Enhancement of Region Merging Algorithm for Image Segmentation

Transcription

1 nternatonal Conference on Advances n Engneerng and Technology CAET'04) March 9-30, 04 Sngapore Enhancement of Regon Mergng Algorthm for mage Segmentaton Tn Tn Htar, and Soe Ln Aung Abstract Effcent and effectve mage segmentaton s one of the most mportant tasks n computer vson and obect recognton. Segmentaton of an mage s the dvson or separaton of the mage nto dsont regons of smlar attrbute. Fully automatc mage segmentaton s usually very hard for natural mages. There may be oversegmentaton problem or undersegmentaton problem durng the segmentaton process. When the watershed segmentaton s appled, oversegmentaton occurs due to hgh contrast of regonal mnma. Although marker-controlled watershed segmentaton overcomes ths problem, t cannot stll produce the meanngful regons. n ths paper, a new enhanced regon mergng algorthm based on dynamc regon mergng method s proposed. n the proposed algorthm, neghborng regons are progressvely merged f there s an evdence for mergng accordng to the mnmum edge weghts between those regons and ther homogenety. By mergng the only necessary adacent regons, the mplemented system can produce meanngful regons of the nput mage whch are useful for mage annotaton, obects detecton n an mage, mage retreval systems and so on. Keywords mage Segmentaton, Nearest Neghbor Regon Mergng, Marker Controlled Watershed Transform.. NTRODUCTON N mage processng, segmentaton s a basc problem n dfferent felds such as pattern recognton, scene analyss and mage analyss. mage segmentaton s the process of dvdng mages nto regons accordng to ts characterstc e.g., color and obects present n the mages. These regons are sets of pels and have some meanngful nformaton about obect. The result of mage segmentaton s n the form of mages that are more meanngful, easer to understand and easer to analyze. n order to locate obects and boundares n mages feature etracton of obect shape, optcal densty, and teture, surface vsualzaton, mage regstraton and compresson mage segmentaton s used. Correct segmented results are very useful for the analyss, predcaton and dagnoses. n partcular, many of the estng technques for mage vsualzaton, mage descrpton and recognton and obect based mage compresson hghly depends on segmentaton. Tn Tn Htar, Student, Faculty of nformaton and Communcaton Technology, Unversty of Technology Yatanarpon Cyber Cty), Pyn Oo Lwn, Mandalay Dvson, Myanmar; emal : tthar.yadanarpon@gmal.com Soe Ln Aung, Prncpal and Assocate Professor, Computer Unversty, Magway Dvson, Myanmar; emal: slnaung@gmal.com The parttonng of a gven mage nto a number of homogenous regons spatally connected groups of pels) became the general problem of mage segmentaton. The unon of any two neghbourng regons may yeld a heterogeneous regon. Alternatvely, segmentaton can be consdered as a pel labellng process n the sense that all pels that belong to the homogenous regon are assgned label []. Watershed segmentaton s a morphologcal based method of mage segmentaton. The gradent magntude of an mage s consdered as a topographc surface for the watershed transformaton. Watershed lnes can be found by dfferent ways. The complete dvson of the mage through watershed transformaton reles mostly on a good estmaton of mage gradents. The result of the watershed transform s degraded by the background nose and produces the oversegmentaton. The proposed technque focuses on the soluton of oversegmentaton problem of mages by applyng preprocessng on the nput mage. The dvson of ths paper s as follows, n Secton, some related work s gven whch descrbes the prevous research about the remedy of watershed ssues. n Secton 3, basc theory for mage segmentaton s gven. n Secton 4, the proposed framework and the mplementaton of proposed system are gven. Epermental results are shown n Secton 5. Fnally, n Secton 6, the concluson s gven.. RELATED WORK Regon growng segmentaton technques were eamned to etract semantcally meanngful obects from an mage []. Regon growng algorthm was proposed that performs on a semantc level, drven by the knowledge of what each regon represents at every teraton step of the mergng process. Ths approach utlzed smultaneous segmentaton and labelng of regons leadng to automatc mage annotaton. The term watershed-lke was used because the decson for whch regons to be merged depends on both ther confdence value and ther dstance from the seed such as catchment basn, n watershed segmentaton termnology and the teraton keeps on untl two epanded regons meet. A novel approach for small obect detecton by usng watershed-based transformaton was proposed [3]. n ths paper, the small movng obects were detected from the mage. n order to mprove the detecton results from the 87

2 nternatonal Conference on Advances n Engneerng and Technology CAET'04) March 9-30, 04 Sngapore prevous technques a nose removal technque was frst appled to the mage whch removed the nose from the mage and mprove the mage qualty. The detecton system ncluded two man modules, frst one was regon of nterest RO) locatng and the other was contour etracton. After nose removal accurate RO could be located. n contour etracton process, a rough canddate obect n the mages could be detected by applyng some dfferencng technque on two contagous mage frames. A method for mage segmentaton whch conssted of watershed segmentaton usng pror shape and appearance knowledge was proposed [4]. Watershed segmentaton was a common technque for mage segmentaton but had problems of over segmentaton and senstvty to nose. The method had two stages, frst was tranng stage and the other was segmentaton stage. n tranng stage, a pror shape and appearance knowledge model was developed by usng shape hstogram and mage ntensty statstcs. The segmentaton stage was an automatc teratve procedure and conssted of four steps: classcal watershed transformaton, mproved k- means clusterng, shape algnment, and refnement. Other researchers also proposed dfferent method to remedy the problem of watershed. An mproved mage segmentaton approach based on level set and mathematcal morphology was presented [5]. The gradent magntude of the smoothed mage was nput to the watershed transformaton, the result of watershed was used for rough appromaton of the desred contour n the mage, and gude for the ntal locaton of the seed ponts used n the followng level set method. Although the researchers mentoned above have ther advantages, gettng meanngful regons of a segmented mage s stll a challengng problem n the feld of mage processng. Therefore, n ths paper, ths research work s epected to overcome the oversegmentaton problem and produce the meanngful regons.. THEORETCAL BACKGROUND A regon s a connected component, and the boundary, also called the borders or contour, of a regon s the set of pels n the regon that have one or more neghbours that are not n the regon. Ponts not on boundary or regon are called background ponts. ntally, only n bnary regons, regon or boundary ponts are represented by s and background ponts by 0s. The boundary s connected set of ponts. The boundary s sad to be mnmally connected. A. Watershed Transform n geography, a watershed s the rdge that dvdes areas draned by dfferent rver systems. A catchment basn s the geographcal area dranng nto a rver or reservor. The watershed transform apples these deas to gray-scale mage processng n a way that can be used to solve a varety of mage segmentaton problems. A gray-scale mage s consdered as topologcal surface, where the values of f,y) are nterpreted as heghts. f ran fell on the three-dmensonal surface, t s clear that water would collect n the two areas labelled as catchment basns. Ran fallng eactly on the labelled watershed rdge lne would equally lkely to collect n ether of the two catchment basns. The watershed transform fnds the catchment basns and rdge lnes n a gray-scale change the startng mage nto another mage whose catchment basns are the obects or regons requred to dentfy []. B. Thresholdng mage thresholdng enoys a central poston n applcatons of mage segmentaton. t s useful n dscrmnatng foreground from the background. By selectng an adequate threshold value T, the gray level mage can be converted to bnary mage. The bnary mage should contan all of the essental nformaton about the poston and shape of the obects of nterest foreground). The advantage of obtanng frst a bnary mage s that t reduces the complety of the data and smplfes the process of recognton and classfcaton. The segmentaton problem becomes one of selectng the proper value for the threshold T. A frequent method used to select T s by analyzng the hstograms of the type of mages that want to be segmented. The deal case s when the hstogram presents only two domnant modes and a clear valley bmodal). n ths case the value of T s selected as the valley pont between the two modes. n real applcatons, hstograms are more comple, wth many peaks and not clear valleys, and t s not always easy to select the value of T [6]. C. Regon Mergng The obectve of segmentaton s to partton an mage nto regons. Regons are sets of pels wth homogenous propertes and they are teratvely grown by combnng smaller regons or pels, pels beng elementary regons. Regon mergng technques usually work wth a statstcal test to decde the mergng of regons. A mergng predcate uses ths test, and bulds the segmentaton on the bass of essentally) local decsons. A good regon mergng algorthm has to fnd a good balance between preservng ths unt and the rsk of overmergng for the remanng regons [7]. Regon mergng s a post-processng technque that merges adacent regons. Many technques have been employed for regon mergng ncludng the use of smple thresholds, sze based thresholds and teratve methods. V. MPLEMENTATON OF ENHANCED REGON MERGNG ALGORTHM Ths secton descrbes how to mplement the proposed system. A. Overall System Desgn Fg. descrbes the steps for the overall system desgn of the proposed system. The nput mage s preprocessed to enhance the mage. The preprocessed mage s segmented wth proposed segmentaton algorthm and regon adacency graph RAG) s acqured. The regons n RAG are teratvely merged accordng to the system and the output mage s produced. 88

3 nternatonal Conference on Advances n Engneerng and Technology CAET'04) March 9-30, 04 Sngapore nput mage Preprocessng Segmentaton RAG Regon Mergng Output mage Fg. Overall system desgn B. Proposed System Desgn Ths system computes the gradent magntude upon the nput mage. Net, t constructs the label matr usng markercontrolled watershed transform and creates the regon adacency graph. nput mage Gradent computaton Openng-closng by reconstructon Assess regonal mnma of reconstructed mage Acqure label matr usng Marker-controlled Watershed Transform Construct RAG and Determne Threshold T Calculate edge weght of each regon par n RAG Etract mnmum par for each regon Get one of mnmum par whch has not been chosen before Evaluate mean m and standard devaton std of mnmum par m < T* std Yes Merge mnmum par accordng to NNR untl all of the mnmum par has been chosen Segmented mage wth meanngful regons Fg. Flow Dagram of Proposed System No n regon mergng, there are two predcates. The frst predcate s mnmum par for each regon. The second one s color homogenety dstrbuton. From RAG, mnmum edge pars for each regon are dscovered. Color homogenety dstrbuton can be obtaned by mean and standard devaton. Only f the regon par can satsfy these two predcates, the regon par s actually merged. f not, ths regon par s gnored. Fg. shows the flow dagram of ths system. C. Enhanced Regon Mergng Algorthm n ths secton, the enhanced regon mergng algorthm s proposed. t overcomes the oversegementaton problem. Table descrbes the steps of the enhanced regon mergng algorthm. TABLE ENHNACED REGON MERGNG ALGORTHM nput : RGB mage Output : Segmented mage wth meanngful regons. Compute the gradent magntude of the mage.. Openng-closng followed by reconstructon. 3. Assess the regonal mnma of the reconstructed mage. 4. Acqure label matr usng Marker-controlled Watershed Transform. 5. Construct Regon Adacency Graph, RAG based on adacent regon pars and determne threshold T. 6. Calculate the edge weght of each regon par n RAG. 7. Etract mnmum par for each regon. 8. Get one of mnmum par whch has not been chosen before. 9. Evaluate mean m and standard devaton std of mnmum par. 0. Check mnmum par wth respect to m, T and std.. Merge the mnmum par accordng to Nearest Neghbor Regon. Step : Gradent computaton n construct to classcal area based segmentaton, the watershed transform s eecuted on the gradent mage. The gradent s defned the frst partal dervatve of an mage and contans a measurement for the change of gray levels. The gradent values G,y)) of the ntal segmented mage are obtaned usng frstly the appromaton of the gradent operator n,y drectons as two 3*3 masks. ) h = fspecal' sobel') hy = h ' ) = mflter double ), h ',' replcate') 3) y = mflter double ), hy ',' replcate') 4) G, y) = + y The gradent mage values G,y)) are calculated n ). The gradent values on the border of the nput mage are the same as n ts nner pels. The gradent values are useful to calculate the edge strength values. Step : Openng-closng followed by reconstructon Drect applcaton of the watershed transform to a gradent mage usually leads to oversegmentaton due to nose and other local rregulartes of the gradent. The resultng problems can be serous enough to render the result vrtually 5) 89

4 nternatonal Conference on Advances n Engneerng and Technology CAET'04) March 9-30, 04 Sngapore useless. A practcal soluton to ths problem s to lmt the number of allowable regons by ncorporatng a preprocessng state desgned to brng addtonal knowledge nto the segmentaton procedure. An approach used to control oversegmentaton s based on the concept of markers. A marker s connected component belongng to an mage. nternal markers are nsde each of the obects of nterest whle eternal markers are contaned wthn the background. These markers are used to modfy the gradent to overcome oversegmentaton problem. Step 3 : Assess regonal mnma of reconstructed mage For both nternal and eternal markers, the gradent mage can be modfed usng a procedure called mnma mposton. The mnma mposton technque modfes a gray-scale mage so that regonal mnma occur only n marked locatons. Other pel values are "pushed up" as a necessary to remove all other regonal mnma. Step 4 : Acqure Label Matr Usng Marker-controlled Watershed Transform n ths step, the mage s segmented usng Markercontrolled watershed transform and the label matr s acqured. Step 5 : Construct Regon Adacency Graph RAG and Determne Threshold Value To represent an mage, regon adacency graph s used. Let G=V,E) be an undrected graph, where v V s a set of nodes correspondng to an mage element such as super-pels or regons. E s a set of edges connectng the pars of neghborng nodes. f the nodes are adacent, there ets an edge between these two nodes. Each edge v, v ) E has correspondng weght w v, v )) to measure the dssmlarty of the two nodes connected by that edge. Moreover, correspondng threshold value s determned n ths step. not be merged. Therefore, mnmum par s chosen one by one to be merged only f the second predcate s satsfed. Step 9 : Evaluate mean m and standard devaton std of mnmum par The arthmetc mean m and standard devaton std of a regon R havng n = R pels: m R) = r, 7) n r, R std R) = r, m R)) 8) n r, R where n = number of pels n R r, = ntensty value of a pel at ponts r and c R = regon of mage as well as R and R n chosen regon par Step 0 : Check mnmum par wth respect to m, T and std The second predcate : mr ) mr ) < T*mn{stdR ), stdr )}, s used to decde f the mergng of the two regons R, R s allowed,.e., f mr ) mr ) <T*mn{stdR ), stdr )}, two regons R, R are merged. f the mnmum par does not satsfy the second predcate, ths mnmum par s gnored and the mnmum par s chosen agan. Step : Merge the mnmum par accordng to Nearest Neghbor Regon NNG) untl all of the mnmum par has been chosen f the mnmum par satsfes the second predcate, ths mnmum par s merged. By mergng only the necessary pars untl all of the mnmum par has been chosen, fnally segmented mages wth meanngful regons can be obtaned. V. EXPERMENTAL RESULTS The results of the proposed system are descrbed n ths secton. Step 6 : Calculate the edge weght of each regon par n RAG To obtan the dssmlarty between two neghborng regons R, R V as the mnmum edge weght connectng them, R R = w v v 6), ) mn v R, v R, v, v ) E, )) Step 7 : Etract mnmum par of each regon Ths step etracts all the mnmum par of each regon among the adacent regon pars n the above step. Step 8 : Get one of mnmum par whch has not been chosen before Although the par regon s the mnmum regon par accordng to RAG, there may be much dfferent on color dstrbuton. For ths reason, every mnmum par may or may Fg. 3 Results of Regon Mergng 90

5 nternatonal Conference on Advances n Engneerng and Technology CAET'04) March 9-30, 04 Sngapore Fg. 3a) descrbes the orgnal colorful mage brd.pg). ts grayscale mage s shown n Fg. 3b) and when t s segmented usng watershed transform, oversegmented mage s obtaned n Fg. 3. Therefore, ts gray scale mage s segmented wth marker-controlled watershed transform as shown n Fg. 3d). Although ths segmented mage can solve the oversegmentaton problem, all the regons n the mage stll need to be meanngful for the purposes such as mage annotaton, mage detecton and so on. Therefore, the regons are merged accordng to the proposed algorthm to make them meanngful as shown n Fg. 3e). Fnally, the proposed system can generate the meanngful regons of the mage as shown n Fg. 3f) whch are useful for annotatng the mage. got n 005 at Computer Unversty, Magway, Myanmar. n 006, the author attended the Master course and Master thess started n 007. Master thess ttle was "Mult-modal Data Fuson wth Smlarty-based Agglomeratve Clusterng''. The feld was Data Mnng. Master degree of computer scence M.C.S was got n 00. At the end of 00, the author became one of the Ph.D canddates. The maor feld of study was dgtal mage processng. n 0, Ph.DT) research was started. mage Segmentaton was very nterestng category among the felds of studyng. Now many dfferent mages are beng tested wth the proposed technque and the accuracy of the system s evaluated wth false acceptance rate FAR) and false reecton rate FRR). The second author Soe Ln Aung s supervsor of frst author. He got Ph.DT) from Russa. He s now assocate professor and prncpal of Computer Unversty, Magway, Myanmar. V. CONCLUSON n ths paper, regon mergng algorthm for mage segmentaton s successfully enhanced. Usng openng-closng reconstructon and fndng mnmal mnma, the oversegmentaton problem s overcome. Regon Adacency Graph s constructed and the edge weght of adacent regon par s found. Consstency of the adacent regons wth mnmum edge weght s calculated by randomly choosng half-sze of the pels of each regon par wth respect to Nearest Neghborng Regon. Accordng to the epermental results, t can be obvously seen that the mplemented system can generate the segmented mage wth meanngful regons whch s very helpful to the mage annotaton. REFERENCES [] Rafael C.Gonzalez, Rchard E.Woods & Steven L.Eddns, "Dgtal mage Processng usng MATLAB". [] Thanos Athanasads and Stefanos Kollas, "A Graph Based, Semantc Regon Growng Approach n mage Segmentaton, mage, Vdeo and Multmeda Systems Laboratory, School of Electrcal and Computer Engneerng, Natonal Techncal Unversty of Athens. [3] Hseh, Han,Wu.Chuangc, and Fana, "A novel approach to the detecton of small obects wth low contrast". [4] Ghassan Hamarneh and Xaong L, "Watershed segmentaton usng pror shape and appearance knowledge", mage and Vson Computng 7 009) [5] P.R. Hll, C.N. Canagaraah and D.R. Bull, "mage Segmentaton usng a Teture Gradent Based Watershed Transform", EEE TRANSACTONS ON MAGE PROCESSNG, 00. [6] Salem Saleh Al-amr, N.V. Kalyankar, and Khamtkar S.D, "mage Segmentaton by Usng Threshold Technques", Journal of Computng, Volume, ssue 5, May 00, SSN [7] Rchard Nock and Frank Nelsen, "Statstcal Regon Mergng", EEE TRANSACTONS ON PATTERN ANALYSS AND MACHNE NTELLGENCE, VOL. 6, NO., NOVEMBER 004. [8] Bo Peng, Le Zhang, Davd Zhang, EEE, "Automatc mage Segmentaton by Dynamc Regon Mergng", 0. Tn Tn Htar was born on st, October, 983 n Taungdwngy Townshp, Magway Dvson, Myanmar. The author s now one of the Ph.D canddates of Faculty of nformaton and Communcaton Technology of Unversty of Technology Yatanarpon Cyber Cty), Myanmar. The author had got Bachelor of Computer Scence B.C.S n 004 and B.C.Sc Hons) had been 9