A New Rate Control Algorithm for MPEG-4 Video Coding

Transcription

1 A New Rae onrol Algorhm for MPEG-4 Vdeo odng Sun Yu and Ishfaq Ahmad Deparmen of ompuer Scence Hong Kong Unversy of Scence and Technology ABSTRAT Ths paper proposes a new MPEG-4 rae conrol algorhm for sngle or mulple objec vdeo sequences. The algorhm ams o acheve an accurae b rae wh he maxmum pcure qualy whle effcenly handlng buffer fullness and scene change. In addon o esmang he b budge of a frame based on s global codng complexy, he algorhm dynamcally dsrbues he arge bs for each objec whn a frame accordng o s codng complexy. Even hough he soluon and oher algorhms adop a smple proporonal buffer conroller, her conrol ably s raher neffecve. The proposed algorhm explos a novel Proporonal Inegraed Dfferenal (PID) buffer conroller o effecvely mnmze he buffer overflow or underflow. The PID based conroller reduces he devaon beween he curren buffer fullness and he arge buffer fullness, mgaes he overshoos, and mproves he ransen response. The combned effec s a more smooh and effecve buffer conrol. Furhermore, he algorhm defnes a new and effecve codng complexy of an objec and dynamcally opmzes several parameers. Overall, he proposed algorhm successfully acheves accurae arge b rae, provdes promsng codng qualy, decreases buffer overflow/underflow and lowers he mpac of a scene change. Keywords: MPEG-4 vdeo codng, rae conrol, b allocaon, mulple vdeo objecs, PID buffer conrol. 1. INTRODUTION MPEG-4, due o s affluen funcons for supporng objec-based hgh qualy codng s a he forefron of he vdeo compresson echnology and s becomng ncreasngly popular for presen and emergng mulmeda applcaons 1. In MPEG-4 mulmeda, a me-varable vsual eny wh an arbrary shape can be ndvdually manpulaed and combned wh oher smlar enes o produce a scene. The scene s compressed no a bsream ha can be ransmed hrough eher consan or varable rae channels. To make he ransmsson as effcen and accurae as possble, a varey of codng facors should be jonly consdered, for example, encodng rae, channel rae, and scene conen, ec. Ths resuls n new research challenges n b allocaon and rae conrol schemes, whch mus sasfy a specrum of applcaon requremens. Mos vsual communcaon applcaons use a fxed rae ransmsson channel, whch means he encoder s oupu b rae mus be regulaed o mee he ransmsson bandwdh. The rae conroller of he encoder adjuss he quanzaon parameers (QPs) n order o mee he desred encodng b rae for a source vdeo. A he same me, he encoder mus mnmze he loss of he codng qualy. The presence of mulple vdeo objecs exacerbaes complexy of he encodng ask as he rae conroller mus dsrbue bs among dfferen objecs accordng o he applcaon requremens. Typcal rae conrollers esmae he arge b-rae by measurng he buffer fullness. A buffer s placed beween he encoder and he channel o smooh ou he b rae varaon oupu from he encoder. The encoder generaes bs and sores hem n he buffer whle he channel removes he bs from he buffer. When he source rae exceeds he ransmsson rae, he buffer emporarly sores he encoded bs so ha hey may be ransmed laer allowng he encodng operaon o connue. However, when he buffer s full, he encoder mus cease generang bs by droppng frames hereby causng an nerrupon o he smoohness of he vdeo. On he oher hand, when he buffer s empy, he communcaon bandwdh s wased and he codng qualy s lower han s possble arge. The buffer sze s deermned by he maxmum delay allowed. A large buffer sze ends o allow smooher vdeo bu causes longer delay, whle a small buffer sze guaranees low delay bu may be more lkely o skp frames due o overflow. Some rae conrol algorhms for MPEG-4 based encodng have been proposed n he pas 2-7, for example, he rae conrol algorhms n of MPEG-4 8. hang and Zhang have proposed a rae conrol algorhm ha s scalable for varous b raes, spaal and emporal resoluon, and can be appled o boh DT and wavele-based coders 7. Ths algorhm s based on a quadrac model

2 ha descrbes he relaon beween he requred bs for codng he exure and he quanzaon parameer, he arge bs of a frame s nally se o a weghed average of he number of bs used n prevous frame and R/F. Vero, Sun and Wang exended he R-D model o mulple objec rae conrol 3, such he oal arge bs of a frame are dsrbued proporonal o he relave sze, moon and varance of each objec. To provde a proper rade-off beween spaal and emporal codng, he algorhm swches beween a hgh rae codng mode and a low rae one. In he low rae mode, a mechansm o conrol he parameers for shape codng s ncluded. Ronda, Ecker, Jaureguzar and Garca focus on rae conrol for real-me applcaons 5. Ther algorhms rely on he modelzaon of he source and he opmzaon of a cos creron based on sgnal qualy parameers. Algorhms are nroduced o mnmze he average dsoron of he objecs, o guaranee desred quales o he mos relevan ones, and o keep consan raos among he objec quales. Snce her earler work can only deal wh sngle objec rae conrol 7, Lee, hang and Zhang exended o mulple objec rae conrol 2. Nunes and Perera presened a scene level and objec rae conrol algorhm amng a performng bs allocaon for he several VOs composng a scene, encoded a dfferen VOP raes 4. Even hough hese algorhms can guaranee a relavely good codng performance, hey are no effcen enough o smulaneously achevng he goals of an accurae arge b rae, hgh pcure qualy, avodng buffer overflow/underflow, and admrably dealng wh a scene change. Snce MPEG-4 allows he codng of arbrarly shaped objecs, mulple objecs and asynchronous VOP rae, he encoder mus consder he sgnfcan amoun of bs ha are used o code he shape nformaon, bs allocaon among mulple objecs, bs allocaon for each me slo, ec. Ths paper proposes a new MPEG-4 rae conrol algorhm called Re-adjusng Adapve wh Proporonal Inegraed Dfferenal (). The algorhm ams o acheve an accurae b rae wh he maxmum pcure qualy whle a he same me handlng buffer fullness and scene change. The specfc characerscs of he algorhm nclude: (a) In addon o esmang he b budge of a frame based on s global codng complexy, he algorhm dynamcally dsrbues he arge bs for each objec whn a frame accordng o s codng complexy; (b) The algorhm explos a Proporonal Inegraed Dfferenal (PID) buffer conroller o effecvely mnmze he buffer overflow or underflow; (c) The algorhm defnes a new and effecve codng complexy of an objec; (d) The algorhm proposes several adapaon mehods o auomacally opmze parameers. The remander of hs paper s organzed as follows: In Secon 2, we descrbe he basc phlosophy of he proposed adapve rae conrol algorhm for sngle/mulple vdeo objec. In he same secon, we dscuss a new buffer conrol mehod named PID conroller o manan a sable buffer level. In Secon 3, we presen some opmzaon mehods ha furher fne une he effcency of he proposed algorhm. Secon 4 summarzes he algorhm and descrbes s funconaly. Secon 5 ncludes he expermenal resuls ha demonsrae he performance of he proposed algorhm. Fnally, Secon 6 concludes he paper by provdng some fnal remarks and observaons. 2. FOUNDATIONS OF THE PROPOSED ALGORITHM The proposed rae conrol algorhm consss of a number of seps. In hs secon, we descrbe he prncples and foundaons of hese seps Inalzaon Sage The nalzaon sage ncludes seng up of he encodng parameers and buffer sze. The buffer sze s nalzed based on laency requremen, whle he buffer fullness s nalzed as he mddle level of he buffer sze. We assume ha he requred b rae s consan, mulple VOs are synchronous wh he same VOP rae, and a frame s defned as a se of VOPs of dfferen objecs wh a common presenaon me. The oal arge number of bs generaed by he encoder durng G s: T G = b_ rae. G The maxmum number of VOPs ha can be encoded durng G s: N = VOP _ rae. G G Thus, he numbers of I-VOPs, P-VOPs and B-VOPs n he gven sequence durng G can be compued by: N IVOP N G + L = In, N G + K N PVOP = In N, IVOP L + 1 K + 1 N N G + K = In K, K + 1 BVOP 1

3 where L s he number of VOPs beween wo consecuve I-VOPs, K s he number of B-VOPs beween wo consecuve P-VOPs or P-VOP and I-VOP. Furhermore, we should know he weghed average number of bs o be oupu from he buffer per frame: Rr Bp = α K, α( I) N + α( B) N + α( P) N ) ( I B P where N I, N P and N B are he numbers of I-VOPs, P-VOPs and B-VOPs whch reman o be coded respecvely, α (I), α (B) and α (P) are her wegh facors, R r s he oal number of bs avalable for he res of he mage sequence, α s α (I), α (B) or α (P) correspondng o he codng ype of curren VOP. k 2.2 Inal Targe B Esmaon Based on avalable bs, he percepual effcen approach, he pas hsory of each VO and he curren me nsan characerscs (codng complexy), a combnaon of sraeges s used o esmae he nal arge bs 2-5, 9. A codng complexy of VOP a me o be encoded s calculaed accordng o he followng formula: 2 ( ) 1, n P NVO j P n j = = wh 1 P = P, (1) j = 1 n n j= 1 where P j s he lumnance value of pxel j n he h Marco-Block MB of a moon-compensaed resdual VOP, P s he arhmec average pxel value of MB, n s he number of non-ransparen pxels n he MB, NVO s he number of nonransparen macro-blocks n he VOP. The codng complexy compued by (1) naurally combnes he objec sze (NVO ) and average varance of each macro-block n a VOP, and, herefore, can reflec he nsananeous characerscs of hs VOP. The codng complexy dcaes how many bs can be approprae for VOPs before really encodng hem. Ths s specally useful when a VO changes s feaures rapdly, or when a scene change 10,11 happens, because he codng complexy of he VOP can reflec hese changes. In he soluon of MPEG-4, arge bs are allocaed o he curren frame only accordng o he sascal nformaon of s prevous frame, whou any consderaon o he real complexy of he curren frame. Ths may resul n napproprae allocaon of bs o he curren frame, whch can lead o flucuang and overall degraded vsual qualy. A global complexy of curren frame a me can be obaned by: 1 / 4 M G, = NW,, ) = 1 (, where, denoes he codng complexy of VOP n he curren frame, NW, s he normalzed wegh of VOP whch wll be dscussed laer. M s he number of VOs n he curren frame. The average global complexy of prevous n frames before me can be compued by: ave, = n n G,, ( =, -1,, -n+1). Accordng o he ype of he curren frame, s arge number of bs s nally se o a weghed average bcoun: T ave, = α r k α ( I ) N I + α ( B) N B + α ( P) N. P The oal arge b budge of he curren frame o be encoded s hen esmaed by: T R G, = Tave,, (2) ave,

4 where α s α (B) or α (P) accordng o he codng ype of he curren VOP. k The number of arge bs s esmaed only for P-VOPs and B-VOPs for each me nsan. We do no esmae arge bs for I-VOPs, whch wll be explaned laer. Ths bs allocaon essenally follows a basc prncple: f G, s hgher han ave,, more bs should be allocaed o he curren frame han he weghed average bs T ave, ; on he conrary, f G, s lower han ave,, fewer bs should be allocaed. Hence, approprae bs can be adapvely allocaed o he curren frame and codng qualy can be kep consan. 2.3 Targe Bs Adjusmen Based on he Buffer Occupancy The nal b arge s furher refned based on he buffer fullness o ge a more accurae arge b esmaon. The goal of he buffer conrol s ry o keep buffer fullness n he mddle level o reduce chances of buffer overflow or underflow: f buffer occupancy exceeds he mddle level, he arge bs are decreased o some exen; smlarly, f buffer occupancy s below he mddle level, he arge bs are ncreased a lle. Even hough he soluon and oher algorhms adop a smple proporonal buffer conroller, her conrol ably s raher neffecve. As shown n our expermen, when he complexy of a sequence changes drascally, he buffer rends o be ou of conrol, especally n he low b rae cases. The proporonal acon can reduce he devaon beween he curren buffer fullness and he arge buffer fullness (ypcally, mddle level), bu canno fully elmnae hs devaon. An Inegral onroller has he effec of elmnang he devaon by hs way: when he devaon lass, can auomacally enhance he conrol srengh. Bu may make he ransen response worse. A Dfferenal onroller has he effec of ncreasng he sably of he sysem, reducng he overshoo, and mprovng he ransen response. The hree-mode Proporonal-Inegral-Dfferenal (PID) conroller 12,13 (see Fgure 1) combnes he advanages of each ndvdual conroller and hus, s more smooh and effecve. Here we apply hs echnque o our buffer conrol problem. I can keep buffer fullness around he mddle level and sgnfcanly reduce he chances of buffer overflow or underflow. R= B s / 2 E PID Buffer PID Buffer Y= B f onroller Fgure 1: The PID buffer onrol Sysem The varable E represens he error sgnal (devaon) a me, he dfference beween he desred value R ( half level of buffer) and he acual oupu Y (buffer occupancy) a me, s defned as: ( B s / 2 B f ) E =, B / 2 where B s s he buffer sze, B f s he curren buffer fullness a me. Ths error sgnal E s sen o he PID conroller: PID s de = K ( E + K E d + K ), (3) p d d where K p, K and K d are he Proporonal, Inegral and Dfferenal conrol parameer respecvely. In he expermens, K p, K, and K d have been se o 1.0, 0.15 and 0.2 respecvely for mulple objec codng; and o 1.0, 0. and 0.3 respecvely for sngle objec codng. Then he nal arge bs can be furher adjused by: T = T 1 + PID ). (4) ( To manan a mnmum accepable vsual qualy, he encoder mus allocae a mnmum number of bs o he curren frame, ha s: T R = max{, T }. 4 F Smlarly, o avod buffer overflow, a maxmum number of bs s gven o : 2 R T = mn, T, F

5 where R and F are he b rae and frame rae requred by he applcaon. 2.4 Dynamc Targe Bs Dsrbuon among Mulple VOs In order o maxmze he overall qualy of he decoded scene wh a gven amoun of resources, s mporan o effecvely dsrbue he oal arge bs among mulple objecs for a frame 5,14. Normally, a rae conrol scheme should allocae more bs o mporan VOs (e.g., foreground VO) han oher areas (e.g., background VOs). Vsual qualy should be bad f mproper bs were allocaed o VOs. For example, he background VOs may have excellen qualy, whle he foreground VOs may have low qualy, or here may be unbalanced quales among VOs. The proposed algorhm dsrbues he b budge a me for VOP accordng o he codng complexy n he followng manner: NW,, T, = G, where T, represen he arge bs allocaed o VOP a me. 2.5 Quanzaon Parameer alculaon T, (5) The quanzaon Parameer (QP) for exure encodng s compued based on he Rae Dsoron model of each VO for he correspondng VOP codng ype 3,15. Once he number of arge bs T, for VOP s obaned, he number of arge bs for codng he exure of he h objec can be compued by : T T H, exure, =,, 1 where H,-1 denoes he number of bs acually used for codng he moon, shape and header for VOP a me -1. T exure, represens he arge bs o encode exure nformaon of VOP. The proposed rae conrol algorhm also adop hs Rae-Dsoron Model 2,3 : X 1 MAD X 2 MAD Texure, = +, (6) 2 Q Q where MAD s compued usng moon-compensaed resdual for he lumnance componen, Q denoes quanzaon level used for VO, X1 and X2 s he frs and second order model coeffcens. One problem of s ha Inra coded VOPs are ypcally encoded wh lower qualy han Iner coded VOPs, hs resul n a larger qualy varaons and qualy decay. I ndcaes ha he b allocaon sraegy of s no very effcen. The paral reason s explaned as follows: A good codng qualy depends on an accurae R-D model, and he accuracy of R-D model depends on he qualy and quany of he daa se used o updae. Generally speakng, more updang daa pons n a codng process are lkely o yeld a more accurae model o reflec he vdeo conens. A he begnnng of he codng process, he R-D models of all ypes of VOPs are very rough. Along wh he codng process, more and more VOPs are seleced o updae hese R-D models and R-D models become more and more accurae han he orgnal ones. Though hs adapve procedure s ruly successful for P-VOPs and B-VOPs, s no very suable for updang I-VOPs R-D model smply because I-VOPs are que sparse n a codng sequence. Even enough quany of I- VOPs can be accumulaed afer many coded I-VOPs, mos of hem canno represen he change of he comng I-VOP. Thus he R-D model of I-VOPs s less accurae han hose of he ner-coded VOPs and, hus, he codng qualy of I- VOPs rends o flucuae. To avod he above problem and acheve a consan codng qualy beween Inra coded VOPs and Iner coded VOPs, a novel way s adoped here: We only esmae he number of arge bs and calculae QPs for B- VOPs and P-VOPs bu no for I-VOPs. Insead, when codng an I-VOP, we jus employ he average QP of s prevous l Iner coded VOPs wh some adjusmen. Though hs mehod s que smple, s very effcen o overcome vsual qualy flucuaon or degradaon of I-VOPs. The QP s lmed o vary beween 1 and 31. To smooh qualy flucuaon, QP s only allowed o change whn % of he prevous QP. 2.6 Encodng and Updang Afer encodng vdeo objecs whn a frame, he encoder updaes he R-D model of each VO for he correspondng VOP codng ype based on he encodng resuls of he curren objecs as well as he pas objecs. Prevous QPs and correspondng exure b couns are used n he R-D model updang. The frs and second model parameers, X1 and X2, are solved by usng lnear regresson echnque 2,7. Oher parameers adapaon s descrbed n he nex secon.

6 2.7 Frame-Skppng onrol To effecvely avod buffer overflow, he encoder needs o examne he curren buffer fullness before encodng he nex frame: If he buffer occupancy exceeds 80 percenage of he buffer sze, he encoder skps he encodng of he nex frame, and he buffer fullness s updaed by he channel oupu rae. Snce frame skppng can sgnfcanly reduce he overall percepual qualy, a good rae conrol algorhm should avod frame skppng as bes as can. 3. OPTIMIZATION OF THE RATE ONTROL PARAMETERS To furher mprove he sysem performance, some codng parameers should be consdered and dynamcally adjused n he codng process. Ths secon descrbes hese echnques. 3.1 Wegh Adjusmen for VOP Types α (I), α (B) and α (P) are weghs of I-VOP, B-VOP and P-VOP, respecvely; her nal values are se o 3.0, 0.5 and 1.0, respecvely. To acheve a smooh vsual qualy, α (I) and α (B) are updaed based on coded I- and B-VOPs, whle α (P) s fxed o 1.0. In prncple, f he average codng qualy of prevously coded B-VOPs (B ) s lower han ha of prevous coded P-VOPs (P ), we ncrease α (B) by a small amoun. Then B-VOP o be coded nex me can be allocaed more bs, and hus mprove s qualy gradually o keep conssen wh he qualy of P-VOPs. On he conrary, f he average of he coded B-VOPs s hgher han ha of he coded P-VOPs, we decrease α (B) by a small amoun o ge fewer arge bs for he nex B-VOP, hus decrease s codng qualy gradually o keep close o s of P-VOPs. P B γ B avebs α ( B ) = e, (7) Pavebs where P avebs and B avebs denoe he average number of bs used n codng prevous n_p P-VOPs and n_b B-VOPs, respecvely; P and B are her average!#"$&%'(*)+,(-.'(!#/"0% 1243/"$/5&6(!#57+' α (I) s also updaed by: P I γ I avebs α( I) = e. (8) Pavebs For he reason o keep sably and rapdly reflec he nfluence of scene varaons, (n_i + n_p + n_b) should no be oo shor or oo long. Here a lengh of frames s chosen o make a compromse o calculae he average values. 3.2 Wegh Adjusmen among Mulple Objecs Smlarly, o acheve comparable and balanced qualy among mulple objecs whn a frame, or n oher words, o avod large percepual qualy dfferences among mulple objecs, wegh for each objec s furher adjused accordng o he dfference of prevous coded VOPs.,-1 of VO (=2..M) s compared o he 1,-1 of VO 1, f,-1 s lower han 1,-1, he wegh of he VO a me, W,, s ncreased a lle, hus VO obans more arge bs and hus acheves a hgher qualy; oherwse, W, s decreased a lle and acheves lower qualy. We nalze W,0 o 1.0 for all VO and adop he frs objec as a referenal base, hen he weghs of oher objecs are updaed:, 1 1, 1, 1 θ W = W e for >1, (9), where 8 = 16, whch s deermned by expermens. Noe, W 1 =1.0 forever. Then he normalzed weghs for all objecs are calculaed by:, = M Obvously, a furher mprovemen could be easly made o provde dfferen pror levels for VOs: NW W j= 1, W j,.

7 , 1 1, 1, 1 + P θ W, = W e for >1, (9a) where P s he prory of VO. P >0 (db) means a hgher prory whle P <0 (db) corresponds o a lower prory. For example, f one lkes he foreground objec VO 2 o have a PNSR 3 db hgher han ha of he background objec VO 1, one can se P 1 =0.0 and P 2 = Quanzaon Parameer Updang for I-VOP Snce QP of I-VOP for an objec s obaned drecly by averagng QPs of prevous l ner coded VOPs, o beer manan he conssen qualy beween I-VOP and s prevous ner coded VOPs, balance adjusmen s appled as followng: QP = QP + _ I, (10) I, ave, β where QP I, s he QP of I-VOP ; QP ave, s average QP of l ner coded VOPs before I-VOP ; nally, β _ I = 1.0 and s updaed as follows: I, j ave. j β _ I = β _ I +, (11) λ where I,j s he of las I-VOP j and ave,j s he average of l ner coded VOPs before las I-VOP j ; l=3 n he expermen; λ s 4 for sngle objec and 16 for mulple objec. The s because f an I-VOP s s hgher han he average of s prevous l ner coded VOPs, The QP for I-VOP should be ncreased n order o lower s codng qualy. Oherwse, f he of an I-VOP s lower han he average of l ner coded VOPs, The QP of I-VOP s should be decreased n order o ncrease s codng qualy. Ths adjuss he qualy of I-VOP o be closer o hose of s prevous ner coded VOPs. 4. THE Rae onrol Algorhm Here, we summarze he prevous secons as he algorhm. The algorhm has he followng seps: 1) Inalze he parameers for he encoder. 2) Esmae he number of arge bs for a frame usng Equaon (1), (2). 3) Adjus arge bs for a frame based on he buffer occupancy usng Equaon (3), (4). 4) Dsrbue arge bs among mulple VOs n a frame usng Equaon (5). 5) alculae he Quanzaon Parameer usng Equaon (6), (10). 6) Encode frame/objecs. 7) Updae R-D Model and adjus oher parameers usng Equaon (7), (8), (9), (11). 8) Apply frame-skppng conrol, f necessary. New VOPs Frame skppng conrol Esmae arge bs for a frame (1), (2) Adjus arge bs based on he buffer occupancy (3), (4) MVO? Y Dsrbue arge bs among VOs n a frame (5) N Encodng Updae R-D models and adjus weghs (7), (8), (9), (11) alculae QPs (6), (10) Buffer Inalzaon oded Bsream ndcae daa flow, are conrol flow and, are jus used n nalzaon. Fgure 2: The funconal dagram of.

8 5. EXPERIMENTAL RESULTS Ths secon presens he performance of he proposed algorhm. We conduced wo ses of expermens: one for encodng a sngle objec wh recangular or arbrary shape, and he second for encodng mulple objecs. The resuls acheved here are compared wh hose acheved usng he rae conrol algorhm suggesed by he MPEG-4 vsual sandard. Snce a skpped VOP s represened n he decoded sequence by repeang he prevously coded VOP accordng o MPEG-4 core expermens, he of a skpped VOP s compued by usng he prevous encoded VOP 5,16. I s obvous ha he of a skpped VOP s ypcally much lower han ha of a normal one. 5.1 Sngle Objec Rae onrol The resuls of encodng varous esng sequences usng I-VOP, P-VOP and B-VOP for one recangular or arbrary shape VO are repored n Table 1. For nsance, Fgure 3a and 4a llusrae curves and Fgure 3b and 4b show he correspondng buffer occupancy curves for wo sequences respecvely. In hese expermens, he Inra perod s se o one second; he number of B-VOPs s se o 2 beween wo P-VOPs or beween I-VOP and P-VOP; he number of P-VOPs s se o 4 beween wo I-VOPs. The nal values of α (I), α(b) and α (P) are 3.0, 0.5, and 1.0, respecvely; he values of α (I) and α (B) are dynamcally adjused durng he encodng process. All sequences are encoded a 15 frames/sec (fps). Each sequence n Table 1 s esed usng a relavely hgher b-rae and a lower b-rae. Vdeo Sequence Algorhms Table 1: Sngle VO rae conrol usng I-VOPs, B-VOPs, and P-VOPs. B Rae (Kbps) # oded VOPs Targe Acual Targe Acual (db) oasguard (qcf) oasguard (qcf) oaner (cf) oaner (cf) oaner (cf) Bream2_ (qcf) Bream2_ (qcf) Slen (qcf) Slen (qcf) Slen (qcf) News (qcf,) News (qcf) Moble (qcf) Moble (qcf) Tran_&_T_R (qcf) Tran_&_T_R (qcf)

9 20 Buffer Occupancy Buffer Sze (a) urves (b) Buffer Occupancy Fgure 3: The resuls for he oasguard sequence (QIF) encoded a 128 kbps, 15fps (IBBP IBBP). 20 (a) urves (b) Buffer Occupancy Fgure 4: The resuls for he Tran_&_Tunnel_Rgh sequence (QIF) encoded a 64 kbps, 15fps (IBBP IBBP). Buffer Occupancy Buffer Sze Vdeo Sequence Table 2: Sngle VO rae conrol, only I-VOPs and P-VOPs are used n codng. Algorhms B Rae (Kbps) # oded VOPs Targe Acual Targe Acual (db) oasguard (qcf) oasguard (qcf) oaner (cf) oaner (cf) Bream2_1 VM (qcf) Bream2_ (qcf) Slen (qcf) Slen (qcf) Slen (qcf) News (qcf) News (qcf) Moble (qcf) Moble (qcf) Tran_Rgh (qcf) Tran_Rgh (qcf)

10 Table 2 shows he encodng resuls of sngle VO whch only I-VOPs and P-VOPs are used. Fgure 5a and 5b shows curves and buffer curves for sequence Bream2_1 respecvely. The Inra perod s se o one second. Inally, α (I) =3.0 and α (P) = Buffer Occupancy Buffer Sze (a) urves (b) Buffer Occupancy Fgure 5: The resuls for he Bream2_1 sequence (QIF) encoded a 64kbps, 15fps whou usng B-VOPs (IP IP). By examnng he resuls n Table 1 and Table 2, s obvous ha he acheves more accurae arge b rae and arge frame rae wh hgher average as compared o he soluon. From Fgure 3a, 4a and 5a, we observe ha n he algorhm, nra coded VOPs ypcally have lower quales han ner coded VOPs or here are large flucuaons beween hem, ndcang a less effcen b allocaon sraegy. From Fgure 3b, 4b and 5b, one can see he buffer occupancy curves of are que sable; hey are around 50% of he buffer sze wh a small varaon. However, by examnng he buffer occupancy curves produced by, s evden ha has less conrol ably and resuls n more frame skppng cases. 5.2 Mulple Objec Rae onrol The resuls for mulple VO encodng are shown n Table 3. The Inra perod s se o 0.5 second, and B-VOP s no used. Inally, α (I) = 3.0 and α (P) = 1.0. α (I) s updaed durng he encodng process. All sequences are QIF forma and are encoded a fps. Wh he same condons, he soluon skps much more frames han (see Table 3), ndcang ha s buffer conrol ably s relavely less effcen and b allocaon s no very accurae. Ths s crucal for low b raes where he b resources are scarce. These resuls also show ha qualy dfferences among VOs of are smaller han hose of, hs llusraes he mer of he proposed auomac adapaon mehodology. Fgure 6 and 7 show he and buffer fullness curves for and. Noe ha, large degradaons of I-VOPs exs n soluons, whch cause qualy flucuaon. The buffer fullness of s around 50% of he buffer sze wh a smaller varaon, hus s more sable han he soluon. Vdeo Sequence Table 3: Mulple objec rae conrol, boh I-VOP and P-VOP are adoped. Algorhms B Rae (Kbps) # oded VOPs (db) Targe Acual VO1 VO2 Targe Acual VO1 VO2 News_1 (Balle) News_2 (Speakers) News_1 (Balle) News_2 (Speakers) Bream2_0 (Background) Bream2_ Bream2_0 (Background) Bream2_ hldren2_ hldren2_ hldren2_ hldren2_

11 20 45 VO1 VO1 Buffer Occupancy Buffer Sze (a) curves of VO1. (b) curves of VO2. (c) Buffer Occupancy. Fgure 6: The resuls for he News sequence (QIF) wh 2VOs encoded a 128 kbps, fps (IP IP). VO1 VO VO2 VO2 VO2 VO (a) curves of VO1. (b) curves of VO2. (c) Buffer Occupancy Fgure 7: The resuls for he News sequence (QIF) wh 2VOs encoded a 6 kbps, fps (IP IP). The resuls gven n Table 4 are under a specal condon ha only frs VOP s I-VOP and he remanng VOPs are all P-VOPs. Ths s he smples case n rae conrol. All sequences are QIF forma and are encoded n fps. The resuls n Table 4 also ndcae ha he performance of s beer han or a leas equal o he soluon. Buffer Occupancy Buffer Sze Vdeo Sequence Table 4: Mulple objec rae conrol, only P-VOPs are used n es sequences excep frs I-VOP. Algorhms B Rae (kbps) # oded VOPs (db) Targe Acual VO1 VO2 Targe Acual VO1 VO2 News_1 (Balle) News_2 (Speakers) News_1 (Balle) News_2 (Speakers) Bream2_0 (Background) Bream2_ Bream2_0 (Background) Bream2_ hldren2_ hldren2_ hldren2_ hldren2_ As he algorhm s very sensve o nal values of QP, unsuable values of QP can resul n many frame skppng, whle s que robus, whch can work wh a wde range of nal QP values whou any frame skppng. In all expermens, nal values of QP are always seleced for opmzng he soluon, and hen hese nal values of QP are also used n. As a resul, s more robus and can handle scene change by quckly adjusng unsuable values of QP o adap he new scene. In some cases he frame skppng acvy s very frequen n soluon, especally when he arge b rae s very low. Bu, can delver good performances whou any

12 frame skppng under he same condons. Ths ndcaes ha he conrol range of arge b-rae of s wder han ha of. 6. ONLUSIONS In hs paper, we proposed a rae conrol scheme for effcen b allocaon for MPEG-4 vdeo codng. We proposed a number of deas: For example, our scheme consders he codng complexes of boh objec and frame and hen performs b allocaon among frames and among VOs whn a frame based on codng complexes. A PID buffer conrol mechansm s used o adjus he global b rae. Fnally, he algorhm performs adjusmens for I-VOP as well as among mulple VOs whn a frame. The performance resuls for boh sngle VO and mulple VOs encodng auhencae ha ouperforms he soluon by: (a) provdng more accurae rae regulaon; (b) achevng beer pcure qualy; (c) reducng qualy flucuaon; (d) balancng among boh frames and mulple VOs; (e) allowng hgher prory o favore VOs; (f) mananng a more sable buffer level; (g) coverng a wde b-rae conrol range; (h) n addonal, olerang unsuable nal QPs and scene change. REFERENES 1. R. Koenen, Ed., Overvew of he MPEG-4 sandard, Doc. ISO/IE JT1/S29/WG11 N27 Seoul, Korea, Mar Hung-Ju Lee, Thao hang, and Ya-Qn Zhang, Scalable Rae onrol for MPEEG-4 Vdeo, IEEE Trans.On rcus and Sysems for Vdeo Technology, VOL. 10, NO. 6, pp , Sep Anhony Vero, Hufang Sun and Yao Wang, MPEG-4 Rae onrol for Mulple Vdeo Objecs, IEEE Trans. On rcus and Sysems for Vdeo Technology, VOL. 9, NO. 1, pp , Feb Paulo Nunes, Fernando Perera, Scene Level Rae onrol Algorhm for MPEG-4 Vdeo odng, n Vsual ommuncaons and Image Processng, Proc. SPIE 4310, pp , Jose I.Ronda, Marna Ecker, Fernando Jaureguzar, and Narcso Garca, Rae onrol and B Allocaon for MPEG-4, IEEE Trans. On rcus and Sysems for Vdeo Technology, VOL.9, NO. 8, pp , Dec J.I.Ronda, M. Ecker., S. Reke, F.Jaureguzar, and A. Pacheco, Advanced Rae conrol for MPEG-4 oders, n Proc. SPIE Vsual ommun. & Image Processng, pp , San Jose, A. Jan T.hang and Y.-Q. Zhang, A new rae conrol scheme usng quadrac rae-dsoron modelng, IEEE Trans. On rcus and Sysems for Vdeo Technology, VOL.7, NO. 1, pp , Feb MPEG-4 vdeo verfcaon model V8.0, ISO/IE JT1/S29/WG11 odng of Movng Pcures and Assocaed Audo MPEG97/N1796, July 1997, Sockholm, Sweden. 9. ISO/IE JT1/S29/WG11, MPEG93/457 MPEG vdeo es model 5, Draf, Apr Sanggyu Park, Youngsun Lee, and Hyunsk hang, A New MPEG-2 Rae onrol Scheme Usng Scene hange Deecon, ETRI Journal, VOL 18, NO. 2, pp , Jul L-Jun Luo, a-rong Zou, and Zhen-Ya He, A New Algorhm on MPEG-2 Targe B-Number Allocaon a Scene hanges, IEEE Trans. On rcus and Sysems for Vdeo Technology, VOL.7, NO. 5, pp , Oc Vance J.VanDoren, Tunng Fundamenals, Bascs of Proporonal-Inegral-Dervave onrol," onrol Engneerng, March 1998, hp:// 13. "Tunng a PID (Three-Mode) onroller, " hp:// 14. H.-J.Lee, T.hang, and Y.-Q.Zhang, Mulple-VO rae conrol and B-VO rae conrol, Doc. ISO/IE JT1/S29/WG11 M54, Sockholm, Sweden, July Descrpon of rae conrol and exure codng core expermens on codng effcency n MPEG-4 vdeo, Doc. ISO/IE JT1/S29/WG11 N28, Sockholm, Sweden, July Jord Rbas-orbera, Shawmn Le, Rae onrol n DT Vdeo odng for Low-Delay ommuncaons, IEEE Trans. On rcus and Sysems for Vdeo Technology, VOL.9, NO. 1, pp , Feb