Theodoros Alexandropoulos, Vassili Loumos, Eleftherios Kayafas

Transcription

1 Estmaton of ambent llumnaton varaton between colour mages n the presence of content changes for real-tme llumnaton-nvarant change detecton Theodoros Alexandropoulos, Vassl Loumos, Eleftheros Kayafas atonal Techncal Unversty of Athens School of Electrcal and Computer Engneerng Multmeda Technology Laboratory 9 Heroon Polytechneou st, 5773, Zographou Campus, Athens, reece Tel/Fax: thalexan@telecom.ntua.gr, loumos@cs.ntua.gr, kayafas@cs.ntua.gr Abstract-The task of detectng changes between two mage frames s obstructed by the nfluence of nose and by the exstence of ambent llumnaton varatons between the mage frames. The former s an nherent property of all electronc magng devces. The latter appears when changes n camera exposure or whte balance settngs occur and tends to degrade the effcency of change detecton, f left untreated pror to frame dfferencng. An addtonal dffculty les n the fact that a change detecton process s requred to accurately detect llumnaton changes n the presence of both lumnance and content changes. In ths paper, a lumnance nvarant change detecton method s proposed. It conssts of a three-channel brghtness correcton stage employed for the brghtness normalzaton process and followed by a block-based clusterng method, whch ams to detect content changes from changes caused by the nfluence of nose. I. Introducton Change detecton technques are algorthms used n nspecton applcatons such as ntellgent ndoor survellance systems and ntruder detecton systems. They are expected to perform the change detecton task, whch s not feasble by the straghtforward procedure of mage dfferencng, due to the nfluence of nose and llumnaton changes. Illumnaton changes pose dffcultes n the change detecton process snce an underlyng lumnance offset tends to swamp content changes, f t s not accounted for pror to mage dfferencng. Ambent llumnaton changes may be caused ether by the change n the brghtness of the surveyed scene or by a scene change whch may trgger adjustment of the gan n the mage acquston devce. Moreover, brghtness changes may not be of equal weght to all colour channels. Ths s the case when whte balance settngs of the mage acquston devce are reconfgured or when the scene s subjected to lght of tme-varyng colour temperature. The change of daylght colour temperature s an example of the latter case. Therefore, n order to address these factors, one must take nto account the applcaton of llumnaton change detecton methods n each colour channel separately. Wth the llumnaton offset ssue treated, the second obstacle n the change detecton process les n the dscrmnaton of changes related to content alteraton from changes related to the effect of nose. For ths task, the applcaton of a block-based statstc analyss combned wth clusterng technques s proposed. II. Ambent llumnaton correcton The frst step appled to the proposed change detecton process focuses on correctng the dfferences n ambent llumnaton. Varatons n global lumnance when no other changes occur, are accurately detected wth the use of mage hstograms. A change n the global llumnaton of an mage causes shftng of ts hstogram towards brghter or darker regons. Therefore, one may expect that, wth no content changes present, all pxels of the hstogram of the mage dfference would concentrate n one peak. In practce, the nose effect dctates that the largest percentage of pxels wll spread n a regon around the hstogram peak. The extent of ths regon depends on nose varance. In real condtons, the dffculty that a change detecton applcaton encounters n lumnance normalzaton s the fact that t needs to perform trackng of ambent lumnance varatons when both llumnaton and content changes exst. It s possble that content changes wll ntroduce addtonal peaks to the hstogram of the mage dfference. In such cases, plan peak detecton does not suffce. An

2 example of ths case s llustrated n the hstograms (Fg 2a,2b,2c) of the absolute dfference of the sample frames dsplayed n Fgures a and b. a) b) Fgure.a,b) Sample frames used for the applcaton of change detecton algorthms a) b) c) Fgure 2: Hstograms of the absolute dfference of frames a b correspondng to the (a)red, (b) green and (c) blue colour channel. Apart from a peak denotng the lumnance offset n each colour channel, addtonal peaks are present due to content changes. The brghtness correcton technque proposed n the present paper s based on the assumpton that there s no content change that occupes more than half of the mage plane and causes homogeneous change of brghtness to ts entre extent at the same tme. Ths assumpton s vald n most cases and fals n the exceptonal case of large objects wth homogeneous lumnance coverng regons of also homogeneous lumnance. The nose nfluence dctates that a large percentage of the mage dfference pxels wll be strongly concentrated to a regon around the peak of the hstogram wth ndex that les close to the lumnance offset. Therefore, one may take advantage of the fact that the peak correspondng to the lumnance offset wll be characterzed by larger pxel concentraton compared to the peaks caused by content changes. Consequently, nstead of the ampltude of a specfc peak of the hstogram, the amount of pxels concentrated to ts neghbourhood s consdered a relable crteron for lumnance dfference detecton. The aforementoned process can also be used to address the ssues of lght temperature change and whte balance readjustment. These scenaros represent a problem of unequal change of ambent lumnance between the colour channels, therefore they consttute a problem of a smlar nature. In order to cope wth such changes, t s enough to apply the lumnance correcton technque on each colour channel of the mage dfference ndependently. Let D denote the mage dfference of two frames I, I 2 subjected to comparson:

3 ( x I I ( x D =, (2.), 2 Frames I and I h [] n denote the hstogram func ton correspondng to the mage dfference D 2 are consdered to present varatons n both content and lumnance. Let. The hstogram functon defnton whch s adopted n the present paper dffers from the classc defnton of the mage hstogram, snce t extends to the support [-255,255]. Ths varant of the mage hstogram s formulated n order to enable dscrmnaton of postve from negatve brghtness changes. The hstogram functon, n ts ntal form, contans numerous spurous peaks, whch need to be suppressed pror to peak detecton. Ths removal s accomplshed by aussan smoothng, thus e ng the l h S n : xport ow-pass fltered hstogram functon [ ] h S [] n = h[] n g[ n] (2.2) In the experments presented n ths paper, a 9-sample auss sequence wth standard devaton σ = 3 has been appled n the convoluton process: g [] n = e ( n ) 4 2, n = 0,,...,8 8 (2.3) Peaks of the fltered hstogram functon are located by trackng changes from negatve to postve values n ts frst dervatve. In order to dstngush the local maxmum whose ndex corresponds to the approprate brghtness offset, the pxel amount concentrated to the neghbourhood of each peak s calculated: w l h x h n= x [] n E = h (2.4) P x, [ n] l S here x correspond to the ndces of the local mnma of h S whch le closest to peak n ts left and rght sde respectvely. The hghest pxel count dstngushes the wnner-peak. The correspondng peak ndex denotes the brghtness offset for the respectve colour channel. Therefore, when processng colour mages, three offsets δ L, δl, δl are acqured. At ths pont, the normalzed colour components I, I, I R B of the corrected reference frame can be calculated as follows: I I I R = IR + δlr = I + δl = I B + δlb B (2.5) Fgure 2 shows the ambent brghtness condtonng flowchart appled to each colour channel. P P R B

4 Fgure 3: Flowchart of the ambent llumnaton condtonng algorthm appled to each colour channel of the mage dfference III. Change mask extracton The second stage n the change detecton process ams to dscrmnate changes caused by nose from content changes. Ths task can be based on the calculatons of localzed statstc nformaton. Such calculatons are performed at block-level and are used to export a statstc map of the mage absolute dfference. A decson rule that dscrmnates the regons where sgnfcant changes have occurred focuses on the comparson of localzed propertes aganst nose propertes. Areas wth a mean value exceedng the nose mean value by a certan threshold are consdered changed. Obvously ths comparson requres knowledge of nose propertes and the choce of a sutable threshold. For small scale changes, one may assume that the overall mean value of the absolute dfference represents the mean value of nose. However, for large scale changes, ths s not the case.[] In order to cope wth ths ssue, a block-based change detecton approach whch s tolerant to large scale changes s proposed. Ths method ams to detect nose propertes by groupng mage blocks of the absolute dfference, based on the smlarty of ther mean values. [2] In the proposed approach, the centrod of each cluster s descrbed by an average mean value and standard devaton m, σ, whch are defned by the equaton set: m σ = = k= k= m σ k k (3.) The proposed clusterng procedure forms clusters and then calculates ther propertes by the followng teratve procedure: a) The centrod of the frst cluster s defned by the parameters of the frst mage block: m = m σ = σ (3.2) b) Successve mage block values are compared aganst the mean value of cluster. Let B denote an mage block characterzed by mean value Bj mj between block and cluster s calculated: and standard devaton σ j the mean value dstance j

5 d = m m (3.3) j, j ) If dj, < σ, block s added to cluster and the cluster centrod coordnates are Bj recalculated as dctated by Eq. 3.. denotes the number of mage blocks grouped n cluster up to the current pont of the algorthm executon. ) If d > σ, a new cluster s ntalzed, wth centrod parameters: j, 2 m 2 = m j σ = σ 2 j (3.4) c) The procedure s repeated untl all subsequent mage blocks have been grouped to clusters. Ther parameters are compared aganst the parameters of each cluster formerly created. Fg 4. Flowchart of the mage block clusterng algorthm After the clusterng procedure the largest cluster s consdered to represent nose parameters. Specfcally: mn = m max (3.5) σ = max σ, =,..., n ( ) max The decson rule appled for the mask creaton stage conssts of a comparson of each block mean value aganst the nose mean value: a) If m j σ n > mn, mage block B j s added to the change mask. b) If m j σ n < mn, mage block B j s dscarded. In order to detect changed regons whch preserve smlar lumnance level but dffer n chromnance, the aforementoned clusterng process s appled on each colour channel ndependently. The results are combned nto a sngle change mask. IV. Expermental results The proposed change detecton applcaton has been developed n the LabVew programmng platform. The algorthms have been appled on colour mages at a resoluton of pxels (llustrated n fgures a and b). Block sze has been preset to 6 6 pxels. Fgure a presents the reference background frame and Fgure b presents an altered frame of the surveyed scene. The frames have been shot wth dfferent exposure settngs. Addtonally, whte balance adjustment has been changed, n order to ntroduce dfferent amount of lumnance alteraton to each colour channel. Drect applcaton of the block clusterng technque wthout ambent llumnaton correcton provdes the result llustrated n fgure 5a. Applcaton of the lumnance condtonng algorthm on the

6 background frame exports the adjusted frame dsplayed n fgure 5b. Applcaton of the blockblusterng method on the frame par b-5b detects the content changes shown n Fgure 5c. Apparently, suppresson of llumnaton dfferences ads to the detecton of a largest percentage of the content change, as t removes a constant brghtness offset, whch would otherwse be consdered as nose of ncreased mean value by the block clusterng procedure. It manly contrbutes to the detecton of content changes of a weaker effect. An advantage n the presented mplementaton of the brghtness correcton technque les n ts executon speed. Snce the algorthm s based on plan mathematc procedures (two mage addtons, lmted summatons n the hstogram values and one low-pass flterng of a sngle dmenson sgnal), ts applcaton ntroduces mnmal computatonal cost. a) b) Fgure 5: a) Changes detected between the mage par a. and b wthout llumnaton correcton b) Adjusted reference frame after the applcaton of the three-channel llumnaton correcton technque on frame a c) Content changes detected after the comparson of frames 5b and b. c) V. Conclusons - Open ssues A technque for the suppresson of ambent llumnaton dfference n the presence of content changes has been proposed, amng to mprove change detecton effcency. Expermental results confrm ts necessty n change detecton, as ths stage ncreases the percentage of changes detected. Moreover, ts computatonal cost s mnmal, because of the plan mathematc operatons nvolved. However, t should be noted that the proposed method compensates only for addtve llumnaton changes. Multplcatve llumnaton s not accounted for. In order to cope wth multplcatve llumnaton, an addtonal refnement stage of homomorphc flterng needs to be mplemented.[3] evertheless, a reduced computatonal cost s predcted, snce the refnement stage s expected to be performed over a lmted search space. References [] Anagnostopoulos C. Vergados D., Kayafas E., Loumos V. and Stassnopoulos, A computer vson approach for textle qualty control, Journal of Vsualzaton and Computer Anmaton, vol. 2, no., p.p. 3-44, February 200 [2] Theodoros Alexandropoulos, Vassl Loumos, Eleftheros Kayafas, A block-based clusterng technque for real-tme object detecton on a statc background, 2 nd Internatonal IEEE Conference on Intellgent Systems, Varna, Bulgara, June 22-24, 2004

7 [3] Danel Toth, Tl Aach and Volker Metzler, Illumnaton Invarant Change Detecton, 4 th IEEE Southwest Symposum on Image Analyss and Interpretaton, p.p. 3-7, Austn, Texas, Aprl 02-04, 2000, ISB: