Detection of Double AVC/HEVC Encoding

Transcription

1 26 24th European Sgnal Processng Conference (EUSIPCO) Detecton of Double AVC/HEVC Encodng Andrea Costanzo, Mauro Barn, Fellow, IEEE Dept. of Informaton Engneerng and Mathematcs, Unversty of Sena, Italy E-mal: Abstract New generaton vdeo codecs are desgned to mprove codng effcency wth respect to prevous standards and to support the latest hardware and applcatons. Hgh Effcency Vdeo Codng (HEVC) s the successor of Advanced Vdeo Codng (AVC), whch s by far the most adopted standard worldwde. To promote the new standard, producers are re-releasng recent moves n HEVC format. In such a scenaro, a fraudulent provder that does not own the orgnal uncompressed data could sell old, lower qualty AVC content re-encoded as f t were natvely HEVC. Furthermore, wth several hundred hours of vdeo content uploaded every mnute, t s not unlkely that re-edted low qualty clps are labelled as HEVC to ncrease popularty and revenues from advertsng. We tackle wth these and smlar ssues by proposng a forensc technque to detect whether a HEVC sequence was obtaned from an uncompressed sequence or by re-encodng an exstng AVC sequence. Index Terms Multmeda Forenscs, Vdeo Forenscs, Vdeo re-encodng, HEVC, AVC. I. INTRODUCTION Hgh Effcency Vdeo Codng (HEVC) [], [2] s the new generaton vdeo compresson standard jontly developed by the ISO/IEC Movng Pcture Experts Group and the ITU-T Vsual Codng Experts Group. HEVC has been developed to overcome the lmtatons of the Advanced Vdeo Codng (AVC) standard [3], whch despte ts ten years of age s stll today the de-facto reference for vdeo compresson. The HEVC standardzaton effort was drven by the growng demand for effcency, n order to enable a set of dverse new applcatons ncludng Ultra HD resolutons (e.g. 4K and 8K), 3D and multvew dsplay. Followng ts thrd release n early 25, HEVC has begun replacng AVC thanks to the growng support from hardware producers and leadng companes n move entertanment, moble communcatons and Internet servces. Durng ths transton perod, exstng vdeo content s beng re-released nto the market n HEVC format to promote the dffuson of the latest standard. New securty ssues rase n such a scenaro. For nstance, a fraudulent servce that does not own the orgnal uncompressed data could attempt to sell old, lower qualty AVC content re-encoded as f t were natvely HEVC. Addtonally, several hundred hours worth of vdeo content are uploaded every mnute to sharng platforms such as YouTube, whch totalses more than four bllon vews overall each day. It s reasonable to hypothesze that a share of such contents are low qualty AVC vdeos, re-encoded as HEVC and wrongly Source: (or delberately) tagged as HEVC to ncrease popularty and, consequently, revenues from advertsng. The Multmeda Forenscs research communty has devoted a sgnfcant effort to the authentcaton and ntegrty verfcaton of vdeo content [4]. Several technques have been devsed to dentfy footprnts left by the acquston devce [5] [7], the employed codec and ts parameters [8] [3], multple encodngs [4], [5] and forgeres such as frame or object removal or duplcaton [6], [7]. In ths paper we contrbute to the Multmeda Forenscs msson wth a method to detect double AVC/HEVC encodng, under the assumpton that the former compresson has a lower qualty than the latter compresson. The man dea behnd the technque we have developed stems from the observaton that the frst AVC encodng tends to alter the statstcal propertes of a vdeo sequence hnderng the flexblty of the subsequent HEVC s quad-tree parttonng. More specfcally, we argue that the frst AVC encodng nfluences the decson strategy of the subsequent HEVC encodng regardng the moton predcton modes n b-drectonally predcted frames. Based on such an observaton, the goal of ths paper s to present a method to reveal double AVC/HEVC encodng and to estmate the frst AVC quantzaton parameter. The rest of the paper s organzed as follows. In Sec. II, we brefly ntroduce the HEVC codec. In Sec. III, we descrbe the footprnt we rely on to detect double AVC/HEVC encodng. In Sec. IV, we valdate our technque and we show how to use t to estmate the quantzaton parameter of the former AVC compresson. II. HIGH EFFICIENCY VIDEO CODING (HEVC) Due to space lmtatons, only the key-features of HEVC requred to understand the method presented n Sec. III are brefly dscussed. The nterested reader s referred to [2] for a thorough descrpton. Lke the majorty of pror vdeo codng standards, HEVC processes all pctures of the nput sequence n block unts, each of whch s compared to a reference block that s computed ether from prevously decoded pctures (nter-predcton) or prevously decoded samples from the same pcture (ntrapredcton). The reference block s then subtracted from the nput block to obtan the resduals, whch are transformed (usually n the frequency doman), quantzed and entropy coded. Specfcally, HEVC parttons each pcture nto Codng Tree Unts (CTU), whose fxed sze can be ether 6 6, or, more commonly, Each CTU conssts of /6/$3. 26 IEEE 2245

2 26 24th European Sgnal Processng Conference (EUSIPCO) three Codng Tree Blocks (CTB): one for the luma samples and two for the chroma samples n YCbCr color space. The sze of luma CTB s the same as the CTU sze, whereas the chroma CTBs sze depends on the adopted samplng scheme 2. To better adapt the codng parameters to the local content, each CTB can be further sub-dvded accordng to a quadtree structure, the same for luma and chroma, nto smaller Codng Blocks (CB) down to a mnmum of 8 8 for luma and 4 4 for chroma. The luma CB and the correspondng chroma CBs are logcally grouped nto a Codng Unt (CU). The predcton type (.e. nter or ntra) s decded at the CU level. To mprove the accuracy of the predcton, each CU can be sub-dvded nto smaller square or rectangular Predcton Unts (PU) dependng on the temporal/spatal predctablty of the content. Once the predcton s made at the PU level, each CU s splt nto Transform Unts accordng to a second quad-tree that may not be algned to the frst one. Each TU s transformed (nteger DCT), quantzed and entropy coded [2]. In HEVC and AVC quantzaton s based on a scalar quantzer whose step s derved from an ndex called Quantzaton Parameter (QP) assumng nteger values n [, 5]. The quantzaton step sze doubles wth every ncrement of 6 n QP accordng to a logarthmc structure. Larger QPs acheve hgher compresson rates at the expense of vsual qualty. Typcally, CTUs are grouped nto slces. The number of slces needs to be constant across the whole vdeo sequence, whereas the number of CTUs n each slce does not. Even though slces can be useful to recover from data losses at the cost of compresson effcency, t s common practce n many HEVC applcatons to use a sngle slce for each pcture. A slce can belong to three dfferent categores, called I-, P- and B-, defnng the predcton types avalable to that slce: CUs n I- slces can only be ntra-predcted; CUs n P- slces can ether be ntra or un-predcted,.e. nter-predcted from a sngle reference pcture; CUs n B- slces can ether be nterpredcted or b-predcted,.e. nter-predcted by resortng to more than one prevously encoded reference pcture. To further ncrease codng effcency n P- and B- slces, HEVC can resort to the skp mode. A Skp CU conssts of a sngle PU whose moton data s derved from neghbourng CUs. No resduals are transmtted n Skp mode. III. THE PROPOSED FOOTPRINT HEVC mproves AVC n nearly all aspects. Arguably, the most sgnfcant leap forward conssts of the frame parttonng scheme. AVC sub-dvdes each pcture nto blocks of sze 6 6, whch can be further splt only nto 4 blocks of sze 8 8 or nto 6 blocks of sze 4 4. Conversely, HEVC chooses block szes n a more flexble and dynamc fashon accordng to a quad-tree structure whch depends on the statstcal propertes of the content. The more (the less) the moton n a regon, the smaller (the larger) the sze of the CUs coverng the regon. In ths way, HEVC sgnfcantly boosts 2 For example, n the wdely used 4:2: samplng scheme, each CTU conssts of one luma CTB and two chroma CTBs. moton predcton accuracy and ether provdes much better perceptual qualty at the same bt-rate or hgher compresson effcency at the same qualty. In fact, HEVC bt-rate reducton wth respect to AVC s about 5% accordng to [2]. A. Footprnt Descrpton Suppose that a vdeo sequence s obtaned from an uncompressed source by encodng t twce, frst wth AVC and then wth HEVC. Suppose also that the AVC quantzaton parameter s larger than the one adopted by HEVC,.e. the latter compresson s hgher qualty than the former. We hypothesze that the former AVC alters the statstcal propertes of the vdeo sequence enough to break the flexblty of the subsequent HEVC s quad-tree parttonng, even when the two QPs are close (and thus the perceptual qualty s smlar) and no vsual clues of double encodng can be notced. If ths were true, then n prncple t would be possble to dstngush between such a sequence and a sequence obtaned by encodng the same source once wth HEVC. To verfy our hypothess, we focused on the moton predcton modes chosen for CUs by the HEVC encoder n P- and B- frames (I- frames have no moton nformaton). More specfcally, we checked the frequency of moton predcton modes over all B- frames of vdeo sequences that were compressed once wth HEVC and twce accordng to the sequence AVC/HEVC. Interestngly, we observed that n B- frames the frequency of b-predcted CUs s sgnfcantly hgher than that of un-predcted CUs when the HEVC sequence s obtaned by startng from an uncompressed clp. Conversely, when the HEVC sequence s obtaned by re-encodng a lower qualty AVC sequence, P- CUs represent the large majorty of all the unts. We beleve that ths happens because the prevous lower qualty AVC encodng reduces the temporal dfferences between blocks, thus preventng the HEVC codec from usng the more accurate b-predcton n favour of the coarser unpredcton. An example of such a behavour s gven n Fg. for all the B- frames of the Tenns sequence 3 : frequences of the sngle (resp. double) compressed CUs are shown on the left (resp. rght). As t can be seen, P- CUs sgnfcantly ncrease f the sequence was orgnally compressed wth AVC. Frequency n B- frames.9.8. Tenns, QP_hevc = 5 P- CU B- CU Skp Frequency n B- frames Tenns, QP_avc = 2, QP_hevc = P- CU B- CU Skp Fg.. Frequency of moton predcton modes n B- frames (Tenns). 3 Ths s a Hgh Defnton (HD 92 8p) sequence belongng to the Class B benchmarks of the JCT-VC (Jont Collaboratve Team on Vdeo Codng). It should not be confused wth the homonym sequence. 2246

3 26 24th European Sgnal Processng Conference (EUSIPCO) In the followng, we wll refer to the footprnt of a vdeo sequence V as F V = [P, B, Skp], whch s the array wth the frequences of each type of moton predcton. B. Detecton of AVC/HEVC compresson sequence In order to explot the footprnt descrbed n the prevous secton to detect a sequence of AVC/HEVC encodng, we rely on the concept of vdeo compresson dempotency frst ntroduced n [2], [8]. Idempotency of lossy codng states that whenever a vdeo codng scheme s appled twce wth the same parameters to a vdeo, t produces approxmately the same output as f t were appled once. Hence, assumng that the codec s known, one can re-encode the sequence under analyss wth dfferent parameters and verfy whether the result s smlar to the nput sequence. If that s the case, then most lkely the parameters used for re-encodng correspond the parameters used to orgnally code the vdeo. We now apply the above deas to solve the followng problem: gven a sequence V whose last QP hevc s known, determne whether V has been compressed once wth HEVC or twce accordng to the sequence AVC/HEVC wth unknown QP avc >QP hevc. Specfcally, the soluton we are proposng works as follows: Extract the footprnt F V from V ; Re-encode V wth AVC wth ncreasng QP avc (d) = {QP hevc +,..., 5} followed by HEVC wth QP hevc to obtan a sequence of vdeos V, =,..., length(qp (d) avc ); Extract the footprnt F V from each V ; Measure the dstances d(f V, F V ) = F V F V, where each dfference s element-wse. Use the dstances computed at the prevous pont to detect AVC/HEVC re-encoded vdeo. Wth regard to the last pont, f V was compressed just once wth HEVC, we expect d(f V, F V ) to be large for all QP (d) avc. Ths s due to the fact that the frequences of moton predcton modes n all the sequences V have been altered wth respect to the orgnal sngle compressed HEVC vdeo as n Fg.. Conversely, f V was compressed twce, we expect d(f V, F V ) to be mnmum n correspondence of the QP avc (d) that concdes wth the orgnal QP avc used to encode V. To show that ths s ndeed the case, let us consder the example n Fg. 2 (a more detaled analyss wll be gven n n Sec. IV-B): the sold (resp. dashed) lne plots the set of dstances for the sngle (resp. double) compressed Foreman sequence (QP avc = 3, QP hevc = ). For both the sngle and the double-compressed sequence there exsts a crtcal past whch the dstance between the footprnts starts ncreasng; n the case of sngle compresson, ths happens n correspondence of the QP hevc declared n the bt-stream nformaton of the vdeo under scrutny; n the case of double compresson, n correspondence of the orgnal QP avc. Let QP crt be such crtcal value, adopt the followng decson QP (d) avc.8. Foreman (352x288) Sngle: QP_hevc = Double: QP_avc = 3, QP_hevc = Sngle QP crt QP_avc (dempotency) Double QP crt Fg. 2. Example of dstances followng dempotency analyss. rule to classfy V : { V HEVC f QP crt = QP hevc AVC/HEVC f QP crt > QP hevc. In the latter case, the quantzaton parameter of the old AVC compresson concdes wth QP crt. IV. EXPERIMENTAL VALIDATION In ths secton, we frst provde a qualtatve analyss of the proposed footprnt on two vdeo sequences, then we evaluate the performance of the method n terms of detecton accuracy on a data set of 2 YUV test sequences 4. Unless specfed otherwse, all sequences have resoluton and ther frame count ranges from 5 to 2. Smlarly to other works, we focused on low resoluton vdeos to keep the tme complexty under control. We compressed the vdeos by means of the open-source lbx264 (AVC) and lbx265 (HEVC) codecs ncluded n the well establshed FFmpeg multmeda framework 5. We used a commercal HEVC bt-stream analyser and a n-house C++ parser to extract all the nformaton. We made two assumptons regardng the quantzaton parameters: () QP avc {,, 2,..., 4} (2) QP avc > QP hevc. (3) Eq. (2) restrcts QP avc to the range of compressons that do not mpact too much on the vsual qualty (otherwse, the tobe-consdered HEVC vdeo would hardly fool anyone), whle Eq. (3) s n lne wth the scenaro of Sec. III. To determne QP crt, we computed the frst order forward dfference of the dstance vector,.e. f(k) = d(k + ) d(k), k [, length(d) ]; then, we selected the QP avc (d) correspondng to the frst k such that f(k) > T, where T s a fxed threshold expermentally derved n Sec. IV-B. 4 Namely: Akyo, BasketBallDrll (qualtatve analyss only), BrdgeClose, BrdgeFar, Bus, Carphone, Coastguard, Contaner, Flower, Football (qualtatve analyss only), Foreman, Ice, MssAmerca, Moble, MotherDaughter, News, Pars, Slent, Soccer, Stefan, Tempete, Tenns (Sec. III only), Waterfall. Download: and 5 Software downloadable from: (FFmpeg); (lbx264); (lbx265). 2247

4 26 24th European Sgnal Processng Conference (EUSIPCO) A. Qualtatve analyss of double AVC/HEVC detecton In ths experment, we compressed the Football sequence wth HEVC wth QP hevc = {,, 2, 3, 4} to obtan fve sngle compressed vdeos. Then, we re-encoded each vdeo wth AVC wth ncreasng QP avc (d) (step 5 to reduce computatonal complexty) followed by HEVC wth the orgnal QP hevc. Fnally, we computed the dstance between the nput footprnt and the footprnt of the re-encoded vdeos. The results we obtaned are shown n Fg. 3, where each curve corresponds to a sequence 6. Expectedly, when QP avc (d) QP hevc no valuable nformaton can be obtaned, snce double encodng can not be revealed when the frst AVC compresson s not strong enough to alter HEVC moton predcton. Conversely, when QP avc (d) > QP hevc, the dstance between footprnts rapdly ncreases.. Orgnal QP_hevc = Orgnal QP_hevc = Orgnal QP_hevc = 2 Orgnal QP_hevc = 3 Orgnal QP_hevc = QP_avc (dempotency) Fg. 3. Dstance between the sngle compressed nput sequence V and dempotency s outputs V. Each curve corresponds to a dfferent nput sequence obtaned by varyng the orgnal HEVC quantzaton parameter. In the second experment, we compressed the Football sequence twce, wth AVC wth QP avc = {5, 2, 25, 3, 35} followed by HEVC wth fxed QP hevc =. Then, we re-encoded each of the fve double compressed sequences wth AVC wth ncreasng QP avc (d) followed by HEVC wth QP hevc =. By observng the results shown n Fg. 4, we notce that the dstance between footprnts s null untl the QP avc (d) used for re-encodng s lower than the orgnal QP avc. Agan, ths happens because re-compresson s hgher qualty than the orgnal compresson, thus producng a sequence that s extremely smlar to the nput sequence. When QP avc (d) s greater or equal than the real QP, the dstance starts growng. One advantage of the proposed method s that t does not requre many frames to obtan a relable footprnt. In Fg. 5, we can observe that the shape of d(f V, F V ) does not depend sgnfcantly on the number of frames, ether for sngle (left) or double (rght) compressed sequences (BasketBallDrll, , QP avc = 3, QP hevc = ). Recall that we resort only to B- frames, whch, accordng to our analyss over all the compressed vdeos, represent on average about the 7% of the totalty when x265 default optons are used [9]. Then, we do not need more than a few seconds of content to classfy a sequence as sngle or double compressed. 6 For example, the dashed blue lne wth trangle markers (Fg. 3) s the dstance between the footprnt of the nput sequence, orgnally compressed once wth HEVC QP hevc =, from all the sequences that are re-encoded wth ncreasng QP avc followed by QP hevc =.. Orgnal QP_avc = 5 + QP_hevc = Orgnal QP_avc = 2 + QP_hevc = Orgnal QP_avc = 25 + QP_hevc = Orgnal QP_avc = 3 + QP_hevc = Orgnal QP_avc = 35 + QP_hevc = QP_avc dempotency (QP_hevc = ) Fg. 4. Dstance between the double compressed nput sequence V and V. Each curve corresponds to a dfferent nput sequence obtaned by varyng the frst AVC quantzaton parameter BasketBallDrll, HEVC QP_hevc = Frames = (B- frames = 69) Frames = 2 (B- frames = 39) Frames = 3 (B- frames = 29) Frames = 4 (B- frames = 279) Frames = 5 (B- frames = 348) QP_avc (dempotency).9.8. BasketBallDrll, QP_avc = 3, QP_hevc = Frames = (B- frames = 64) Frames = 2 (B- frames = 29) Frames = 3 (B- frames = 2) Frames = 4 (B- frames = 272) Frames = 5 (B- frames = 34) QP_avc (dempotency) Fg. 5. Dstance between footprnts dependng on the number of analysed frames (BasketBallDrll). Left: HEVC; rght: AVC/HEVC. B. Detecton accuracy of double AVC/HEVC detecton We evaluated the detecton accuracy of the method n terms of ROC (Recever Operatng Characterstc) curves as follows. We encoded the 2 YUV test sequences to create 2 sngle compressed and 2 double compressed versons; we computed the dstance as n Sec. III-A; we vared the decson threshold T n the nterval [, ] wth step.5; for each T, we carred out the classfcaton task as n Eq. (). Results are shown n Fg. 6, where we have repeated the experment twce for (QP avc, QP hevc ) = (3, ) and (2, ) (sold/dashed lne). Expectedly, the closer the QPs, the harder the classfcaton. Nevertheless, results are ndeed satsfactory, consderng that the AuC (Area Under Curve) s.973 and.926 respectvely. True Postve Rate.9.8. QP_avc = 3, QP_hevc = QP_avc = 2, QP_hevc =..8.9 False Postve Rate Fg. 6. Accuracy of sngle vs. double classfcaton. ROC curves obtaned on the data set of 4 vdeo sequences by varyng the decson threshold of Eq. (), wth fxed QP avc = 3 (sold lne) / 2 (dashed lne) and QP hevc =. Eventually, we evaluated the accuracy of the estmaton of the old AVC quantzaton parameter. To do so, we fxed 2248

5 26 24th European Sgnal Processng Conference (EUSIPCO) T = 5, correspondng to a false alarm rate of., and we focused on those vdeos that were correctly classfed as double compressed (only n the case QP avc = 3, QP hevc = ),.e. 8 out of 2 sequences. We found out that for 6 of them, QP crt concdes wth the true value 3, whereas for the remanng two, we got 2. Interestngly, the outcome of QP estmaton does not change as T ncreases: nstead, t seems related to the smooth content and very small moton of such two vdeos. Arguably, a correct estmaton would requre more B- frames than those among the frames under analyss. V. CONCLUSION We have proposed a new forensc technque to dscrmnate between uncompressed vdeo sequences encoded once wth HEVC and lower-qualty AVC sequences re-encoded wth HEVC. Dscrmnaton s based on the presence of alteratons ntroduced by the ntal AVC compresson to the decson strategy of the subsequent HEVC encodng regardng the moton predcton modes. We showed that, n the case of double encodng, we can also estmate the former AVC quantzaton parameter. We plan to devote further research to assessng the performance of our method on Hgh Defnton content; determnng the relatonshp between the amount of vdeo moton and the number of frames for fngerprnt stablty; consderng dfferent codecs for the frst compresson. Fnally, we wll adapt our method locally to the detecton of splcng of HEVC and AVC sequences. [] M. Taglasacch and S. Tubaro, Blnd estmaton of the QP parameter n H. 264/AVC decoded vdeo, n Image Analyss for Multmeda Interactve Servces (WIAMIS), 2 th Internatonal Workshop on. IEEE, 2, pp. 4. [2] P. Bestagn, A. Allam, S. Mlan, M. Taglasacch, and S. Tubaro, Vdeo codec dentfcaton, n Acoustcs, Speech and Sgnal Processng (ICASSP), 22 IEEE Internatonal Conference on. IEEE, 22, pp [3] T. Gloe, A. Fscher, and M. Krchner, Forensc analyss of vdeo fle formats, Dgtal Investgaton, vol., pp. S68 S76, 24. [4] S. Mlan, P. Bestagn, M. Taglasacch, and S. Tubaro, Multple compresson detecton for vdeo sequences, n Multmeda Sgnal Processng (MMSP), 22 IEEE 4th Internatonal Workshop on. IEEE, 22, pp [5] D. Vazquez-Padn, M. Fontan, T. Banch, P. Comesaña, A. Pva, and M. Barn, Detecton of vdeo double encodng wth GOP sze estmaton, n Informaton Forenscs and Securty (WIFS), 22 IEEE Internatonal Workshop on. IEEE, 22, pp [6] W. Wang and H. Fard, Exposng dgtal forgeres n vdeo by detectng duplcaton, n Proceedngs of the 9th workshop on Multmeda & securty. ACM, 27, pp [7] J. Zhang, Y. Su, and M. Zhang, Exposng dgtal vdeo forgery by ghost shadow artfact, n Proceedngs of the Frst ACM workshop on Multmeda n forenscs. ACM, 29, pp [8] P. Bestagn, S. Mlan, M. Taglasacch, and S. Tubaro, Vdeo codec dentfcaton extendng the dempotency property, n Vsual Informaton Processng (EUVIP), 23 4th European Workshop on. IEEE, 23, pp [9] MultcoreWare x265 documentaton, Tech. Rep. MultcoreWare Inc. Revson e95ab673bc+. [Onlne]. Avalable: org/en/default/ REFERENCES [] G. J. Sullvan, J.-R. Ohm, W.-J. Han, and T. Wegand, Overvew of the hgh effcency vdeo codng (HEVC) standard, Crcuts and Systems for Vdeo Technology, IEEE Transactons on, vol. 22, no. 2, pp , 22. [2] V. Sze, M. Budagav, and G. J. Sullvan, Hgh Effcency Vdeo Codng (HEVC). Sprnger, 24. [3] T. Wegand, G. J. Sullvan, G. Bjøntegaard, and A. Luthra, Overvew of the H. 264/AVC vdeo codng standard, Crcuts and Systems for Vdeo Technology, IEEE Transactons on, vol. 3, no. 7, pp , 23. [4] S. Mlan, M. Fontan, P. Bestagn, M. Barn, A. Pva, M. Taglasacch, and S. Tubaro, An overvew on vdeo forenscs, APSIPA Transactons on Sgnal and Informaton Processng, vol., p. e2, 22. [5] M. Chen, J. Frdrch, M. Goljan, and J. Lukáš, Source dgtal camcorder dentfcaton usng sensor photo response non-unformty, n Electronc Imagng 27. Internatonal Socety for Optcs and Photoncs, 27, pp. 65 5G 65 5G. [6] S. Bayram, H. T. Sencar, and N. Memon, Effcent technques for sensor fngerprnt matchng n large mage and vdeo databases, n IS&T/SPIE Electronc Imagng. Internatonal Socety for Optcs and Photoncs, 2, pp [7] M. Vsentn-Scarzanella and P. L. Dragott, Vdeo jtter analyss for automatc bootleg detecton, n Multmeda Sgnal Processng (MMSP), 22 IEEE 4th Internatonal Workshop on. IEEE, 22, pp. 6. [8] S. Tjoa, W. S. Ln, and K. Lu, Transform coder classfcaton for dgtal mage forenscs, n Image Processng, 27. ICIP 27. IEEE Internatonal Conference on, vol. 6. IEEE, 27, pp. VI 5. [9] W. Luo, Y. Wang, and J. Huang, Detecton of quantzaton artfacts and ts applcatons to transform encoder dentfcaton, Informaton Forenscs and Securty, IEEE Transactons on, vol. 5, no. 4, pp. 8 85, 2. [] G. Valenzse, M. Taglasacch, and S. Tubaro, Estmatng QP and moton vectors n H. 264/AVC vdeo from decoded pxels, n Proceedngs of the 2nd ACM workshop on Multmeda n forenscs, securty and ntellgence. ACM, 2, pp