Chapter 2 - Digital Image Fundamentals

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1、§2-DigitalImageFundamentalsDr.DirkSchniedersDepartmentofComputerScienceTheUniversityofHongKongElementsofVisualPerception★IntheDIPfield,humanintuitionandanalysisplayacentralroleinthechoiceofonetechniqueversusanother★Subjectivevisualjudgmentisinvolved★Knowingthemechan

2、icsandparametersrelatedtohowimagesareformedintheeyesisthustheveryfirststepindevelopingabasicunderstandingofhumanvisualperception★Inparticular,wehavetodeterminethephysicallimitationsofhumanvision2013-2014COMP75022HumanVisionSystem★Iris:controlstheamountsoflightenteri

3、ngtheeye★Lens:focusestheimageontotheretina★Retina:coveredbylightreceptors(i.e.,Cones&Rods)thatconverttheimageintochemicalandnervoussignalsthataretransmittedtothebrainviatheopticalnerves2013-2014COMP75023Photopic(Bright-Light)Vision★Conesintheretina★Numberbetween6and

4、7millions★Locatedprimarilyinthefovea★Eachoneisconnectedtoitsownnerveend★Highlysensitivetocolorandfinedetails2013-2014COMP75024Scotopic(Dim-Light)Vision★Rodsintheretina★Numberbetween75and150millions★Distributedovertheretinalsurface★Severalrodsareconnectedtoasinglener

5、ve★Servetogiveageneral,overallpictureofthefieldofview2013-2014COMP75025ResolvingPower★Thefoveaisacircularindentationintheretinaofabout1.5mmindiameter★Letusapproximatethefoveaasasquaresensorarrayof1.5mmx1.5mm★Thedensityofconesis~150,000elementspermm2★Basedontheseappr

6、oximations,thenumberofconesintheregionofhighestacuityintheeyeis~337,000elements★Intermsofrawresolvingpower,acharge-coupleddevice(CCD)imagingchipofmediumresolutioncanhavethisnumberofelementsinareceptorarraynolargerthan5mmx5mm2013-2014COMP75026ImageFormationintheEye★T

7、heshapeofthelensiscontrolledbytensioninthefibersoftheciliarybody★Tofocusadistantobjects,thecontrollingmusclescausethelenstobeflattenedtomakeitlessrefractive★Tofocusanearobject,thesemusclesallowthelenstobecomethickertomakeitmorerefractive★Thedistancebetweenthecentero

8、flensandtheretina(calledthefocallength)variesfrom~17mmto~14mm★Wecancalculatethesizeoftheretinaimage★Intheexamplebelow,15m/100m=h/17mm=>h=2