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CS332VisualProcessingDepartmentofComputerScienceWellesleyCollege
CS332VisualProcessinginComputerandBiologicalVisionSystems
AnalysisofColor
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Content-basedimageretrieval
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Applicationsofcolorimagesegmentation
medicine,surveillance,inspection,recognition,…
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Colorconstancy
Colorofasurfacelooks(roughly)thesameunderdifferentilluminations…
http://www.echalk.co.uk/amusements/OpticalIllusions/illusions.htm
…althoughcolorscanbeinfluencedbycontext
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Surfacereflectance
R(x,y,λ)Surfacereflectancevarieswithpositionandwavelength
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Illumination
Evenwithuniformsceneillumination,castshadowsandreflectionoflightfromothersurfacescreatesspatiallynon-uniformsurfaceillumination
E(x,y,λ)
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Surfacereflectancemeetsilluminant
E(x,y,λ)
R(x,y,λ)
L(x,y,λ)=
E(x,y,l)*R(x,y,l)
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“TheDress”– whatcolorisit?
white-gold?(30%) blue-black?(57%)???Lafer-Sousa,Hermann,Conway(2015)
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Itdependsontheilluminant...
coolilluminant(bluesky)
warmilluminant(incandescent)
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Measuringcolorbyretinalcones
AbsorptionspectraforS(short),M(medium),L(long)wavelengthcones
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Luminancemeetsthecones
L(x,y,λ)
S(λ)M(λ)L(λ)
IS(x,y)=∫L(x,y,λ)S(λ)dλ
IM(x,y)=∫L(x,y,λ)M(λ)dλ
IL(x,y)=∫L(x,y,λ)L(λ)dλ
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Land’scolor“Mondrian”experiments
PietMondrianCompositionA, 1923EdwinLand
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Land’sRetinex theory*
L(x,y)=E(x,y)*R(x,y)
L(x,y):luminance
E(x,y):illuminant
R(x,y):surfacereflectance
Goal: recoversurfacereflectance
*ignorecolorfornow…
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Land’sRetinex theory(1D)
R(x)
E(x)
L(x)
log(L(x))
derivativeoflog(L(x))
computedR(x)
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Retinex theoryfor2Dcoloranalysis2Dextensions:• Land&McCann:multiple1Dpaths• Horn:2DanalysisbasedonLaplacianÑ2L• Jobson,Rahman&Woodell:appliedtoimageenhancement
Color:• PerformsameanalysisforIS(x,y),IM(x,y),IL(x,y)• TripletofvaluesRS(x,y),RM(x,y),RL(x,y)Þ color
Restrictions(Assumptions):• Reflectancechangesabruptly,illuminationchangesslowly•Matte(Lambertian)reflectancecharacteristics
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Robson,Rahman&WoodellMorel,Petro&Sbert
Imageenhancement,Retinex style