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-1723- 1723 GestureRecognitionwithDepthImages-ASimpleApproachUpalMahbub1,HafizImtiaz1,TonmoyRoy1,Md.ShafiurRahman1,andMd.AtiqurRahmanAhad21DepartmentofElectricalandElectronicEngineering,BangladeshUniversityofEngineeringandTechnology,Dhaka-1000,BangladeshE-mail:omeecd@eee.buet.ac.bd;hafiz.imtiaz@live.com;tonmoyroy@live.com;shafeey@live.com2DepartmentofAppliedPhysics,Electronics&CommunicationEngineeringUniversityofDhaka,BangladeshEmail:atiqahad@univdhaka.eduAbstract:Anovelapproachforgesturerecognitionisdevelopedinthispaperbasedontemplatematchingfrommotiondepthimage.Theproposedmethodusesasingleexampleofanactionasaquerytofindsimilarmatchesfromagoodnumberoftestsamples.Nopriorknowledgeabouttheactions,theforeground/backgroundsegmentation,oranymotionestimationortrackingisrequired.Anovelapproachtoseparatedifferentgesturesfromasinglevideoisalsointroduced.Theproposedmethodisbasedonthecomputationofspace-timedescriptorsfromthequeryvideowhichmeasuresthelikenessofagestureinalexicon.Thedescriptorextractionmethodincludesthestandarddeviationofthedepthimagesofagesture.Moreover,twodimensionaldiscreteFouriertransformisemployedtoreducetheeffectofcamerashift.Classificationisdonebasedoncorrelationcoefficientoftheimagetemplatesandanintelligentclassifierisproposedtoensurebetterrecognitionaccuracy.Extensiveexperimentationisdoneonavastandverycomplicateddatasettoestablishtheeffectivenessofemployingtheproposedmethod.Keywords:gesturerecognition,depthimage,standarddeviation,2DFouriertransform.1.INTRODUCTIONHandgesturerecognitionandanalysisisveryimpor-tantincomputervisionandman-machineinteractionsforvariousapplications[1],[2].WuandHuang[3]presentedasurveyongesturerecognition,whereas,actionrecogni-tionissuesareanalyzedin[1].Theapplicationarenasaregaming,computerinterfaces,robotics,videosurveil-lance,bodypostureanalysis,actionrecognition,behavioranalysis,signlanguageinterpretationsfordeaf,facialandemotionunderstanding,etc.[4][5],[6].Thereareseveralmethodsproposedbyagoodnumberofresearchersinthefieldofactionandgesturerecog-nition[4],whicharealreadyimplementedinpracticalapplications.Thebasisofrepresentationoftheaction,whichisusuallydifferentineachmethod,iscommonlyrelatedtoappearance,shape,spatio-temporalorientation,opticalflow,interest-pointorvolume.Forexample,inthespatio-temporaltemplatebasedapproaches,anim-agesequenceisusedtoprepareamotionenergyimage(MEI)andamotionhistoryimage(MHI),whichindicatetheregionsofmotionsandpreservethetimeinformationofthemotion[5],[2]aswell.Anextensionofthisap-proachtothreedimensionisproposedin[7].Recently,someresearchersexploredopticalflowbasedmotionde-tectionandlocalizationmethodsandfrequencydomainrepresentationofgestures[8],[9].Amongothermeth-ods,theBag-of-wordsmethodhasbeenusedsuccessfullyforobjectcategorizationin[10].[11]proposedamethodforsegmentingaperiodicactionintocyclesandtherebyclassifytheaction.Eventhoughitisveryimportanttounderstandvar-ioushandgestures,itisextremelydifficulttodosoduetovariouschallengingaspects:dimensionalityandvarietiesofgestures,partialocclusion,boundaryselec-tion,headmovement,background,culturalvariationsinsignlanguages,etc.Usually,mostoftheexistingworksongesturerecognitionandanalysiscoveronlyafewclassesortypesofgesturesandusually,thesearealsonotcontinuousandnotinrandomorders.Exist-ingmethodscanbeclassifiedintoseveralcategories,suchasview/appearance-based,model-based,space-timevolume-based,ordirectmotion-based[1]methods.Template-matchingapproaches[2],[12]aresimplerandfasteralgorithmsformotionanalysisorrecognitionthatcanrepresentanentirevideosequenceintoasingleimageformat.Recently,approachesrelatedtoSpatio-TemporalInterestfeaturePoints(STIP)arebecomingprominentforactionrepresentation[13].However,duetovariouslimitationsandconstraints,nosingleapproachseemstobeenoughforwiderapplicationsinactionunderstandingandrecognition.Hence,thechallengingaspectsofges-turerecognitionstillremainsinitsinfancy.Moreover,thoughanumberoftheavailablegesturerecognitionmethodshaveacquiredhighaccuracyindif-ferentdatasets,mostofthemdependonagoodamountinputtotrainthesystemanddonotperformverywellifthenumberoftrainingdatawerelimited.[14]proposedamethodforactionrecognitionusingapatchbasedmotiondescriptorandmatchingschemewithasingleclipasthetemplate.TheKinectsensorwithamounteddepthsensoren-couragedtheresearcherstodevelopsomealgorithmstotakeadvantageofthedepthinformation.Amongsuchworks,[15]proposedamethodforhanddetectionbyintegratingRGBanddepthdatawhichinvolvedfindingpossiblehandpixelsbyskincolordetectionbasedonthefactthatneckandotherskincoloredclothspartsofthebodywillbebehindthehandinmostofthecases.SICE Annual Conference 2012 August 20-23, 2012, Akita University, Akita, Japan PR0001/12/0000- ¥ 400 ©2012 SICE
-1724- Fig.1ExtractsfromtheChaLearnGestureDataset:Toprow(lefttoright)hastenRGBgesturedatafromdevel01todevel10lexicons;bottomrowcontainsthecorrespondingdepthimagedataFig.2Backgroundsubtractedfromdepthimage.Anotherrobustmethodfordetectionoffingerstoeasilyidentifysophisticatedandconfusinggestureswaspro-posedin[16],namelytheFinger-EarthMoverDistance(FEMD)[17].Howevergesturerecognitionsinthesecasesweresimplersinceasmallsetofgestureswereusedwithverylittlevariationintypes.Inthispaper,anefficientgesturerecognitionmecha-nismisdevelopedbasedonasequenceofdepthimagesofhumangestures.ThedepthimagesareobtainedbyutilizingtheinfrareddepthsensorpresentinaMicrosoftKinectTMSensor.Motionbasedtemplatematchingtech-niquesareemployedonthetrainingvideostoobtainthekeyfeaturesfromthegesturesequences.Thefeaturevectorisformedemployingstatisticaloperationsbothintemporalandspectraldomains.Thetestingphaseisdi-videdintoseveralsteps.First,differentgesturesaresep-aratedfromthelongtestsequences.Then,similartothetrainfeaturevectors,testfeaturevectorsaregener-atedforeachgesture.Finally,everytestfeaturevectoriscomparedtoeachtrainfeaturevectorsfordifferentges-turesandaclassifierisemployedtofindthebestpossiblematchofagesturefromthegiventrainingvocabulary.Theperformanceoftherecognitionalgorithmisevalu-atedwithrespecttothepercentageofLevenshteindis-tance(LD)[18].Throughextensiveexperimentationsat-isfactoryrecognitionperformanceforawidevarietyofgesturesincludingsignlanguages,clutteredbackgroundscenarios,partiallyvisiblehumanfigurescenariosetc.isachieved.2.PROPOSEDMETHODFORGESTURERECOGNITIONInthispaper,amethodofgesturerecognitionfromasmallvocabularyofgesturesisdeveloped.Itisevidentthatmanyconsumerapplicationsofgesturerecognitionwillbecomepossibleonlyifsystemscanbetrainedtorecognizenewgestureswithveryfewexamples,and,inthelimit,justone.HereitisassumedthatthedepthimageisavailableinadditiontotraditionalRGBimage.Thegestureshadtoberecognizedfromasetofgesturesandmatchedwithaknownvocabulary.2.1GestureDatasetTherearesomeclearly-definedhandorbodyorheadgesturedatasets,e.g.,Cambridgegesturedataset[19],NavalAirTrainingandOperatingProceduresStandard-ization(NATOPS)aircrafthandlingsignalsdatabase[20],Keckgesturedataset[21],KoreaUniversityGes-ture(KUG)database[22],etc.Thoughallthesedatasetsarewellknownfortheircontentsandcomplexities,allofthemaddressesaparticulartypeofgestureslimitedtoveryfewclassesandapplicationdomains.Therefore,forthesimulationpurposeoftheproposedmethod,averyrichbutextremelycomplicateddataset,namely,theChaLearnGestureDataset(CGD2011),isconsideredinthispaper[23].Itisaverychallengingdatabase.Ithasdevelopmentdataaswellasvalidationdata.Thedevel-opmentdataconsistedofbatchesdevel01,devel02:::devel20,etc.ForthedevelXXbatches,allthelabelsareprovidedbythedatasetcreators.Eachsub-datasetorlexi-conconsistsof47videosequencesof1to5gestures,hav-inginitialswith‘K’todenotedepthimagesfromKinectsensorand‘M’astheoriginalRGBvideos.So,thereareactually472numberofvideofilesunderalex-icon.Eachvideohas8to13uniqueactions,andthenumbersarevariedfromonelexicontoanother.Therearearound50;000gesturesrecordedwiththeKinectTMcamerawithimagesizes240320pixelsat10framespersecond.Thevideosarerecordedby20differentusersandgroupedin500batchesof100gestures.Thedataareavailablefrom[23]in2formats:AlossycompressedAVIformatandaquasi-losslessAVIformat.Togetasuf-ficientspacialresolution,onlytheupperbodyisframed.Somemoreattributesaboutthisdatabaseare:fixedcamera,availabilityofdepthdata,singleuserwithinabatch,homogeneousrecordingconditionswithinabatch,smallvocabularywithinabatch,gesturesperformedmostlybyarmsandhands,cameraframingmostlytheupperbody(someexceptions),onlyonelabeledexampleofeachuniquegestures,variationsinrecordingcondi-tions(variousbackgrounds,clothing,skincolors,light-ing,temperature,resolution),somepartsofthebodymaybeoccluded,someusersarelessskilledthanothers,someusersmadeerrorsoromissionsinperformingtheges-tures,etc.Ithaslexiconsfromninecategoriescorre-spondingtovarioussettingsorapplicationdomains;theyinclude(1)bodylanguagegestures(likescratchinghead,crossingarms),(2)gesticulationsperformedtoaccom-panyspeech,(3)illustrators(likeItaliangestures),(4)emblems(likeIndianMudras),(5)signs(fromsignlan-
-1725- guagesforthedeaf),(6)signals(likerefereesignals,div-ingsignals,ormarshalingsignalstoguidemachineryorvehicle),(7)actions(likedrinkingorwriting),(8)pan-tomimes(gesturesmadetomimicactions),and(9)dancepostures.SampleframesofbothRGBanddepthdataofthefirst10lexiconsofthedevelopmentdataareshowninFig.1.2.2FeatureExtractionandTrainingThedepthdataprovidedinthedatasetareinRGBfor-matandthereforeneedtobeconvertedtograyscalebe-foreprocessing.Thegrayscaledepthdataisatruerepre-sentationofobjectdistancefromthecamerabyvaryingintensityofpixelsfromdarktobrightforneartofarawayobjects,respectively.NextinordertoseparatethepersonandthebackgroundofthegrayscaleimageOtsu’smethod[24]isemployed.Inthismethodathresholdischosenbasedonminimizationoftheintra-classvarianceoftheblackandwhitepixelsformakingablackandwhitebi-naryimage.Utilizingthebinaryimagethebackgroundfromthedepthimageisfilteredoutandtherebythehu-mansubjectisseparatedasshowninFig.2.Intheproposedmethodsofgesturerecognitiontwotypesofoperationsareperformedforobtainingfeaturevectorsfromtrainingsamples:a.calculatingstandarddeviation(STD)oneachpixelvaluesacrossalltheframesandb.employingtwodimensionalFouriertransform(2D-FFT)oneachframe.Theprocessoffeatureextrac-tion,trainingandclassificationareshowninbriefinFig.3.Forthen-thgestureatlexiconLconsistingofDframes,thestandarddeviationD2n(x;y)ofpixel(x;y)acrosstheframesisgivenbyD2n(x;y)=∑(Ixy(d)Ixy)2D:(1)HereIxy(d)isthepixelvalueofthelocation(x;y)oftheframed.Wherex=1;2;3:::r,y=1;2;3:::sandd=1;2;3:::D.randscorrespondstothemaximumpixelvaluealongxandyaxis,respectively.Ixyistheav-erageofallIxy(d)valuesfromframes1tod.Therefore,forthewholeframe,thematrixobtainedisdefinedasD∆Ln=D2n(1;1)D2n(1;2):::D2n(1;s)D2n(2;1)D2n(2;2):::D2n(2;s)............D2n(r;1)D2n(r;2):::D2n(r;s)(2)Thesilhouettesobtainedbyperformingstandardde-viationacrossframesonthetrainingsamplesofdevel-opmentdatalexicon1oftheChaLearngesturedatasetisshowninFig.4.Itisevidentfromthefigurethatperform-ingstandarddeviationacrossframesenhancestheinfor-mationofmovementacrosstheframeswhilesuppressesthestaticparts.Therefore,informationaboutthepathofmotionflowandtypeofthegestureiseasilyobservable.Anotherfeaturecanbeobtainedfromthegesturesam-plesbytakingtheabsolutevalueofthetwo-dimensionalDiscreteFourierTransform(2DDFT)usingfastFourierTransform(FFT)oneachbackgroundsubtracteddepthimageframeandthencalculatingthestandarddeviationofeachpointatthespectraldomainacrossframes.Foranimageofsizersthe2DdiscreteFouriertransformisgivenbyFkl(d)=1rsr1∑x=0s1∑y=0Ixy(d)ej2(kxr+lys);(3)where,theexponentialtermisthebasisfunctioncorre-spondingtoeachpointFkl(d)intheFourierspaceandIxy(d)isthevalueofthe(x;y)pixeloftheimagegiveninspatialdomainforthed-thframe[25].PerformingFouriertransformpriortotakingSTDhascertainadvan-tages.Whenthedepthvaluesofthepersonaretakentothefrequencydomain,thepositionofthepersonbecomesirrelevant,i.e.anyslightmovementofthecameraverti-callyorhorizontallywouldbenullifiedinthefrequencyresponse.Forexample,ifthecameraissteadyinthetrainingsamplesbutmovesalittlebitinthetestsamples,theSTDontemporalvaluesofthetestdatawouldbedif-ficulttoprojectonthatofthetrainingdataforfindingamatch.However,spectraldomaintransformationofthedatabeforetakingstandarddeviationacrossframeswillsuppressthetimeresolutionandtherebyreducetheeffectofcameramovement[4].2.3FindingActionBoundaryWhiletesting,thetestdatasequencescontainingoneoneormoregesturesareneededtobeseparatedfordif-ferentgestures.ThegesturesprovidedintheChaLearngesturedatasetareseparatedbyreturningtoarestingpo-sition[23].Thus,subtractingtheinitialframefromeachframeofthemoviecanbeagoodmethodforfindingthegestureboundary.ForarsdepthimageifIxy(d)isthepixelvalue(depth)ofposition(x;y)atthed-thframe,thenthefollowingformulacanbeusedtogenerateavari-ablePdwhichgivesanamplitudeofchangefromtheref-erenceframep1=(I11(d)I11(1))2++(I1s(t)I1s(1))2p2=(I21(d)I21(1))2++(I2s(t)I2s(1))2p3=(I31(d)I31(1))2++(I3s(t)I3s(1))2::::::::::::::::::pr=(Ir1(d)Ir1(1))2++(Irs(t)Irs(1))2(4)Thus,thesumsquaredifferenceofthecurrentframefromthereferenceframecanbeobtainedbyPd=r∑k=0pk(5)ThevalueofPdisthennormalized.ThesmoothedgraphofnormalizedPdvs.disshowninFig.5.aand5.bfortwodifferentdevelopmentdatasamples.Inthefigure,boththeactualboundary(determinedbyobservingthe
-1726- Fig.3ProposedMethodofGestureRecognitionFig.4STDoutcomeoneachpixelacrosstimeFig.5ActionBoundaryDetectiongesturesequence)andtheboundarydetectedbythepro-posedalgorithmareshown.Everycrestinthesecurvesrepresentagestureboundary.Usingthelocationofthesecrestsintimealongwithsomeintelligentdecisionmak-ingonthecurve,theactionboundariescanbefoundwithfairyhighaccuracy.2.4ClassificationAfterextractingthefeaturesfromthegesturesinthetrainingdatasetofaparticularlexiconafeaturevectorta-bleisformedforthatlexiconasshowninFig.3.Then,forthetestsamples,firstthegesturesareseparatedifmul-tiplegestureexists.Thenfeaturesareextractedforeachgestureinamannersimilartothatdonefortrainingsam-ples.Forgesturesequencesofframesizersthefeaturevectorobtainedfromthetestgesturesarealsorsma-tricesforeachtypesoffeatures.Nextthecorrelationco-efficientiscalculatedbetweenthefeaturesobtainedfromthetestgesturetothatsimilarfeatureobtainedfromeachofthetraingestures.Acoefficientofcorrelation,,isamathematicalmeasureofhowmuchonenumbercanexpectedtobeinfluencedbychangeinanother.Itisawidelyusedmeasureforimageandgesturerecognition[25].ThecorrelationcoefficientbetweentwoimagesAandBisdefinedas=r1∑k=0s1∑l=0(AklA)(BklB)vuutr1∑k=0s1∑l=0(AklA)2vuutr1∑k=0s1∑l=0(BklB)2;(6)where,Aklistheintensityofthepixel(k;l)inimageAandBklistheintensityofthepixel(k;l)inimageB.AandBarethemeanintensityofallthepixelsofimageAandB,respectively.If=1thenthereisastrongpositive/negativecorrelationbetweenthetwoimages,i.e.theyareidentical/negativeofoneother.Ifiszerothenthereisnocorrelationamongthematrices.Ahighvalueofthecorrelationcoefficientbetweenthesamefeatureofthetestsampleandthatofoneofthetrainingsampleindicatesahigherprobabilityofthetwogesturestobeidenticalandviceversaforalowvalueofcorrelationcoefficient.Intheproposedmethod,forusingcombinationofmorethenonefeaturessimilarfeaturesarematchedusingcorrelationcoefficientseparatelyandthendecisionistakenfromthetwoarraysofcorrelationcoefficientstoidentifytheappropriatematchinawaythatresultofbothmatchesinfluencethedecisionmaking.3.EXPERIMENTALRESULTSForthepurposeofevaluatinggesturerecognitionper-formanceoftheproposedmethod,theLevenshteindis-tance(LD)measureisemployed.TheLevenshteindis-tance,alsoknownaseditdistance,isnumberofdeletions,insertions,orsubstitutionsrequiredtomatchanarraywithanother[18].TheLDmeasurehasawiderangeofapplications,suchasspellcheckers,correctionsystemsforopticalcharacterrecognitionandothersuchsystems[26],[27].Fortheproposedgesturerecognitionsystem,
-1727- Fig.6LevenshteinDistances(%)fordifferentlexiconsifthelistoflabelsoftruegestureinatestsequenceisTwhilethelabelscorrespondingtotherecognizedges-turesforthesamesequenceifR,thentheLevenshteindistanceL(R,T)willrepresenttheminimumnumberofeditoperationsthatonehastoperformtogofromRtoT(orviceversa).Theevaluationcriteriaisthepercent-ageLDwhichisthesumofalltheLDsobtainedfromalexicondividedbythetotalnumberoftruegesturesinthatlexiconandmultipliedby100.ItisevidentthatthehigherthevalueofLD,themoreisthenumberofwrongestimations[18].Forthepurposeofanalysis,thefirst20lexicons,namelyDevel01toDevel20fromCGD2011[23]datasetareconsidered.Inthisdataset,differentactionsinasetareassignedanumberasalabelandastringcon-tainingthelabelsofactionsareprovidedforeachgesturesequence.Fortheexperimentationpurpose,threesetsoffeaturesareformedbycombiningthetwobasicopera-tions.Also,methodofclassificationdeviatedalittlebitwiththetypesoffeatures.Thus,threedifferentmethodsareproposed.Thesemethodsare,1.takingSTDofthedepthimageandusingmaximumvalueofthecorrelationcoefficientasbestmatch,2.takingSTDoftheabsolutevalueofthefrequencydo-mainspectrumofthedepthimageandusingmaximumvalueofthecorrelationcoefficientasbestmatch3.measuringSTDofbothtimeandspectraldomainim-ages,measuring(a)correlationcoefficientsforthespec-traldomainfeatureand(b)multiplicationofcorrelationcoefficientforbothfeatures.Then,makingintelligentse-lectionfromthethreehighesvaluesof(a)byobservingcorrespondingvaluesat(b)SimulationarerunonthedatasetusingthesemethodsandtheresultobtainedintermsofpercentageofLeven-shteinDistance(LD)isshowninFig.6foreachlex-iconofthefirst20developmentdata.Itcanbeseenfromthefigurethattheresultvarieswidelyfromsettoset.Thewidevariationiscausedbythedifferenttypesofmotionsindifferentsets.Forexample,indevel03,devel07,devel10,etc.percentageLDishigherbecauseofthegesturesbeingmostlysignlanguagesandthereforeTable1AVERAGELEVENSHTEINDISTANCE(%)FORPROPOSEDMETHODSMethodAverageLev.dist%Method148.73Method238.95Method343.43verydifficulttodifferentiatewhileotherlexiconsincludeeasilydifferentiablegestures.Theaccuracyofthepro-posedmethodarealsolimitedbytheaccuracyofsepara-tionofdifferentactionsfromthevideosandunexpectedmovementsoftheperformer.ThebestresultsareobtainedforMethod2whichisverymuchexpected(Fig.6)becauseduetothespec-traldomainoperationthefeatureinthismethodisrobustenoughtoperformwellagainstcameramovement.TheaveragepercentageLDisshowninTable1.4.CONCLUSIONAnovelapproachforgesturerecognitionemployingcombinationsofstatisticalmeasuresandfrequencydo-maintransformationobtainedfromdepthmotionimagesisproposedinthispaper.Afterseveralpre-processingsteps,featuresfromgesturesequencesareextractedbasedontwobasicoperations-calculatingstandardde-viationacrossframesandtakingtwodimensionalFouriertransform.Themeasuresarethencombinedtofindthemostusefulmethodforrecognizinggestures.Theuseful-nessofusingintelligentclassifierbasedonthecorrelationcoefficientfromthestandarddeviationofthe2dimen-sionalFouriertransformoftheimageisapparentfromtheresults.Thedataset,namely,theChaLearnGestureDataset2011,targetedforexperimentalevaluationisarichbutdifficultdatasettohandlehavingalotofvariationandclasses.However,throughextensiveexperimentationitisprovedthatevenforoneofthemostcomplexdatasettheproposedmethodcanprovideasatisfactorylevelof
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