Chapter 3 - Spatial Image Enhancement

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1、§3-ImageEnhancementDr.DirkSchniedersDepartmentofComputerScienceTheUniversityofHongKongIntroduction★Objective:★Processanimagesothattheresultismoresuitablethantheoriginalimageforaspecificapplication★Approaches:★Spatialdomaintechniquesarebasedondirectman

2、ipulationofpixelsinanimage★FrequencydomaintechniquesarebasedonmodifyingtheFouriertransformationofanimage★Evaluationofimagequalitybyvisualinspection(asubjectiveprocess)★Evaluationoftheperformanceofmachineperceptione.g.successrateincharacterrecognition2

3、013-2014COMP75022SpatialDomainProcess★f(x,y)istheinputimage(s)★g(x,y)istheoutputimage★Tisanoperationonf,definedoversomeneighborhoodof(x,y)★Tutilizesonlythepixelsintheareaoftheimagespannedbytheneighborhoodof(x,y)toproduceg(x,y)★Theneighborhoodshapesare

4、usuallysquareorrectangularduetotheeaseofimplementation2013-2014COMP75023GrayLevelTransformation★Simplestform:Twithaneighborhoodofsize1×1sandrdenoteintensityofgandfatanypoint(x,y)★Referredtoaspointprocessingsincetheenhancementatanypointinanimagedepends

5、onlyonthatpoint★Fordigitalimages,themappingfromrtoscanbeimplementedasalookup-tableusinga1Darray2013-2014COMP75024GrayLevelTransformation★Threebasictypesoffunctionsusedfrequentlyforimageenhancement:★Linear★Logarithmic★Power-law2013-2014COMP75025ImageNe

6、gative★Producestheequivalentofaphotographicnegative★Forenhancingwhiteorgraydetailsembeddedindarkregionsinputoutput2013-2014COMP75026LogTransformation★Forexpanding(spreading)thevaluesofdarkpixelsinanimagewhilecompressingthehigher-levelvaluesinput★Theop

7、positeistrueoftheinverselogtransformation★Especiallyusefulforcompressingthedynamicrangeofimageswithlargevariationsinpixelvaluese.g.Fourierspectrumwithvaluesfrom0to>106output2013-2014COMP75027Power-LawTransformation★Bothcandγareassumedtobesomepositivec

8、onstants★Similartotheeffectsofthelogtransformationforfractionalvaluesofγ★Moreflexiblethanthelogtransformation★Usefulforgeneral-purposecontrastmanipulation2013-2014COMP75028Power-LawTransformation★Example:★Gamma-correction2013-2014COMP75029Powe