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InternationalJournalofPatternRecognitionandArtificialIntelligenceVol.18,No.2(2004)229–246c󰀁WorldScientificPublishingCompany

CHINESEWORDSEARCHINGINIMAGEDDOCUMENTS

YUELU∗andCHEWLIMTAN†

DepartmentofComputerScience,SchoolofComputing,NationalUniversityofSingapore,KentRidge,Singapore117543∗luy@comp.nus.edu.sg†tancl@comp.nus.edu.sg

Anapproachtosearchingforuser-specifiedwordsinimagedChinesedocuments,with-outtherequirementsoflayoutanalysisandOCRprocessingoftheentiredocuments,isproposedinthispaper.AsmallnumberofChinesecharactersthatcannotbesuccess-fullyboundedusingconnectedcomponentanalysisduetolargergapsbetweenelementswithinthecharactersareblacklisted.Asuitablecharacterthatisnotincludedintheblacklistischosenfromtheuser-specifiedwordastheinitialcharactertosearchforamatchingcandidateinthedocument.Onceamatchedcandidateisfound,theadja-centcharactersinthehorizontalandverticaldirectionsareexaminedformatchingwithothercorrespondingcharactersintheuser-specifiedword,subjecttotheconstraintsofalignment(eitherhorizontalorverticaldirection)andsizesimilarity.AweightedHausdorffdistanceisproposedforthecharactermatching.Experimentalresultsshowthatthepresentmethodcaneffectivelysearchtheuser-specifiedChinesewordsfromthedocumentimageswiththeformatofeitherhorizontalorverticaltextlines,orbothappearingonthesameimage.

Keywords:Chinesedocumentimage;wordsearching;charactersegmentation;charactermatching;weightedHausdorffdistance.

1.Introduction

Withtheadventandrapiddevelopmentofcomputertechnology,digitaldocuments

havebecomepopularforstorageandtransmissioninsteadofthetraditionalpaper

documents.Themostwidespreadformatofthesedigitaldocumentsistextinwhich

thecharactersofthedocumentsarerepresentedbymachine-readablecodes(forex-

ample,ASCIIcodes).Undoubtedly,thetextformatfacilitatesnotonlythestorage

andtransmissionofdocuments,butalsodocumentinformationretrieval.There

hasbeenactiveresearchonWebcontentextractionusingtext-basedtechniques,on

whichthetextretrievalcommunityhasmadesignificantprogress.20,21,28

Althoughmostofthenewlygenerateddocumentsareintextformat,billionsof

volumesoftraditionalnewspapers,books,magazines,etc.areinthepaperformat,

andneedtobetransferredtothedigitaldomain.Ingeneral,thesepaperdocu-

mentsareeasilyscannedandconvertedtodigitalimagesbythedigitizationequip-

ment.ThetechnologyofOpticalCharacterRecognition(OCR)maybeutilizedto

229230Y.Lu&C.L.Tan

automaticallytransferthedigitalimagesofthesepaperdocumentstotheirmachine-

readabletextformat.However,OCRhasitsinherentweaknessesintherecognition

ability,especiallyfortherecognitionofChinesecharacters.Althoughmanycom-

mercialproductsofmachineprintedChinesecharacterrecognitionsystemshave

appearedonthemarketinrecentyears,5theprocessstillcarriesasignificantcost.

Anobviousreasonistheneedformanualcorrection/proofingofOCRresults,which

istypicallynotcosteffectivefortransferringahugeamountofpaperdocumentsto

theirtextformat.

ItisgenerallyacknowledgedthattheOCRaccuracyrequirementsforin-

formationretrievalareconsiderablylowerthanformanydocumentprocessing

applications.23Motivatedbythisobservation,themethodwiththeabilityoftol-

eratingrecognitionerrorsofOCRhasbeenresearchedrecently.10,19Additionally,

somemethodsarereportedtoimprovetheretrievalperformancebyusingtheOCR

candidates.12,13IntheseOCR-basedmethods,thelayoutanalysisandcharacter

segmentationareunavoidable.However,theresearchonlayoutanalysisandunder-

standingisstillimmature,especiallyfordocumentswithcomplexlayout.

Nowadays,manydocumentsarestoredintheimageformatduetovariousrea-

sons.Itisofsignificantmeaningtostudythestrategiesofretrievinginformation

fromthesedocumentimagesdirectly.Animage-basedapproachtosearchingand

locatingauserspecifiedwordinthedocumentimageshasitspracticalvalueforre-

trievinginformationfromthedocumentrepositories,suchasdigitallibraryarchives

ortheInternet.Forexample,byusingthistechniquetheusercandetectaspeci-

fiedwordinthedocumentimageswithouttherequirementofOCRingtheentire

document.

InthecasewheretherearealargenumberofimagedocumentsintheInternetor

digitallibraries,theuserhastodownloadeachonetoseeitscontentsbeforeknowing

whetherthedocumentisrelevanttohisinterest.Theimage-basedwordsearching

technologyiscapableofnotifyingtheuserwhetheradocumentimagecontains

wordsofinteresttohim,andindicatingwhichdocumentsareworthdownloading,

priortotransmittingthedocumentimagesthroughtheInternet.

Theimage-basedwordsearchingmethodissomewhatdifferentfromthemethod

ofOCRingthedocumentimagesfollowedbyanalysisoftheresultingtext.The

formerconcentratesonlyonthepresenceoftheuserspecifiedwordwhilepaying

noattentiontotheunrelatedwords.Thelattermustassigneachcharacterinthe

documenttoadefiniteclassintheOCRprocedurenomatterwhetherthecharacter

isrelevanttotheuser’sinterestornot.Ofcourse,comparingwiththeimage-based

approach,OCRhasanadvantageofproducingmachinereadabletextforsubsequent

use.However,ifthegoalisonlytoretrieveahandfulofdocumentsfromahuge

collectionofdocumentimages,itmakessensenottoOCRtheentirecollection

whichwillneverbeusedagain.

Inrecentyears,anumberofattemptshavealsobeenmadetoavoidtheuseof

OCRforvarioustextprocessingapplications,1,6buthavebeenmainlyonEnglish

textdocuments.Chineselanguageisquitedifferentfromthewesternlanguages.