Conversion Scheme for DCT-domain Transcoding of MPEG-2 to H.264/AVC

Transcription

1 Converson Scheme for DC-doman ranscodng of MPEG-2 to H.26/AVC Joo-kyong Lee 1, K-dong Chung 1 1, Dept. of Computer Engneerng, Pusan Natonal Unv., Jangjeon-dong, Geumjeong-gu, Busan, , Korea jklee@melon.cs.pusan.ac.kr, kdchung@pusan.ac.kr Abstract. he approxmate dscrete cosne transform (DC of H.26/AVC [1] makes t dffcult to transcode the pre-coded vdeo contents wth the prevous vdeo codng standards to H.26/AVC n DC doman. hs s due to the dfference between 8 8 DC used prevous standards and DC n H.26/AVC. In ths paper, we propose an effcent algorthm that converts the quantzed 8 8 DC block of MPEG-2 nto newly quantzed four DC blocks of H.26/AVC to support DC-doman transcodng. Expermental results show that the proposed scheme mproves computatonal complexty by 5~11% and vdeo qualty by 0.1 ~ 0.5 db compared wth cascaded pxeldoman transcodng that explots nverse quantzaton (IQ, nverse DC (IDC, DC, and re-quantzaton (re-q. 1 Introducton As the number of networks, types of devces, and vdeo representaton formats ncrease, nteroperablty between dfferent systems and dfferent networks s becomng more and more mportant [2]. o provde a seamless nteracton between producers and consumers, dverse research on vdeo transcodng such as bt-rate reducton [3], spatal resoluton reducton [], frame skppng [5][6], and smple vdeo format converson [7], has been conducted. Recently, DC-doman transcodng approaches have been studed to mprove computatonal complexty and to avod DC and IDC msmatch problem [8][9]. hese approaches are based on 8 8 DC used n most vdeo codng standards such as MPEG-2, MPEG-, and H.263/AVC. However, t s mpossble to drectly apply them to H.26/AVC, because t uses transforms. In ths paper, for non-ntra-coded blocks, we propose an effcent algorthm to convert an 8 8 DC block of MPEG-2 to four DC blocks of H.26/AVC. Addtonally, we propose a quantzaton converson algorthm that changes quantzaton step sze between the two standards. hs algorthm mproves vdeo qualty by reducng the quantzaton error caused by cascaded IQ/re-Q. However, for ntra-coded blocks, we follow the cascaded re-encodng method n pxel doman nstead of DC converson to avod heavy computatonal complexty n predcton mode selecton. Our scheme can be easly appled to other standards by slghtly modfyng the quantzaton converson algorthm. hs paper s organzed as follows. In Secton 2 we

2 descrbe transformaton for H.26/AVC, respectvely. In Secton 3 we present the Qstep converson scheme and DC converson scheme. Expermental results wll be presented n Secton, and the concluson s shown n Secton 5. 2 ransformaton for H.26/AVC [10] [11] he transformaton of H.26/AVC approxmates the true DC. he transform matrx, H of the true DC can be llustrated by Eq. 1, where a, b, c represent the matrx elements. H.26/AVC, On the other hand, uses the modfed transform matrx H, where b and b/c are changed to 2 5 and 1, respectvely. In actual 2 H.26/AVC mplementatons, the nteger transform matrx s used for transformaton, and the resdual scalng factors are absorbed nto quantzaton process to avod multplcaton operatons, and mantan DC/IDC accuracy. See [10] as well as [11] for more detal. In ths paper we use H to convert the 8 8 DC block nto four DC blocks of H.26/AVC, and we cover the resdual scalng factors usng quantzaton step sze (Qstep converson tool. hroughout ths paper, we represent the result of the modfed DC usng H as approxmate DC. Snce there s a dfference between true DC and approxmate DC, we should compensate t when we convert the 8 8 DC block nto four approxmate DC blocks. a a a a b c c b 1 1 π 1 3π Y = HXH, H =, where a =, b= cos(, c = cos( a a a a c b b c (1 3 Proposed Converson Schemes In ths secton we present two converson schemes, Qstep converson scheme and the DC converson scheme. he Qstep converson scheme changes the quantzed DC coeffcents of MPEG-2 to newly quantzed coeffcents of H.26/AVC. hs scheme covers the H.26/AVC quantzer that s assocated wth transformaton by ncorporatng the resdual scalng factors nto t. DC converson scheme converts the true 8 8 DC nto the approxmate DC. 3.1 Qstep Converson In MPEG-2 encoder, ntra coded macroblock (MB and non-ntra coded MB are quantzed by dfferent formula and quantzer matrces. Intra coded MB coeffcents are quantzed as shown n Eq. 2, where B (, j refers to the (,j th coeffcent of the DC block B, QB ( (, j refers to the quantzed value of B (, j, and q refers to Qstep value.

3 he DC coeffcents of the lumnance and chromnance components of the ntra coded MB are dvded by one of the Qstep values 8,, 2 and 1. For AC coeffcents, frst, the coeffcents are scaled by 32 and correspondng elements of the ntra quantzer matrx ntra W, and secondly, they are quantzed by Qstep value q rangng from 1 (, j to 112. B / q for = j = 0 {(32 B + sgn( B qw }/(2 qw for all, jexcept = j= 0 (, j ntra ntra (, j (, j (,j (,j (, j = 1, B (, j > 0 QB (, where sgn( B (, j = 0, B(, j == 0 1, B (, j < 0 (2 B qw sgn( B ntra (, j (, j = { Q( B(, j (, j } 16 2 (3 ntra B (, j q W(,j sgn( B(, ( j = { QB ( (, j } q' q' 16 2 QB ( = (32 B /(2 qw, 0, j 7 (5 non-ntra (, j (, j (, j q non-ntra (6 B(, j = W(,j Q( B(, j 16 non-ntra B(, j q W(,j = QB ( (, j q q 16 Now let us change Eq. 2 to transcode the quantzed coeffcents, QB ( (, j, to newly quantzed values of H.26/AVC. Frst, the DC coeffcents are consecutvely dequantzed by q and then re-quantzed by new Qstep q. Secondly, the unquantzed AC coeffcents, B (, j, of MPEG-2 as descrbed n Eq. 3, derved from Eq. 2, can be calculated the newly quantzed coeffcents usng q as descrbed n Eq.. Brefly, usng Eq., we convert quantzed AC coeffcents of MPEG-2 nto newly quantzed coeffcents of H.26/AVC. hs method has an effect of skppng MPEG-2 quantzaton. hat s, we can calculate the newly quantzed coeffcents of H.26/AVC as f we performed quantzaton only once wth the unquantzed coeffcents, B wthout any quantzaton for MPEG-2. As a result, the proposed (, j Qstep converson scheme can reduce the dequantzaton errors of MPEG-2 bt-stream compared wth cascaded pxel-doman scheme. As n the case of ntra-mb converson, Eq. 6 for unquantzed coeffcents s derved from the MPEG-2 quantzaton formula, Eq. 5, where non-ntra W ndcates (,j th (, j (7

4 element of the quantzer matrx for non-ntra MB, and the newly quantzed coeffcents B can be calculated by dvdng (, j q B (, j by q. 3.2 DC Converson he 8 8 DC block converson nto four approxmate DC blocks s extended from the pxel-doman extractng. In the pxel-doman, extractng of the subblock B from B s defned by Eq. 8, where B s an 8 8 moton compensated (MC block or a ntra block n the pxel-doman, and L, R, = 0,..,3 s the dsplacement matrces that perform vertcal and horzontal flterng, respectvely. he order of subblock B s a raster scan order n B. he matrces L sze of 8 and R sze of 8 are defned n Eq. 9, where I s dentty matrx of sze. he pre-multplcaton by L vertcally extracts a sub-block sze of 8 from B whereas, the post-multplcaton by R horzontally extracts block from the resultant block. B = L B R(0 3 (8 ( ( L0= L1= I 0, L 8 2= L3= 0 I, 8 I 0 R = R =, R = R = I 8 8 he DC block can be calculated by performng DC on the extracted sub-block, B of Eq. 8. Because of the dstrbutve property of matrx multplcaton wth respect to the DC, transformed matrx of B s gven by B = L B R as represented n Eq. 10, where B,,, and L B R are the DC representatons of B, L, B and R respectvely. Especally, and L R can be wrtten n the form of H L H 8 8 and H88 R H as descrbed n Eq. 11, where H and H are the transform matrces 8 8 for and 8 8 DC, respectvely, and H ndcates the transpose of H. Consequently, B can be rewrtten as the form of Eq. 11. hrough ths equaton, we can smply change an 8 8 DC block nto four DC blocks, because the transform matrces, H, H, H, 8 8 H 88, and flterng matrces L, R(0 3 are already known matrces and we can make up the look-up table usng them. However, B (0 3 s not dentcal wth the approxmate DC block, B of H.26/AVC, because the transform matrces are dfferent as mentoned n secton 2. Accordngly, t s necessary to compensate the dfference, and Eq. 12 shows our fnal formula. In Eq. 12, we can obtan B by usng H, whch changes elements of H, 1 π b = 2 cos( and c/ b = 0.1 to b = 2 5 and c/ b= 1 2, nstead of H. he specfc dervaton processes are shown n Eq. 13 ~ Eq. 15. In Eq. 1, H88 H88 (9

5 and H88H88 are an dentty matrx, respectvely, due to the orthogonal property of the DC transform matrx. Lastly, we can get the form of Eq. 15 by applyng the assocatve property of matrx multplcaton to Eq. 1. hs Equaton s equal to Eq. 12, because the matrx multplcaton H 8 8 B H 8 8 s dentcal to B. As a consequence, we can exactly converts an 8 8 DC block nto four approxmate DC blocks of H.26/AVC wth the modfed transform matrx H. As stated earler, the product of matrces, H L H 8 8 and H 88 R H n Eq. 12 can be precomputed and stored n the memory as a look-up table. DC ( B = DC ( L B R = DC ( L DC ( B DC ( R (10 = B = L B R, (0 3 B = ( H L H ( B ( H R H (0 3 ( B = ( H L H ( B ( H R H (0 3 ( B = H B H = H ( L B R H = ( H L B ( R H (13 = ( H L ( H H B ( H H ( R H ( = ( H L H ( H B H ( H R H ( Expermental Results hroughout the varous experments we compare the performance of our proposed scheme wth the cascaded re-encodng method of pxel-doman. In ths paper, we call the cascaded re-encodng method cascade method. For smulaton, we mplemented DC-based transcodng archtecture and cascaded pxel-based transcodng archtecture by modfyng the MPEG-2 est Model 5 (M5 codec [12] and adoptng the H.26 Jont Model 8 (JM8 encoder partally [13]. For convenent smulaton, we fxed Qstep for all frames n a vdeo sequence. In experments, we tested three vdeo PSNR(dB MPEG-2 Qstep=5 clare-base clare-conv carphone-base carphone-conv football-base football-conv H.26 Qstep Fg. 1. he comparson of PSNR for test vdeo sequences when Qstep s fxed to 5 n MPEG-2 encodng and the re-qstep s changed from 1 to 0. PSNR(dB MPEG-2 Qstep=30 clare-base clare-conv carphone-base carphone-conv football-base football-conv H.26 Qstep Fg. 2. he comparson of PSNR for test vdeo sequences when Qstep s fxed to 30 n MPEG-2 encodng and the re-qstep s changed from 1 to 0.

6 sequences wth the dfferent moton characterstc: FOOBALL (hgh moton degree, CARPHONE (medum degree, and CLAIRE (low degree. In the smulaton, nput vdeo sequences of Quarter Common Intermedate Format (QCIF were encoded by the MPEG-2 encoder at constant frame rate of 30, GOP sze of 6, and I/P dstance of 3, and then, the output bt-streams of the MPEG-2 encoder are transcoded to H.26 bt-streams. Fg. 1 and Fg. 2 shows the peak sgnal-to-nose rato (PSNR comparson as changng the re-qstep from 1 to 0 of the transcoder at fxed MPEG-2 Qstep 5 and 30, respectvely. In the legend of the fgures, -base refers to cascade method and - conv refers to our proposed scheme. In Fg. 1, we cannot make a dscrmnaton between the performance of the proposed scheme and the cascade method even though our method s numercally hgher than the cascade method by 0.01 ~ 0.0 db. Contrast to ths result, Fg. 2 shows dfferent results that PSNR of our method s hgher than the cascade method by maxmum 0.5 db. hs s due to our quantzaton scheme that has an effect of skppng MPEG-2 quantzaton as explaned n secton 3.1. Fg. 3 and Fg. show PSNR of the frames rangng from 1 to 30 for CARPHONE and CLAIRE sequences, respectvely. Wth ths fgures, we can observe that, as a whole, the proposed scheme shows hgher performance than the cascade method. Especally, for I frames, t s more superor to the cascade method. he reason s due to the magntude of the Moton Compensated DC (MC-DC coeffcents. More specfcally, because the magntude of the ntra-coded block coeffcents s generally larger than the nter-coded block coeffcents, the quantzaton errors of the ntracoded block are larger than the nter-coded block at hgh Qstep. Conversely, because our scheme has an effect of skppng the MPEG-2 quantzaton process, t can mantan the vdeo qualty. Fg. 5 shows the reconstructed real mages carred out the transcodng process at Qstep 20 for MPEG-2 encodng and at re-qstep 6 for H.26/AVC transcodng. he cascade method mages are arranged n the frst row, whle the proposed method mages are n the second row. FOOBALL, CARPHONE, and CLAIRE mages are arranged from left to rght. Let us look hard at the mages and compare each pars. Frst, for the FOOBALL sequence, the second player s hp area of our mage (b s 3.5 CARPHONE Qstep: 20 to CLAIRE Qstep: 20 to 8 PSNR(dB Frame number PSNR(dB Frame number Fg. 3. he PSNR performance for the CARPHONE sequence rangng from frame 1 to frame 30, where Qstep s set to 20 for MPEG-2 encodng and re-qstep s set to 6 for H.26/AVC. Fg.. he PSNR performance for the CLAIRE sequence rangng from frame 1 to frame 30, where Qstep s set to 20 for MPEG-2 encodng and re-qstep s set to 8 for H.26/AVC

7 clearer than that of the cascade method (a. Second, for CARPHONE, we cannot dscover notable dfference. hrd, for CLAIRE, the woman s slhouette and background of our mage (f are clearer wthout blurrng than the cascade method (e. (a (c (e (b (d Fg. 5. he real mages for the sample vdeo sequences at Qstep=20 and re-qstep=6: FOOBALL (a(b CARPHONE (c(d CLAIRE (e(f. he cascade method mages are arranged n the frst row, whle the proposed method mages are n the second row able 1 he requred operatons to transcode a quantzed 8 8 DC block to newly quantzed approxmate DC blocks Functon Cascade method Proposed method Proposed method (Inter or Intra block (Inter (Intra IQ/Q 128M+1D+6A+6 192M+128A+192S 6M+1D (or Qcon S DC converson - 1,02M+896A IDC 1,02M+896A - 102M+896A approxmate DC 256A+6S - 256A+6S otal operatons 1,216M+1,280A+256S 1,088M+1D+896A 1,152M+1D+1,02A +128S Fnally, we represent the computatonal complexty by comparng the number of multplcatons and addtons. We have mplemented IQ, IDC, DC, and re-q for the cascade method, and quantzaton converson and DC converson for the proposed method, respectvely. For the cascade method, we do not consder fast DC/IDC methods, but only consder the conventonal DC/IDC methods. Fast DC/IDC methods can reduce computatonal complexty by re-usng ntermedately calculated values nstead of several multplcatons and addtons for transform. Even though our proposed method also can be mplemented by the same knd of fast method, we do not consder the fast method n ths paper. For ntra-coded block, we use the cascaded re-encodng method n pxel doman to avod too much computaton complexty. able 1 shows the number of operatons to transcode a quantzed block of 8 8 sze of MPEG-2 to newly quantzed four approxmate DC blocks of H.26/AVC, where M stands for multplcaton operatons, A for adds, S for Shft, and D (f

8 for dvson. For a bref comparson of computatonal complexty, we gnore the number of addton, shft, and subtracton operatons because the overhead of the multplcaton and dvson operatons are hgher than any other operaton. Accordng to able 1, the proposed scheme allows a computatonal complexty savng of about 11% for non-ntra-coded blocks and about 5% for ntra-coded blocks, compared wth the cascade method. 5 Conclusons In ths paper, we proposed an effcent and exact converson algorthm for the quantzed 8 8 DC block of MPEG-2 nto newly quantzed four approxmate DC blocks of H.26/AVC to support DC-doman transcodng. Wth the slght modfcaton of quantzaton converson, ths method also can be appled to MPEG-1, MPEG- and H.263, etc. Extensve smulaton results show that the PSNR of the proposed method outperforms the cascaded re-recodng method n the pxel doman by 0.1 ~ 0.5 db and the computatonal complexty can be reduced by 5~11%. Our next subject s to fnd out an effcent half pxel converson algorthm to compensate the dfference of half pxel value between MPEG-2 and H.26/AVC and s to ntegrate the algorthm and our DC converson algorthm. References 1. ISO/IEC :2003, Codng of Audovsual Objects Part 10: Advanced Vdeo codng and IU- Recommendaton H.26 :Advanced vdeo codng for generc audovsual servces. 2. Vetro, A.; Chrstopoulos, C.; Sun, H, Vdeo ranscodng Archtectures and echnques: An Overvew, IEEE Sgnal Processng Magazne, Vol.20, Issue2, pp.18-29, March H. Sun, W. Kwok, and J. Zdepsk, archtectures for MPEG compressed btstream scalng, IEEE rans. Crcuts Syst. Vdeo echnol., vol. 5, pp , Aprl N. Bjork and C. Chrstopoulos, rascoder archtectures for vdeo codng, IEEE rans. Consumer Electron., vol., pp.88-98, February J. Youn, M.. Sun, and C.W. Ln, Moton vector refnement for hgh performance transcodng, IEEE rans. Multmeda, vol.1, pp.30-0, March J.N. Hwang,.D. Wu, and C.W. Ln, Dynamc frame-skppng n vdeo transcodng, In Proc. IEEE Workshop Multmeda sgnal processng, Redondo Beach, CA, pp , December Kalva, H.; Vetro, A.; Sun, H., Performance Optmzaton of the MPEG-2 to MPEG- Vdeo ranscoder, SPIE Conference on VLSI Crcuts and Systems, Vol. 5117, pp , May Hayan Shu, An Effcent Arbtrary Downszng Algorthm for Vdeo, ranscodng, IEEE rans. Crcuts Syst. Vdeo echnol., vol.1, no.6, pp , June Kwang-deok Seo, Jae-Kyoon Km: Fast moton vector re-estmaton for transcodng MPEG-1 nto MPEG- wth lower spatal resoluton n DC-doman, Sgnal Processng: Image Communcaton, vol.19, pp , Ian E.G. Rchardson, H.26 and MPEG- vdeo compresson. Wlley, Henrque S. Malvar, Low-Complexty ransform and Quantzaton n H.26/AVC, IEEE rans. Crcuts. Syst. Vdeo echnol,. vol.13. no.7, pp , July