数字视频特效处理lec02-camera摄影机概述

CamerasDigitalVisualEffectsYung-YuChuangwithslidesbyFredoDurand,BrianCurless,SteveSeitzandAlexeiEfrosCameratrial#1scenefilmPutapieceoffilminfrontofanobject.PinholecamerascenefilmAddabarriertoblockoffmostoftherays.ItreducesblurringThepinholeisknownastheapertureTheimageisinvertedbarrierpinholecameraShrinkingtheapertureWhynotmakingtheapertureassmallaspossible?LesslightgetsthroughDiffractioneffectShrinkingtheapertureHigh-endcommercialpinholecameras$200~$700AddingalensscenefilmlensLensesThinlensequation:ThinlensformulafDD’Similartriangleseverywhere!y’yy’/y=D’/DFrédoDurand’sslideThinlensformulafDD’y’yy’/y=D’/Dy’/y=(D’-f)/fFrédoDurand’sslideSimilartriangleseverywhere!ThinlensformulafDD’1D’D11f+=Thefocallengthfdeterminesthelens’sabilitytobend(refract)light.Itisafunctionoftheshapeandindexofrefractionofthelens.FrédoDurand’sslideAddingalensscenefilmlens“circleofconfusion”AlensfocuseslightontothefilmThereisaspecificdistanceatwhichobjectsare“infocus”otherpointsprojecttoa“circleofconfusion”intheimageThinlensapplet:.tw/java/Lens/lens_e.htmlZoomlensNikon28-200mmzoomlens.200mm28mmsimplifiedzoomlensinoperationFromwikipediaFieldofviewvsfocallengthioGaussianLensFormula:SceneSensorfαα=2arctan(w/(2i))wFieldofView:Example:w=30mm,f=50mm=>α

≈33.4º≈2arctan(w/(2f))

SlidesfromLiZhangFocallengthinpractice24mm50mm135mmDistortionRadialdistortionoftheimageCausedbyimperfectlensesDeviationsaremostnoticeableforraysthatpassthroughtheedgeofthelensNodistortionPincushionBarrelCorrectingradialdistortionfromHelmutDerschVignettingVignettingBAmorelightfromAthanB!SlidesfromLiZhangVignettingVignettingBAmorelightfromAthanB!originalcorrectedGoldman&ChenICCV2005SlidesfromLiZhangChromaticAberrationLenshasdifferentrefractiveindicesfordifferentwavelengths.Speciallenssystemsusingtwoormorepiecesofglasswithdifferentrefractiveindexescanreduceoreliminatethisproblem.

/learn/?/Glossary/Optical/chromatic_aberration_01.htmSlidesfromLiZhangExposure=aperture+shutterspeedApertureofdiameterDrestrictstherangeofrays(aperturemaybeoneithersideofthelens)ShutterspeedistheamountoftimethatlightisallowedtopassthroughtheapertureFExposureTwomainparameters:Aperture(infstop)Shutterspeed(infractionofasecond)Slowershutterspeed=>morelight,butmoremotionblurFastershutterspeedfreezesmotionEffectsofshutterspeeds1/1251/2501/5001/1000WalkingpeopleRunningpeopleCarFasttrainFromPhotography,Londonetal.ApertureApertureisthediameterofthelensopening,usuallyspecifiedbyf-stop,f/D,afractionofthefocallength.f/2.0ona50mmmeansthattheapertureis25mmf/2.0ona100mmmeansthattheapertureis50mmWhenachangeinf-stopoccurs,thelightiseitherdoubledorcutinhalf.Lowerf-stop,morelight(largerlensopening)Higherf-stop,lesslight(smallerlensopening)DepthoffieldChangingtheaperturesizeaffectsdepthoffield.AsmallerapertureincreasestherangeinwhichtheobjectisapproximatelyinfocuslenssensorPointinfocusObjectwithtextureDiaphragmDepthoffieldChangingtheaperturesizeaffectsdepthoffield.AsmallerapertureincreasestherangeinwhichtheobjectisapproximatelyinfocuslenssensorPointinfocusObjectwithtextureDiaphragmDepthoffieldFromPhotography,Londonetal.ExposureTwomainparameters:Aperture(infstop)Shutterspeed(infractionofasecond)Reciprocity ThesameexposureisobtainedwithanexposuretwiceaslongandanapertureareahalfasbigHencesquarerootoftwoprogressionoffstopsvs.poweroftwoprogressionofshutterspeedReciprocitycanfailforverylongexposuresFromPhotography,Londonetal.ReciprocityAssumeweknowhowmuchlightweneedWehavethechoiceofaninfinityofshutterspeed/aperturepairsWhatwillguideourchoiceofashutterspeed?Freezemotionvs.motionblur,camerashakeWhatwillguideourchoiceofanaperture?Depthoffield,diffractionlimitOftenwemustcompromiseOpenmoretoenablefasterspeed(butshallowDoF)Exposure&meteringThecamerameteringsystemmeasureshowbrightthesceneisInApertureprioritymode,thephotographersetstheaperture,thecamerasetstheshutterspeedInShutter-speedprioritymode,photographerssetstheshutterspeedandthecameradeducestheapertureInProgrammode,thecameradecidesbothexposureandshutterspeed(middlevaluemoreorless)InManualmode,theuserdecideseverything(butcangetfeedback)ProsandconsofvariousmodesAperturepriorityDirectdepthoffieldcontrolCons:canrequireimpossibleshutterspeed(e.g.withf/1.4forabrightscene)ShutterspeedpriorityDirectmotionblurcontrolCons:canrequireimpossibleaperture(e.g.whenrequestinga1/1000speedforadarkscene)NotethatapertureissomewhatmorerestrictedProgramAlmostnocontrol,butnoneedforneuronsManualFullcontrol,buttakesmoretimeandthinkingSensitivity(ISO)ThirdvariableforexposureLineareffect(200ISOneedshalfthelightas100ISO)Filmphotography:tradesensitivityforgrainDigitalphotography:tradesensitivityfornoiseFromSummaryinapicturesourcehamburgerfotospots.deDemoSee/simcam/Filmcamerascenefilmlens&motoraperture&shutterDigitalcamerascenesensorarraylens&motoraperture&shutterAdigitalcamerareplacesfilmwithasensorarrayEachcellinthearrayisalight-sensitivediodethatconvertsphotonstoelectronsCCDv.s.CMOSCCDislesssusceptibletonoise(specialprocess,higherfillfactor)CMOSismoreflexible,lessexpensive(standardprocess),lesspowerconsumptionCCDCMOSSensornoiseBloomingDiffusionDarkcurrentPhotonshotnoiseAmplifierreadoutnoiseSLR(Single-LensReflex)Reflex(RinSLR)meansthatweseethroughthesamelensusedtotaketheimage.NotthecaseforcompactcamerasSLRviewfinderlensMirror

(whenviewing)Mirror

(flippedforexposure)Film/sensorPrismYoureyeLightfromsceneColorSofar,we’veonlytalkedaboutmonochromesensors.Colorimaginghasbeenimplementedinanumberofways:FieldsequentialMulti-chipColorfilterarrayX3sensorFieldsequentialFieldsequentialFieldsequentialProkudin-Gorskii(early1900’s)Lanternprojector/exhibits/empire/Prokudin-Gorskii(early1900’s)Multi-chipwavelengthdependentEmbeddedcolorfiltersColorfilterscanbemanufactureddirectlyontothephotodetectors.ColorfilterarrayColorfilterarrays(CFAs)/colorfiltermosaicsKodakDCS620xCMYWhyCMYCFAmightbebetterColorfilterarrayColorfilterarrays(CFAs)/colorfiltermosaicsBayerpatternBayer’spatternDemosaickingCFA’sbilinearinterpolationoriginalinputlinearinterpolationDemosaickingCFA’sConstanthue-basedinterpolation(Cok)Hue:InterpolateGfirstDemosaickingCFA’sMedian-basedinterpolation(Freeman)1.Linearinterpolation2.MedianfilteroncolordifferencesDemosaickingCFA’sMedian-basedinterpolation(Freeman)originalinputlinearinterpolationcolordifference(e.g.G-R)medianfilter(kernelsize5)Reconstruction(G=R+filtereddifference)DemosaickingCFA’sGradient-basedinterpolation(LaRoche-Prescott)1.InterpolationonGDemosaickingCFA’sGradient-basedinterpolation(LaRoche-Prescott)2.InterpolationofcolordifferencesDemosaickingCFA’sbilinearCokFreemanLaRocheDemosaickingCFA’sGenerally,Freeman’sisthebest,especiallyfornaturalimages.FoveonX3sensorlightpenetratestodifferentdepthsfordifferentwavelengthsmultilayerCMOSsensorgets3differentspectralsensitivitiesColorfilterarrayredgreenblueoutputX3technologyredgreenblueoutputFoveonX3sensorX3sensorBayerCFACameraswithX3SigmaSD10,SD9PolaroidX530SigmaSD9vsCanonD30ColorprocessingAftercolorvaluesarerecorded,morecolorprocessingusuallyhappens:WhitebalanceNon-linearitytoapproximatefilmresponseormatchT