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DetectingcatsinimageswithOpenCVbyAdrianRosebrockonJune20,2016inObjectDetection,Tutorials

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DidyouknowthatOpenCVcandetectcatfacesinimages…rightout-of-the-boxwithnoextras?

Ididn’teither.

ButafterKendrickTanbrokethestory,Ihadtocheckitoutformyself…anddoalittleinvestigativeworktoseehowthiscatdetectorseemedtosneakitswayintotheOpenCVrepositorywithoutmenoticing(muchlikeacatslidingintoanemptycerealbox,justwaitingtobediscovered).

Intheremainderofthisblogpost,I’lldemonstratehowtouseOpenCV’scatdetectortodetectcatfacesinimages.Thissametechniquecanbeappliedtovideostreamsaswell.

Lookingforthesourcecodetothispost?Jumprighttothedownloadssection.

DetectingcatsinimageswithOpenCVIfyoutakealookattheOpenCVrepository,specificallywithinthehaarcascadesdirectory(whereOpenCVstoresallitspre-trainedHaarclassifierstodetectvariousobjects,bodyparts,etc.),you’llnoticetwofiles:

BothoftheseHaarcascadescanbeuseddetecting“catfaces”inimages.Infact,Iusedtheseverysamecascadestogeneratetheexampleimageatthetopofthisblogpost.

Doingalittleinvestigativework,IfoundthatthecascadesweretrainedandcontributedtotheOpenCVrepositorybythelegendaryJosephHowsewho’sauthoredagoodmanytutorials,books,andtalksoncomputervision.

Intheremainderofthisblogpost,I’llshowyouhowtoutilizeHowse’sHaarcascadestodetectcatsinimages.

10 75

haarcascade_frontalcatface.xmlhaarcascade_frontalcatface_extended.xml

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CatdetectioncodeLet’sgetstarteddetectingcatsinimageswithOpenCV.Openupanewfile,nameit ,andinsertthefollowingcode:

Lines2and3importournecessaryPythonpackageswhileLines6-12parseourcommandlinearguments.Weonlyrequireasingleargumenthere,theinput thatwewanttodetectcatfacesinusingOpenCV.

Wecanalso(optionally)supplyapathourHaarcascadeviathe switch.We’lldefaultthispathtoandassumeyouhavethe fileinthesamedirectoryasyour

script.

Note:I’veconvenientlyincludedthecode,catdetectorHaarcascade,andexampleimagesusedinthistutorialinthe“Downloads”sectionofthisblogpost.Ifyou’renewtoworkingwithPython+OpenCV(orHaarcascades),Iwouldsuggestdownloadingtheprovided.zipfiletomakeiteasiertofollowalong.

Next,let’sdetectthecatsinourinputimage:

OnLines15and16weloadourinputimagefromdiskandconvertittograyscale(anormalpre-processingstepbeforepassingtheimagetoaHaarcascadeclassifier,althoughnotstrictlyrequired).

Line20loadsourHaarcascadefromdisk(inthiscase,thecatdetector)andinstantiatesthe object.

DetectingcatfacesinimageswithOpenCVisaccomplishedonLines21and22bycallingthe methodoftheobject.Wepassfourparameterstothe method,including:

1. Ourimage, ,thatwewanttodetectcatfacesin.2. A ofourimagepyramidusedwhendetectingcatfaces.Alargerscalefactorwillincreasethespeedofthedetector,butcouldharmourtrue-positivedetectionaccuracy.Conversely,asmallerscalewillslowdownthedetectionprocess,butincreasetrue-positivedetections.However,thissmallerscalecanalsoincreasethefalse-positivedetectionrateaswell.Seethe“AnoteonHaarcascades”sectionofthisblogpostformoreinformation.

3. The parametercontrolstheminimumnumberofdetectedboundingboxesinagivenareafortheregiontobeconsidereda“catface”.Thisparameterisveryhelpfulinpruningfalse-positivedetections.

4. Finally,the parameterisprettyself-explanatory.Thisvalueensuresthateachdetectedboundingboxisatleastwidthxheightpixels(inthiscase,75x75).

The functionreturns ,alistof4-tuples.Thesetuplescontainthe(x,y)-coordinatesandwidthandheightofeachdetectedcatface.

Finally,let’sdrawarectanglesurroundeachcatfaceintheimage:

Givenourboundingboxes(i.e., ),weloopovereachofthemindividuallyonLine25.

cat_detector.py

--image

--cascadehaarcascade_frontalcatface.xml haarcascade_frontalcatface.xmlcat_detector.py

cv2.CascadeClassifier

detectMultiScaledetector detectMultiScale

grayscaleFactor

minNeighbors

minSize

detectMultiScale rects

rects

123456789101112

#importthenecessarypackagesimportargparseimportcv2#constructtheargumentparseandparsetheargumentsap=argparse.ArgumentParser()ap.add_argument("-i","--image",required=True, help="pathtotheinputimage")ap.add_argument("-c","--cascade", default="haarcascade_frontalcatface.xml", help="pathtocatdetectorhaarcascade")args=vars(ap.parse_args())

141516171819202122

#loadtheinputimageandconvertittograyscaleimage=cv2.imread(args["image"])gray=cv2.cvtColor(image,cv2.COLOR_BGR2GRAY)#loadthecatdetectorHaarcascade,thendetectcatfaces#intheinputimagedetector=cv2.CascadeClassifier(args["cascade"])rects=detector.detectMultiScale(gray,scaleFactor=1.3, minNeighbors=10,minSize=(75,75))

242526272829303132

#loopoverthecatfacesanddrawarectanglesurroundingeachfor(i,(x,y,w,h))inenumerate(rects): cv2.rectangle(image,(x,y),(x+w,y+h),(0,0,255),2) cv2.putText(image,"Cat#{}".format(i+1),(x,y-10), cv2.FONT_HERSHEY_SIMPLEX,0.55,(0,0,255),2)#showthedetectedcatfacescv2.imshow("CatFaces",image)cv2.waitKey(0)

DetectingcatsinimageswithOpenCV Python



WethendrawarectanglesurroundingeachcatfaceonLine26,whileLines27and28displaysaninteger,countingthenumberofcatsintheimage.

Finally,Lines31and32displaytheoutputimagetoourscreen.

CatdetectionresultsTotestourOpenCVcatdetector,besuretodownloadthesourcecodetothistutorialusingthe“Downloads”sectionatthebottomofthispost.

Then,afteryouhaveunzippedthearchive,youshouldhavethefollowingthreefiles/directories:

1. :OurPython+OpenCVscriptusedtodetectcatsinimages.2. :ThecatdetectorHaarcascade.3. :Adirectoryoftestingimagesthatwe’regoingtoapplythecatdetectorcascadeto.

Fromthere,executethefollowingcommand:

Figure1:Detectingacatfaceinanimage,evenwithpartsofthecatoccluded(source).

Noticethatwehavebeenabletodetectthecatfaceintheimage,eventhoughtherestofitsbodyisobscured.

Let’stryanotherimage:

cat_detector.pyhaarcascade_frontalcatface.xmlimages

1 $pythoncat_detector.py--imageimages/cat_01.jpg

1 pythoncat_detector.py--imageimages/cat_02.jpg

DetectingcatsinimageswithOpenCV Shell


Figure2:AsecondexampleofdetectingacatinanimagewithOpenCV,thistimethecatfaceisslightlydifferent(source).

Thiscat’sfaceisclearlydifferentfromtheotherone,asit’sinthemiddleofa“meow”.Ineithercase,thecatdetectorcascadeisabletocorrectlyfindthecatfaceintheimage.

Thesameistrueforthisimageaswell:

Figure3:CatdetectionwithOpenCVandPython(source).

OurfinalexampledemonstratesdetectingmultiplecatsinanimageusingOpenCVandPython:

1 $pythoncat_detector.py--imageimages/cat_03.jpgDetectingcatsinimageswithOpenCV Shell

Figure4:DetectingmultiplecatsinthesameimagewithOpenCV(source).

NotethattheHaarcascadecanreturnboundingboxesinanorderthatyoumaynotlike.Inthiscase,themiddlecatisactuallylabeledasthethirdcat.Youcanresolvethis“issue”bysortingtheboundingboxesaccordingtotheir(x,y)-coordinatesforaconsistentordering.

AquicknoteonaccuracyIt’simportanttonotethatinthecommentssectionofthe files,JosephHowedetailsthatthecatdetectorHaarcascadescanreportcatfaceswherethereareactuallyhumanfaces.

Inthiscase,herecommendsperformingbothfacedetectionandcatdetection,thendiscardinganycatboundingboxesthatoverlapwiththefaceboundingboxes.

AnoteonHaarcascadesFirstpublishedin2001byPaulViolaandMichaelJones,RapidObjectDetectionusingaBoostedCascadeofSimpleFeatures,thisoriginalworkhasbecomeoneofthemostcitedpapersincomputervision.

Thisalgorithmiscapableofdetectingobjectsinimages,regardlessoftheirlocationandscale.Andperhapsmostintriguing,thedetectorcanruninreal-timeonmodernhardware.

Intheirpaper,ViolaandJonesfocusedontrainingafacedetector;however,theframeworkcanalsobeusedtotraindetectorsforarbitrary“objects”,suchascars,bananas,roadsigns,etc.

Theproblem?ThebiggestproblemwithHaarcascadesisgettingthe parametersright,specifically and

.Youcaneasilyrunintosituationswhereyouneedtotunebothoftheseparametersonanimage-by-imagebasis,whichisfarfromidealwhenutilizinganobjectdetector.

The variablecontrolsyourimagepyramidusedtodetectobjectsatvariousscalesofanimage.Ifyour istoolarge,thenyou’llonlyevaluateafewlayersoftheimagepyramid,potentiallyleadingtoyoumissingobjectsatscalesthatfallinbetweenthepyramidlayers.

Ontheotherhand,ifyouset toolow,thenyouevaluatemanypyramidlayers.Thiswillhelpyoudetectmoreobjectsinyourimage,butit(1)makesthedetectionprocessslowerand(2)substantiallyincreasesthefalse-positivedetectionrate,somethingthatHaarcascadesareknownfor.

Torememberthis,weoftenapplyHistogramofOrientedGradients+LinearSVMdetectioninstead.

TheHOG+LinearSVMframeworkparametersarenormallymucheasiertotune—andbestofall,HOG+LinearSVMenjoysamuchsmallerfalse-positivedetectionrate.Theonlydownsideisthatit’shardertogetHOG+LinearSVMtoruninreal-time.

Interestedinlearningmoreaboutobjectdetection?

.xml

detectMultiScale scaleFactorminNeighbors

scaleFactor scaleFactor

scaleFactor

1 $pythoncat_detector.py--imageimages/cat_04.jpgDetectingcatsinimageswithOpenCV Shell

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SummaryInthisblogpost,welearnedhowtodetectcatsinimagesusingthedefaultHaarcascadesshippedwithOpenCV.TheseHaarcascadesweretrainedandcontributedtotheOpenCVprojectbyJosephHowse,andwereoriginallybroughttomyattentioninthispostbyKendrickTan.

WhileHaarcascadesarequiteuseful,weoftenuseHOG+LinearSVMinstead,asit’sabiteasiertotunethedetectorparameters,andmoreimportantly,wecanenjoyamuchlowerfalse-positivedetectionrate.

IdetailhowtobuildcustomHOG+LinearSVMobjectdetectorstorecognizevariousobjectsinimages,includingcars,roadsigns,andmuchmoreinsidethePyImageSearchGuruscourse.

Anyway,Ihopeyouenjoyedthisblogpost!

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Downloads:Ifyouwouldliketodownloadthecodeandimagesusedinthispost,pleaseenteryouremailaddressintheformbelow.Notonlywillyougeta.zipofthecode,I’llalsosendyouaFREE11-pageResourceGuideonComputerVisionandImageSearchEngines,includingexclusivetechniquesthatIdon’tpostonthisblog!Soundgood?Ifso,enteryouremailaddressandI’llsendyouthecodeimmediately!Emailaddress:

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classification,haarcascades,objectdetection

16ResponsestoDetectingcatsinimageswithOpenCV

JosephHowseJune20,2016at6:36pm#

Thankssomuchforfeaturingthecatfacecascades!Iamdelightedtoseetheminactionherealongsideyourothergreattutorials!

Funfacts:

Thehaarcascade_frontalcatface_extended.xmlversionusesthe“extended”Haarfeaturesettomakeitsensitivetodiagonalfeaturessuchasearsandwhiskers.>^-^<

BesidestheHaarversions,youwillfindanLBPversion,lbpcascade_frontalcatface.xml,inOpenCV'slbpcascadesfolder.TheLBPversionislessaccuratebutfaster;youmightlikeitforRaspberryPiorotherplatformswithlimitedresources.

Detailsaboutthetrainingofthecatfacecascadescanbefoundinmybook,OpenCVforSecretAgents,andinfreepresentationsonmywebsite'sOpenCVlandingpage(http://www.nummist.com/opencv/).

Ninelives,

Joe

REPLY

AdrianRosebrockJune23,2016at1:34pm#

You’retheoneweshouldbethankingJoe—youtrainedtheactualcascades!

REPLY

KendrickTanJune20,2016at11:46pm#

HiAdrian,I’mabitstokedwhenIsawmynamecomeuponthenewsletter.

Anyway,Ijustcameheretosaythankyou.Yourtutorialsreallygavemeafirmunderstandingofthebasicsofcomputervision,andmachinelearning.Reallyappreciateyourtimeandeffort.

REPLY


HeyKendrick!Thanksfordoingtheinitialawesomepost—withoutyourpost,thecatdetectioncascadewouldhavetotallypassedmeby.Ishouldbetheonethankingyou

REPLY

LinusJune21,2016at7:39am#

Wow.ThanksAdrian,thisissoawesome!I’vethoughtaboutthepossibitityofdetectinganimalssometimeago,butI’veneverfounda

REPLY

solutionWhataboutotheranimals(dogsetc.)?Aretherealsoxmlfilesavailable?Andisusingthisinalivecamerafeedalsopossibe,withamoderateexecutionspeed?


AsfarasIknow,thereisonlytheHaarcascadeforcats—thereisn’tonefordogs.Althoughyoucouldcertainlytrainone.IwouldsuggestusingHOG+LinearSVMinsteadsinceithasalowerfalse-positiverate.

Andyes,thismethodcanbeusedtoruninreal-time.TakealookatPracticalPythonandOpenCVformoredetailsonrunningHaarcascadesinreal-time.

REPLY

RanjeetSinghJune21,2016at12:06pm#

ButnowIwanttoknowwhatthatxmlfileis?Howithasbeenwritten?

REPLY


TheXMLfileisaserializedHaarcascade.Itisgeneratedbytrainingamachinelearningalgorithmtorecognizevariousobjectsinimages.Torecognizeyourownobjects,you’llneedtotrainyourowncustomobjectdetector.

REPLY

swightNovember10,2016at7:23pm#

HowwouldIwritethefinalimagetofileinsteadofdisplayingtoscreen?

REPLY

AdrianRosebrockNovember14,2016at12:16pm#

Youcanjustusethecv2.imwritefunction:

cv2.imwrite("/path/to/my/image.jpg",image)

IfyouneedhelplearningthebasicsofcomputervisionandimageprocessingIwouldsuggestyouworkthroughPracticalPythonandOpenCV.

REPLY

VictorDecember17,2016at9:34pm#

WetrainedanLBPcascadewithbettertimeandaccuracyperformance.Ifanyonewantsthis,wecanpushittoour10imaging/opencvrepoonGitHub.

REPLY

RidgeDecember19,2016at8:01pm#

@Victor–Sure,I’dliketoseeit.

REPLY

GismoSchaMarch15,2017at8:28pm#

ThanksAdrian,itworks!Howcaniextract/copytherectangledareaandwritetofile?

REPLY

AdrianRosebrockMarch17,2017at9:31am#

YouwoulduseNumPyarrayslicingtoextracttheboundingboxandthenusecv2.imwritetowritetheimagetodisk.IdiscussthesebasicoperationsinsidePracticalPythonandOpenCV.IwouldsuggestyoustartthereifyouarenewtoOpenCV.

REPLY

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HarmanSinghMarch18,2017at5:06pm#

ThankAdrian,yourtutorialsaregreatandfunandalsodetectsfairlygoodbutwhiledeployingthisscriptonmultipleimageswithdifferentsize,theoutputisnottoaccurate.

forexamplein1stpic(catrolledinpaper)igot2rectangleonecoveringthefaceandotheronthenose..(toget1rect,iadjustedscaleFactorto1.715)

evenafterconvertingalltheimagetosizestillgettingmixedresultlikementionedaboveandsometimesnone.

doihavetoadjustscalefactoreverytime?orisitmehavingthisproblem?

ThankYou.

REPLY

AdrianRosebrockMarch21,2017at7:35am#

ThisisacommonproblemwithHaarcascades(theyareverypronetofalse-positives).YoucanreadmoreabouttheissueinthisblogpostonHistogramofOrientedGradients.

REPLY

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