ on dstorton and rate. We concentrate on modelng dstorton and rate for moton-compensated predcton MCP) frames. It should be noted that smlar models can be establshed for ntra frames, but the related models wll not be derved here because the ntra frames are not frequently used n low-delay vdeo communcatons. A. Dstorton Modelng In the past decades, many R-D models [22] [24] have been proposed for rate control. For example, n H.264/AVC and MPEG-4, the R-D curves can be modeled by a logarthmc expresson. Specfcally, ths logarthmc model has been proved assumng a hgh-rate envronment e.g., above 0.5 bts/pxel) [25]. However, n the HEVC reference software, snce the encodng effcency s greatly mproved, the tradtonal R-D model s not approprate for the smulaton of the encoder n the low-rate case. In the meantme, as shown n equaton 3), the more accurate the R-D model s, the better λ can be obtaned. Therefore, n HEVC, a more nterestng hyperbolc R-D model s proposed n equaton 4), where ths model was ntroduced by Mallat n 1998 [26]. D R) = K R C, 4) where C and K are the model parameters related to the characterstc of the vdeo source. Puttng equaton 4) nto equaton 3), we now that λ can be re-wrtten as λ = D R = C K R C 1 = C K C 1 C+1 D C = γ D τ, 5) where γ and τ are both codng constants. The dstorton measure D s the mean squared error MSE) between the orgnal and reconstructed CTUs or frames. MSE s a mathematcally tractable and fast-to-compute full-reference FR) qualty metrc, whch has been wdely used n modern bloc-based vdeo compresson. To valdate the relatonshp between D and λ, we have compressed several vdeo sequences n the low-delay case usng the HEVC reference software HM10.0 and the smulaton results are shown n Fg. 1. For several values of λ, we plot Dλ) as a functon of λ and ft the data va the formula n 5), where the average values are used. Ths experment

4 2946 IEEE TRASACTIOS O IMAGE PROCESSIG, VOL. 25, O. 7, JULY 2016 Fg. 1. Relatonshp between D and λ. The color curves are the fttng results of λ = γ D τ. Each sample pont represents one test vdeo, and the data ponts wth the same color denote the encodng results assocated to the same vdeo sequence. confrms that n the HEVC vdeo codng system, the dstorton curves can be approxmated by a power formula. From now on, we wll focus on the consstent vdeo qualty modelng n whch the more nterestng Dλ) relatonshp between two consecutve frames s establshed. The dstrbuton of dstorton between two consecutve frames s crucal n mantanng consstent qualty control n vdeo codng. For consstent qualty vdeos, t s observed that the dstorton of the ent CTU s dstrbuted smlarly to that of the co-located poston n the prevous frame. Hence, we ncorporate the hstory of codng nformaton nto the dstorton model to avod the complexty of parameter estmaton n equaton 5). In addton, to mantan a consstent qualty, we assume D D prev. Puttng equaton 5) D nto ths assumpton, we can get λ D prev, where D and λ are the dstorton and the Lagrange multpler for the th CTU n the ent frame. Smlarly, D prev and are for the co-located CTU n the prevous frame. Wth ths assumpton, we can construct a heurstc lnear dstorton model D D prev λ. In order to remove large fluctuaton durng encodng, both and D prev can be computed as the weghted averages from the prevous frames [14] such as = 1 l l = 1 ω and D prev = 1 l l = 1 ω D prev,where l s the total number of the most recent encoded frames, and ω s the assocated weght. On the other hand, from the mplementaton pont of vew, we need to reduce the computatonal complexty and save the storage of the encoder. Thus, we mae a trade-off between model accuracy and mplementaton complexty,.e., = 1 = 1,andD prev 1 =1 s the actual dstorton of the th CTU n the prevous frame. Consequently, a lnear dstorton model s completed as, D = D prev λ, 6) where s a scalng factor that reduces the speed of dstorton changes between the co-located CTUs. Fg. 2. Dfference between the actual dstorton and the estmated dstorton per CTU of the 11th frame for a) BasetballDrll, b) BQMall, c) BQTerrace and d) Cactus QP=32). TABLE I COMPARISOS OF DIFFERET DISTORTIO MODELS Extensve experments have been performed to verfy the proposed dstorton model, and four typcal results are presented n Fg. 2 that shows the dstorton estmaton results for each CTU n low-delay vdeo applcaton. The encodng frame s the 11th frame. The dstorton of each CTU n the ent frame s estmated by the co-located dstorton n the 10th encoded frame. The scalng factor s chosen to reduce the estmaton error. For example, = 1.30, 1.34, 1.26 and 1.33 are the optmal values n MSE sense) for a), b), c) and d), respectvely. However, n ths paper, we do not explore the use of scalng factor for a better estmaton. Instead, as we wll see later n Secton IV-A, s not requred n the computaton of λ at the CTU level. More detaled numercal experments are tabulated n Table I. Pearson s correlaton coeffcent PCC) and normalzed root mean square error RMSE) are computed to measure the estmaton accuracy and estmaton error, respectvely. The RMSEsdefnedasn RMSE = 1 D act,max D act,mn ) ) 2 =1 Dact, D est,, ctu 7)

5 WAG et al.: LOW-DELAY RATE COTROL FOR COSISTET QUALITY USIG DISTORTIO-BASED LAGRAGE MULTIPLIER 2947 where ctu s the number of encoded CTUs n a frame, D act and D est are the actual dstorton and estmated dstorton of the th CTU, D act,max and D act,mn are the maxmum and mnmum values of D act, respectvely. Table I shows the comparson between the proposed method and a typcal dstorton model that uses the neghborng nformaton to estmate the ent one. Typcally, the neghborng reconstructed error D 1 s consdered as the predcted dstorton of the ent D,.e., D = D 1. The PCCs of the proposed lnear dstorton method 6) range from 0.91 to In addton, the average RMSE of our method s , whle the neghborng method s It can be seen that our dstorton model s able to acheve hgher estmaton accuracy than the tradtonal neghborng estmaton method. B. Rate Modelng In HEVC, the rate of the th CTU n a frame s a functon of λ as shown n equaton 5). For convenence, equaton 5) s re-wrtten as R = α λ β, 8) where R s the target budget of a CTU, and α and β are the correspondng model parameters. Extensve smulaton results n JCTVC-K0103 have shown that Rλ) n equaton 8) s suffcent to represent the relatonshp between R and λ on the HEVC reference software. In HEVC, gven a target budget R for a CTU, the assocated λ s computed usng equaton 8). Consequently, the quantzaton parameter used for quantzng the transform coeffcents s computed by a natural logarthmc formula QP = a lnλ ) + b, 9) where QP s the quantzaton parameter for the th CTU, and both a and b are constants, whch are emprcally set as 4.2 and n the reference software of HEVC HM10.0, respectvely. IV. RATE COTROL WITH DISTORTIO-BASED LAGRAGE MULTIPLIER A. Proposed Dstorton-Based Lagrange Multpler Qualty varaton n a compressed vdeo sgnal has an essental mpact on the human vsual percepton [27] [30], so the goal of the consstent qualty control s to mnmze the dstorton varaton across vdeo frames wthn the constrants of frame rate, bandwdth, and delay requrement. Based on the aforementoned dstorton and rate models, we nvestgate the problem of optmal bt allocaton from the vewpont of mnmzng average MIAVE) dstorton [31] for the HEVC encoder system. Specfcally, we want to fnd an expresson for the Lagrange multplers that mnmzes the average dstorton 1 = 1 D subject to the summaton of the target budgets = 1 R whch s not larger than the frame target budget R max. In addton, replacng D wth the proposed dstorton model n equaton 6) and R wth equaton 8), respectvely, the constraned MIAVE problem can be formulated as: λ 1,...,λ = arg mn λ 1,λ 2,...,λ, = 1 R λ ) R max = arg mn 1 λ 1,λ 2,...,λ, =1 =1 R λ ) R max = 1 D ) λ D prev λ, 10) where λ s the optmal value of λ. Equaton 10) s the ey formulaton for consstent qualty control. In equaton 10), the average dstorton and the target budget constrant are convex functon and convex set of the varable λ, respectvely. Accordng to the Lagrange theory [32], there s a unque soluton λ =[λ 1,λ 2,...,λ ] that can be obtaned from equaton 10). One way to solve t s to use the Karush-Tuhn-Tucer KKT) condton. Unfortunately, t s dffcult to obtan a closed-form soluton of equaton 10) from ts Lagrangan cost functon, snce the complex rate model s nvolved. Another way s to search the possble set of λ that satsfes the KKT condton of equaton 10). One should fnd f there s a certan λ that leads to a soluton satsfyng the KKT condton. Ths exhaustve search method s nfeasble for practcal low-delay vdeo applcatons. Clearly, a more effcent soluton s needed. Based on the observaton that the budget s a power functon wth respect to the varable λ, we propose to relax the constrant ) = 1 R λ Rmax to ) = 1 R λ ) Rmax wth the nequalty of arthmetc and geometrc means. Then, we rewrte the problem n equaton 10) as n equaton 11). In equaton 11), we can fnd a closed-form soluton of λ. However, t s noted that we should consder the buffer state and frame budget, and hence we need to adjust λ to guarantee that ) = 1 R λ Rmax n the practcal encoder HM10.0. The proposed method s formulated as follows. λ 1,...,λ = argmn λ 1,λ 2,...,λ, ) =1 =1 lnr ) ln Rmax D prev λ. 11) The Lagrangan cost functon of equaton 11) can be expressed n equaton 12), whch s an unconstraned problem. In addton, f R s replaced wth equaton 8), then we have argmn λ > 0,= 1,...,,u 0 = argmn L λ 1,λ 2,...,λ, u) λ > 0,= 1,...,,u 0 = 1 Rmax + u = 1 ln R ) ln D prev ) ) λ

6 2948 IEEE TRASACTIOS O IMAGE PROCESSIG, VOL. 25, O. 7, JULY 2016 = arg mn λ >0,α >0,β <0, =1 =1,...,,u 0 + u =1 D prev ln α ) + β ln λ λ ) )) ) Rmax ln, 12) where u s the Lagrange multpler. In Lagrange s theory, t has been shown that f there s a u such that equaton 12) acheves the mnmum value at λ =[λ 1,λ 2,...,λ ],then λ s also an optmal soluton to equaton 11). In addton, snce equaton 12) s mnmzng a convex and dfferentable functon on a convex set, the KKT condton guarantees that the KKT pont λ s an optmum soluton. Consequently, after some straghtforward manpulatons see Appendx), we obtan the optmal λ n equaton 13). ln Rmax ) β ln α =1 D prev λ = β β D prev e =1. 13) As mentoned before, the summaton of the CTU bt budgets R λ ) n equaton 11) should satsfy the frame budget constrant. Thus, we can adjust λ to a proper λ CTU, such that ) = 1 R CTU, λ CTU, R max. In general, adjacent frames n a vdeo sequence have very hgh correlatons that guarantee that we can predct the assocated model parameters for the ent frame from the prevous encoded one. Furthermore, equal bt allocaton scheme s used for each frame n the proposed method. To mantan a proper bt budget for the ent frame, we use a scalng factor χ of the prevous frame to update λ n equaton 14). The scalng factor χ s obtaned from the actual budget Ract prev and target budget Rtar prev of the prevous frame. χ = R tar prev/ R prev. 14) To satsfy the frame budget, we adjust the CTU level target bt budget as R CTU, = χ R. As a result, when replacng R CTU, and R wth equaton 8), we can obtan the CTU level λ CUT, as act ) β λ CUT, = χ 1 β λ, 15) where λ CUT, s used to encode the ent CTU. We conducted experments to evaluate the proposed method 15) n low-delay vdeo communcatons. The standard vdeo sequences wth dfferent resolutons and frame rates are encoded. Tables V and VI summarze the smulaton results of the actual bt rates and the target ones. Expermental results show that the proposed method can acheve a hgher average bts estmaton accuracy. B. Proposed Rate Control Algorthm The tradtonal three-level bt allocaton scheme has been very successful n the H.264/AVC rate control. As a result, JCTVC-K0103 and other rate control proposals n the TABLE II SYMBOLS USED I THE DISTORTIO-BASED LAGRAGE MULTIPLIER ALGORITHM HEVC codec follow the same structure, such as GOP level, frame level and CTU level. Inspred by these poneerng wors, we adopted the same approach of JCTVC-K0103 wth the detaled algorthm as descrbed as Algorthm 1. To easly understand the followng steps, the reader s referred to the JCTVC-K0103 mplementaton n the HM10.0 software. The constant exponents e.g., n Step 2.2 and 3.3) are emprcally set n our paper. Symbols used n the proposed method are tabulated n Table II. V. SIMULATIO RESULTS To evaluate the performance of the proposed consstent qualty control approach, we compare t wth the state-of-theart technques on HM10.0 platform [33]. The experments are conducted on a dual-core @3.10G Hz) worstaton wth RAM 4GB that s also used to measure the computatonal complexty of our method. In the smulaton, the rate control based encoder parameters are set as follows: RateControl enabled), umlcuinunt enabled), LCULevelRateControl enabled), RCLCUSeparateModel enabled), IntalQP dsabled), KeepHerarchcalBt dsabled) and RCForceIntraQP dsabled). All the other encoder settngs are set dentcally for all methods. We have performed the followng four methods. HM10.0: JCTVC-K0103 rate control algorthm has been mplemented and enabled for all tests n ths secton. Cho et al. [10]: Cho s method [10].e., JCTVC-H0213) has been mplemented and compared n HM10.0. Lee et al. [14]: a frame-level rate control method s mplemented and tested n HM10.0. Proposed method: the proposed dstorton-based Lagrange multpler method s mplemented n HM10.0. In the experment, the frst two frames.e., the frst ntra and nter frame) are encoded by the HM10.0 scheme, whch are used to collect the correspondng model parameters. The proposed method was evaluated wth two bandwdths.e., low and hgh bt rate) under the low-delay confguratons.e., P and B Man codng profle). In both codng structures, all the representatve standard sequences wth

7 WAG et al.: LOW-DELAY RATE COTROL FOR COSISTET QUALITY USIG DISTORTIO-BASED LAGRAGE MULTIPLIER 2949 Algorthm 1 Proposed dstorton-based rate control method unque characterstcs n the format of 4:2:0 YUV were used to smulate low-delay communcatons. The ntra frame perod IFP) s set about 0.5 fps. In the results, the standard devaton of Pea Sgnal-to-ose Rato PSR) and Structural SIMlarty SSIM) [34] are the measures for the smoothness of vdeo qualty. Besdes the above measures, the qualty change between adjacent frames s also employed,.e., V avg = 1 L DY, D Y, 1, 16) L 1 =2 where L s the length of the coded vdeo frames, and D Y, can be the lumnance Y-PSR value or Y-SSIM of the th frame. Snce the Y-PSR s frequently used n comparson, we do not explctly dstngush PSR and Y-PSR n ths paper. The addtonal complexty s measured by T avg = T method T anchor / T anchor 100%, 17) where T method and T anchor are the total computatonal complextes of the canddate method and HM10.0, respectvely. The buffer sze s set as B uffer = D elay T arget, 18) where D elay s the delay tme for the real-tme vdeo btstream, and T arget s the channel bandwdth. D elay s set at about 0.3 seconds n the smulaton. Obvously, hgh qualty and low buffer occupancy cannot be acheved conently due to a contradcton between them n low-delay communcatons. In order to acheve low-latency, the buffer sze B uffer s set as small as possble. Accordng to equaton 18), the buffer occupancy s manly determned by the target bts, whch can be adjusted by changng the target bts as well as the vdeo qualty. Fg. 3 shows four typcal buffer occupancy results under the low-delay P confguraton, whch demonstrates the superorty of the proposed approach that can acheve lower buffer occupancy and less buffer fluctuaton n comparson wth HM10.0. Such an mprovement s benefcal to enhance both the vdeo qualty and the buffer occupancy level. The buffer fullness curves n Fg. 3 show that the proposed method has no buffer overflow whereas HM10.0 can lead to the buffer overflow. Meanwhle, our method can provde less buffer varaton e.g., no buffer underflow) n comparson wth Cho et al. [10] and Lee et al. [14]. It s noted that our wor here s not to study the effect of buffer overflow or underflow) and frame-sp, but to examne the role of the low buffer occupancy n the low-delay transmsson. We beleve that f the frame-sp s enabled, HM10.0 would gve a worse qualty fluctuaton due to the loss of hgh-qualty reference frames. Besdes controllng the buffer occupancy, hgh qualty s also desrable n low-latency communcaton. In Fg. 4, we compare the overall rate-dstorton performance of the proposed algorthm wth that of HM10.0, Cho et al. [10] and Lee et al. [14] under the low-delay P confguraton. Experments show that R-lambda based methods.e., HM10.0 and our method) generally gves a better rate-dstorton performance compared to that of R-Q based methods.e., Cho et al. [10] and Lee et al. [14]). Furthermore, t can be seen that our method outperforms the other methods for all of bt rates. In Fg. 4 b), our method can acheve a sgnfcant vdeo qualty mprovement. The reason s that the

8 2950 IEEE TRASACTIOS O IMAGE PROCESSIG, VOL. 25, O. 7, JULY 2016 Fg. 3. Comparson of frame-by-frame buffer occupancy: a) BlowngBubbles 0.8 Mbps), b) BQSquare 0.8 Mbps), c) BQMall 3.2 Mbps) and d) PartyScene 3.2 Mbps). BQSquare sequence contans zoom moton, and HM10.0 has poor estmaton of encodng parameters n the raster-scannng bt allocaton, λ adjustment, and qualty control. On the other hand, our method establshes a novel relatonshp between dstorton and λ. Based on the new dstorton model, we obtan a computatonally feasble soluton to the problem of obtanng optmal λ for consstent vdeo qualty that can avod the use of raster-scannng bt allocaton. Consequently, the proposed method can acheve a better compromse between qualty varaton and buffer occupancy n comparson wth HM10.0. Other smulaton results also verfy smlar performance. Fg. 5 shows the subjectve comparson of vdeo qualty for Basballpass.e., from frame 106 to 115) under the low-delay P confguraton. The comparsons of the reconstructed frames n Fg. 5 valdate the vsual qualty performance of the proposed method. As shown n the top two rows of Fg. 5, t can be seen that our method has a better qualty n the texture area, where the numbers on the scoreboard clearly demonstrate the superorty of our method. In addton, dstorton maps show a subjectve comparson, and small dstorton ndcates hgher vdeo qualty. We can observe that the fourth row of Fg. 5 s more homogeneous compared to the thrd row, where the brghtness s the error measure. Fg. 6 shows the case for BQSquare from frame 106 to 115. In the results of BQSquare, the maxmum SSIM varaton s n HM10.0 whle the maxmum SSIM varaton s n our method. Smulatons have presented smlar performance for all other sequences that we have tested. Snce the SSIM score s consdered to be closer to human evaluaton than the PSR value, smulaton results of subjectve comparson n terms of SSIM are tabulated n Tables III and IV. In the smulaton, we frst compute the frame-by-frame SSIM score for each vdeo sequence, and then the standard devaton of SSIM and the average SSIM change between adjacent frames are used as the measures of vdeo qualty varaton. In Tables III and IV, Avg., Std. dev. and V avg represents the average SSIM, the standard devaton of SSIM and the average qualty change between adjacent frames, respectvely. It can be seen that compared to HM10.0 and Cho et al. [10], our method can acheve the hghest average SSIM scores and the lowest SSIM varatons n both the low-delay P and B Man codng structures. Meanwhle, the average SSIM score of Cho et al. [10] s worse than that

9 WAG et al.: LOW-DELAY RATE COTROL FOR COSISTET QUALITY USIG DISTORTIO-BASED LAGRAGE MULTIPLIER 2951 Fg. 4. Rate-dstorton curves: a) BlowngBubbles, b) BQSquare, c) BQMall and d) PartyScene. Fg. 5. Subjectve vsual qualty comparsons of BasetballPass at bt rate 0.4 Mbps: a) j) HM10.0, proposed method, dstorton map of HM10.0, and dstorton map of the proposed method from top to bottom). of the HM10.0 algorthm. The detaled smulaton results of SSIM can be found n Table III and Table IV. We conducted the subjectve experment to evaluate vsual qualty as suggested n [30]. There are total ffteen subjects who partcpated n the test. The mean opnon score MOS) taes the fve-pont scale rule.e., 5-excellent, 4-good, 3-far, 2-poor, and 1-bad). Fve representatve vdeos are selected as the test source, ncludng the followng content characterstcs, such as fast and slow moton, hgh and low resoluton. The dstorted vdeos are the low bt-rate reconstructed results from Table V. The results are shown n Fg. 7, where the bgger MOS value means the better vsual qualty of the decoded vdeo stream. It can be observed that the proposed method outperforms the other methods. For easy comparson, the smulaton results of PSR and the varatons thereof are tabulated n Table V. In the low-delay P codng structure, the average PSR mprovement of our method s about 0.35 db and 0.59 db n comparson

10 2952 IEEE TRASACTIOS O IMAGE PROCESSIG, VOL. 25, O. 7, JULY 2016 Fg. 6. Subjectve vsual qualty comparsons of BQSquare at bt rate 0.4 Mbps: a) j) HM10.0, proposed method, dstorton map of HM10.0, and dstorton map of the proposed method from top to bottom). TABLE III COMPARISOS OF SSIM VALUES I THE LOW-DELAY PCODIG STRUCTURE TABLE IV COMPARISOS OF SSIM VALUES I THE LOW-DELAY BCODIG STRUCTURE wth HM10.0 and Cho et al. [10], respectvely. The average rate control errors of HM10.0, Cho et al. [10] and our method are 1.54%, 1.40% and 0.08%, respectvely. For the qualty varaton measurement n terms of standard devaton, HM10.0, Cho et al. [10] and our method are 2.52, 2.21 and 1.31, respectvely. In addton, for the qualty varaton measurement n terms of V avg, HM10.0, Cho et al. [10] and our method are 0.50, 1.03 and 0.21, respectvely. As can be observed from the smulaton results, the proposed method generally outperforms HM10.0 and Cho et al. [10] n terms of the average PSR and PSR varatons n the low-delay P codng structure. Fg. 7. Subjectve qualty comparsons between the proposed method and the tradtonal algorthms. In order to obtan a better codng effcency, HEVC supports the low-delay B codng structure. The assocated smulaton results are shown n Table VI. In our method, model parameters updates n equaton 13) only rely on the nearest prevous frame. The average PSR mprovement of our method s about 0.39 db and 1.24 db, compared to HM10.0 and Cho et al. [10], respectvely. Meanwhle, the average standard devatons of PSR are 2.57, 1.74 and 1.34 for HM10.0, Cho et al. [10] and our method, respectvely, whle the qualty varaton V avg s of PSR are 0.48, 0.95 and 0.21, respectvely. It can be seen that the proposed method can provde a more consstent vdeo qualty compared to HM10.0 and Cho et al. [10] n terms of the PSR varatons under the low-delay B codng structure. To get a consstent qualty control, we employ addtonal operatons to compute a dstorton-based λ for each CTU. T avg s consdered as a factor to measure ts complexty. Compared to HM10.0 n the low-delay P codng structure, the T avg value of our method s about 3.90%, whle that of Cho et al. [10] s about 27.57% as shown n Table V. In the low-delay B codng structure, the T avg value of our method s about 4.98%, whle Cho et al. [10] s about 22.52%. It can be seen that the complexty of our method s lower than that of Cho et al. [10] n both the low-delay P and B codng structures. There are two reasons. One s that the model parameters n the

11 WAG et al.: LOW-DELAY RATE COTROL FOR COSISTET QUALITY USIG DISTORTIO-BASED LAGRAGE MULTIPLIER 2953 TABLE V SIMULATIO RESULTS I THE LOW-DELAY PCODIG STRUCTURE TABLE VI SIMULATIO RESULTS I THE LOW-DELAY BCODIG STRUCTURE proposed algorthm can be easly computed. The other beng the computatonal complexty of the R-lambda based rate control s margnal when compared to that of the whole encodng system. Furthermore, t should be ponted out that the real-tme concept s from the algorthm desgn nstead of the computer-based mplementaton here. Therefore, consderng the mprovement n vdeo qualty, one can conclude that the proposed algorthm outperforms HM10.0 n low-delay vdeo communcatons. VI. COCLUSIO Ths paper focuses on consstent qualty control for HEVC and ntroduces an effcent dstorton-based Lagrange multpler approach n low-latency vdeo communcatons. Usng the dstorton of co-located CTU n the prevous frame, a new relatonshp between dstorton and λ s establshed and employed to control the vdeo qualty fluctuatons. Based on the proposed dstorton model, we obtan a computatonally feasble λ n the mnmzaton of the total dstorton subject to a gven bt rate. When consdered jontly wth the buffer state, the CTU level λ s further adjusted such that the target bts satsfy the overall bandwdth of low-delay vdeo communcaton. As demonstrated n the smulaton experments, the proposed rate control method outperforms state-of-the-art technques n terms of bt rate regulaton, vdeo qualty fluctuaton and encoder buffer fullness. APPEDIX The Karush-Tuhn-Tucer KKT) condton of equaton 12) s A.1), as shown at the top of the next page. It s easy to chec that D prev λ = 1 and ln α ) + β ln λ )) ) ln Rmax are convex = 1 functons wth respect to the varable λ. In ths case, we now that f the pont λ 1,λ ) 2,...,λ satsfes the above KKT condtons, t s a globe mnmum soluton for the optmzaton problem 12). In practce, the dstorton and the prevous = 1,...,) s a non-negatve value, and hence we now that D prev = 0forany [1, 2,...,]. In ths case,

12 2954 IEEE TRASACTIOS O IMAGE PROCESSIG, VOL. 25, O. 7, JULY 2016 L λ = D prev + u β λ = 0, = 1,...,. ) u ln α ) + β ln )) λ )) Rmax ln = 0, u 0. ) =1 λ > 0, > 0, D prev > 0, ) >0, α > 0, β < 0, = 1,...,. A.1) we now u = 0 n the condton ). As a result, λ can be expressed as λ = β ) u D prev = u β D prev. A.2) Substtutng equaton A.2) nto the condton ), we can get )) ) u ln α ) + β ln β Rmax λ =1 prev D prev = ln. A.3) Equaton A.3) can be further rewrote as u = e ln Rmax ) =1 Usng equatons A.2) and A.4), λ λ = u = β D prev β D prev e ln Rmax ) =1 ln α β D prev ) β ) =1 β. A.4) ACKOWLEDGMET s formulated as ln α β D prev ) β ) =1 β. A.5) The author would le to than the helpful comments gven by the anonymous revewers. REFERECES [1] G. J. Sullvan, J.-R. Ohm, W.-J. Han, and T. Wegand, Overvew of the hgh effcency vdeo codng HEVC) standard, IEEE Trans. Crcuts Syst. Vdeo Technol., vol. 22, no. 12, pp , Dec [2] T. Wegand, G. J. Sullvan, G. Bjøntegaard, and A. Luthra, Overvew of the H.264/AVC vdeo codng standard, IEEE Trans. Crcuts Syst. Vdeo Technol., vol. 13, no. 7, pp , Jul [3] T. Chang and Y.-Q. Zhang, A new rate control scheme usng quadratc rate dstorton model, IEEE Trans. Crcuts Syst. Vdeo Technol., vol. 7, no. 1, pp , Feb [4] Z. He and S. K. Mtra, Optmum bt allocaton and accurate rate control for vdeo codng va ρ-doman source modelng, IEEE Trans. Crcuts Syst. Vdeo Technol., vol. 12, no. 10, pp , Oct [5] B. L, H. L, L. L, and J. Zhang, Rate control by R-lambda model for HEVC, document JCTVC-K0103, Shangha, Chna, [6] X. Wang and M. Karczewcz, Intra frame rate control based on SATD, document JCTVC-M0257, Incheon, South Korea, [7] M. Wang, K.. gan, and H. L, An effcent frame-content based ntra frame rate control for hgh effcency vdeo codng, IEEE Sgnal Process. Lett., vol. 22, no. 7, pp , Jul [8] H. Cho, J. am, J. Yoo, D. Sm, and I. Bajć, Rate control based on unfed RQ model for HEVC, document JCTVC-H0213, San Jose, CA, USA, [9] Y. Lu, Z. G. L, and Y. C. Soh, A novel rate control scheme for low delay vdeo communcaton of H.264/AVC standard, IEEE Trans. Crcuts Syst. Vdeo Technol., vol. 17, no. 1, pp , Jan [10] H. Cho, J. Yoo, J. am, D. Sm, and I. V. Bajć, Pxel-wse unfed rate-quantzaton model for mult-level rate control, IEEE J. Sel. Topcs Sgnal Process., vol. 7, no. 6, pp , Dec [11] X. Jng, L.-P. Chau, and W.-C. Su, Frame complexty-based rate-quantzaton model for H.264/AVC ntraframe rate control, IEEE Sgnal Process. Lett., vol. 15, pp , 2008, do: /LSP [12] S. Wang, S. Ma, S. Wang, D. Zhao, and W. Gao, Rate-GOP based rate control for hgh effcency vdeo codng, IEEE J. Sel. Topcs Sgnal Process., vol. 7, no. 6, pp , Dec [13] B. Lee and M. Km, Modelng rates and dstortons based on a mxture of Laplacan dstrbutons for nter-predcted resdues n quadtree codng of HEVC, IEEE Sgnal Process. Lett., vol. 18, no. 10, pp , Oct [14] B. Lee, M. Km, and T. Q. guyen, A frame-level rate control scheme based on texture and nontexture rate models for hgh effcency vdeo codng, IEEE Trans. Crcuts Syst. Vdeo Technol., vol. 24, no. 3, pp , Mar [15] M. Jang and. Lng, Low-delay rate control for real-tme H.264/AVC vdeo codng, IEEE Trans. Multmeda, vol. 8, no. 3, pp , Jun [16] M. Jang and. Lng, On Lagrange multpler and quantzer adjustment for H.264 frame-layer vdeo rate control, IEEE Trans. Crcuts Syst. Vdeo Technol., vol. 16, no. 5, pp , May [17] M. Wang and B. Yan, Lagrangan multpler based jont three-layer rate control for H.264/AVC, IEEE Sgnal Process. Lett., vol. 16, no. 8, pp , Aug [18] B. L, H. L, L. L, and J. Zhang, λ doman rate control algorthm for hgh effcency vdeo codng, IEEE Trans. Image Process., vol. 23, no. 9, pp , Sep [19] Z. Chen and K.. gan, Recent advances n rate control for vdeo codng, Sgnal Process., Image Commun., vol. 22, no. 1, pp , [20] A. Ortega and K. Ramchandran, Rate-dstorton methods for mage and vdeo compresson, IEEE Sgnal Process. Mag., vol. 15, no. 6, pp , ov [21] T. Wegand, H. Schwarz, A. Joch, F. Kossentn, and G. J. Sullvan, Rate-constraned coder control and comparson of vdeo codng standards, IEEE Trans. Crcuts Syst. Vdeo Technol., vol. 13, no. 7, pp , Jul [22] L.-J. Ln and A. Ortega, Bt-rate control usng pecewse approxmated rate-dstorton characterstcs, IEEE Trans. Crcuts Syst. Vdeo Technol., vol. 8, no. 4, pp , Aug [23] M. Wang and M. van der Schaar, Operatonal rate-dstorton modelng for wavelet vdeo coders, IEEE Trans. Sgnal Process., vol. 54, no. 9, pp , Sep [24] Y.-K. Tu, J.-F. Yang, and M.-T. Sun, Rate-dstorton modelng for effcent H.264/AVC encodng, IEEE Trans. Crcuts Syst. Vdeo Technol., vol. 17, no. 5, pp , May [25] G. J. Sullvan and T. Wegand, Rate-dstorton optmzaton for vdeo compresson, IEEE Sgnal Process. Mag., vol. 15, no. 6, pp , ov [26] S. Mallat and F. Falzon, Analyss of low bt rate mage transform codng, IEEE Trans. Sgnal Process., vol. 46, no. 4, pp , Apr [27] Z. Chen and K.. gan, Towards rate-dstorton tradeoff n real-tme color vdeo codng, IEEE Trans. Crcuts Syst. Vdeo Technol., vol. 17, no. 2, pp , Feb

13 WAG et al.: LOW-DELAY RATE COTROL FOR COSISTET QUALITY USIG DISTORTIO-BASED LAGRAGE MULTIPLIER 2955 [28] L. Xu, D. Zhao, X. J, L. Deng, S. Kwong, and W. Gao, Wndow-level rate control for smooth pcture qualty and smooth buffer occupancy, IEEE Trans. Image Process., vol. 20, no. 3, pp , Mar [29] J. Hou, S. Wan, Z. Ma, and L.-P. Chau, Consstent vdeo qualty control n scalable vdeo codng usng dependent dstorton quantzaton model, IEEE Trans. Broadcast., vol. 59, no. 4, pp , Dec [30] L. Xu, S. L, K.. gan, and L. Ma, Consstent vsual qualty control n vdeo codng, IEEE Trans. Crcuts Syst. Vdeo Technol., vol. 23, no. 6, pp , Jun [31] G. M. Schuster, G. Melnov, and A. K. Katsaggelos, A revew of the mnmum maxmum crteron for optmal bt allocaton among dependent quantzers, IEEE Trans. Multmeda, vol. 1, no. 1, pp. 3 17, Mar [32] D. A. Perre, Optmzaton Theory Wth Applcatons. ewyor, Y, USA: Dover, [33] 2014). HEVC Reference Software. [Onlne]. Avalable: w.bbc.co.u/trac/browser/jctvc-hm/tags [34] Z. Wang, A. C. Bov, H. R. Sheh, and E. P. Smoncell, Image qualty assessment: From error vsblty to structural smlarty, IEEE Trans. Image Process., vol. 13, no. 4, pp , Apr Maohu Wang S 13 M 16) receved the Ph.D. degree from the Department of Electronc Engneerng, Chnese Unversty of Hong Kong, Hong Kong, n From 2014 to 2015, he was a Vstng Scholar wth the Innovaton Laboratory, InterDgtal Inc., San Dego, CA, USA. He receved the Best Master Thess Award n Shangha 2011) and Fudan Unversty 2012), Chna. He has authored or co-authored numerous techncal papers n nternatonal journals and conferences. Hs ent research nterests cover a wde range of topcs related wth vdeo compresson and transmsson, computer vson and machne learnng, ncludng transform codng, rate control, mage restoraton, and denosng, and deepneuron-networ-based applcatons. He s a member of the IEEE Crcuts and Systems Socety. Kng g gan M 79 SM 91 F 00) receved the Ph.D. degree n electrcal engneerng from Loughborough Unversty, Loughborough, U.K. He was a Full Professor wth anyang Technologcal Unversty, Sngapore, and wth the Unversty of Western Australa, Perth, Australa. He s ently a Char Professor wth the Department of Electronc Engneerng, Chnese Unversty of Hong Kong, Hong Kong. He holds honorary and vstng professorshps wth numerous unverstes n Chna, Australa, and South East Asa. He has publshed extensvely, ncludng three authored boos, sx edted volumes, over 300 refereed techncal papers, and has edted nne specal ssues n journals. He holds ten patents n mage or vdeo codng and communcatons. Dr. gan s a fellow of IET, U.K., and IEAust, Australa, and was an IEEE Dstngushed Lecturer from 2006 to He has served as an Assocate Edtor of the IEEE TRASACTIOS O CIRCUITS AD SYSTEMS FOR VIDEO TECHOLOGY, the Journal on Vsual Communcatons and Image Representaton, the EURASIP Journal of Sgnal Processng: Image Communcaton, and the Journal of Appled Sgnal Processng. He has chared a number of prestgous nternatonal conferences on vdeo sgnal processng and communcatons, and has served on the advsory and techncal commttees of numerous professonal organzatons. He co-chared the IEEE Internatonal Conference on Image Processng, Hong Kong, n Honglang L SM 12) receved the Ph.D. degree n electroncs and nformaton engneerng from X an Jaotong Unversty, X an, Chna, n He was a Research Assocate wth the Vsual Sgnal Processng and Communcaton Laboratory VSPC), Chnese Unversty of Hong Kong CUHK), Hong Kong, from 2005 to From 2006 to 2008, he was a Post-Doctoral Fellow wth VSPC, CUHK. He s ently a Professor wth the School of Electronc Engneerng, Unversty of Electronc Scence and Technology of Chna, Chengdu, Chna. He has authored or co-authored numerous techncal artcles n nternatonal journals and conferences. He s a Co-Edtor of the boo enttled Vdeo Segmentaton and ts Applcatons Sprnger, 2011). Hs research nterests nclude mage segmentaton, object detecton, mage and vdeo codng, vsual attenton, and multmeda communcaton system. Dr. L s a member of the Edtoral Board of the Journal on Vsual Communcatons and Image Representaton, and the Area Edtor of Sgnal Processng: Image Communcaton. He served as a Techncal Program Co-Char n ISPACS 2009, the General Co-Char of the ISPACS 2010, the Publcty Co-Char of the IEEE VCIP 2013, the Local Char of the IEEE ICME 2014, and a TPC Member n a number of nternatonal conferences, such as ICME 2013, ICME 2012, ISCAS 2013, PCM 2007, PCM 2009, and VCIP He serves as a Techncal Program Co-Char of the IEEE VCIP He was selected as the ew Century Excellent Talent n Unversty, Chnese Mnstry of Educaton, Chna, n 2008.