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1、WHITEPAPERRawCapture,LinearGamma,andExposurePerhapsthebiggestdifferencebetweenshootingfilmandshootingdigitalisthewaythetwodifferentmediarespondtolight.Filmrespondstolightthesamewayoureyesdo,butsilicondoesnot.Ifyou’retemptedtojustfilethisinformationawayasamildlyinterestingfactoid,letmepointoutthatyou’db
2、eoverlookingtheimportantimplicationsthequirksofsilicon’sresponsehaveonthewaywesetexposureondigitalcaptures.Ifyouexposedigitalthewayyouexposefilm,youruntwindangersoffailingtoexploitthecamera’sdynamicrange,andcreatingexposureswhoseshadowsarenoisierthantheyneedtobe.Filmmimicstheeye’sresponsetolight,whi
3、chishighlynonlinear.Mostofourhumansensesdisplayasignificantcompressivenonlinearity—abuilt-incompressionthatletsusfunctioninawiderangeofsituationswithoutdrivingoursensorymechanismsintooverload.Ifyouplaceagolfballinthepalmofyourhand,thenaddanotherone,itdoesn’tfeeltwiceasheavy.Ifyouputtwospoonfulsofsug
4、arinyourcoffeeinsteadofone,itdoesn’ttastetwiceassweet.Ifyoudoubletheacousticpowergoingtoyourstereospeakers,theresultingsoundisn’ttwiceasloud.Andifyoudoublethenumberofphotonsreachingyoureyes,youdon’tseethesceneastwiceasbright—brighter,yes,butnottwiceasbright.Thisbuilt-incompressionallowsyoursensestof
5、unctionoveranimmenserangeofstimuli.Youcangofromsubduedroomlightingtofulldaylightwithoutyoureyeballscatchingfire,eventhoughyoumayhavesuddenlyincreasedthestimulusreachingthoseeyeballsbyafactorof10,000orso.Butthesensorsindigitalcameraslackthecompressivenonlinearitytypicalofhumanperception.Theyjustcount
6、photonsinalinearfashion.Linearprocessedrawcaptureslookverydark.Butallthedataisthereintheimage.Thehistogramofthelinearcaptureshowsthevastmajorityofthedataclumpedtowardthedarkerend.Thesamelinearprocessedcapturewithatonecurveappearsnormal.Thisisthecurverequiredtoapplyagammacorrectiontonetothelinearcap
7、ture.Afterthetonecurve,thehisto-gramhasanormaldistributionthroughouttheimage.Thismeansthatifacamerauses12bitstoencodethecaptureinto4,096levels,thenlevel2,048rep-resentshalfthenumberofphotonsrecordedatlevel4
