9/19/17

1

MinH.Kim(KAIST) CS681:ComputationalColorimetry

CS681ComputationalColorimetry

MinH.KimKAISTSchoolofComputing

MinH.Kim(KAIST) CS681:ComputationalColorimetry

COLORTRANSFORMATION

Acknowledgements:someofslidesarecourtesyofProf.LindsayMcDonaldandPhilGreenatLCC,Prof.RoyBernatRIT,andSteveMarschner atCornell.

9/19/17

2

MinH.Kim(KAIST) CS681:ComputationalColorimetry

ColorReproduction• Imaginewehaveaspectrums;wanttomatchonRGBdisplay

• Practically,wecannotachieveaphysicallyidenticalspectrumbecausetheyaredifferentmedia

• Butcouldfindaspectrumsa thatthedisplaycanproduce,whichisametamerofs

ssa

MinH.Kim(KAIST) CS681:ComputationalColorimetry

ColorReproductionasLinearAlgebra

• Wehaveaspectrums, wewanttomatchonanRGBdisplay– Anyspectrumthatprojectstothesamepointinthevisualcolorspaceisagoodreproduction

• Todoso,wewanttofindaspectrumthatthedisplaycanproducethatmatchess– i.e.,wewanttodisplayametamerofs onthescreen

9/19/17

3

MinH.Kim(KAIST) CS681:ComputationalColorimetry

ColorReproductionasLinearAlgebra

• Wewanttocomputesa thecombinationofR,G,B

• whichwillprojecttothesamevisualresponseass

• sa willbeametamerofs

XYZ

Spanofeye’sspectralresponsefunctions

Spanofdisplay’sprimaries

Adaptedfro

mSteveM

arschn

er

Visualresponsetosandsa

Spectrums

Spectrumsa C

V

MinH.Kim(KAIST) CS681:ComputationalColorimetry

ColorReproductionasLinearAlgebra

• Theprojectionontothethreeresponsefunctionscanbewritteninamatrixform:

• or,

XYZ

⎡

⎣

⎢⎢⎢

⎤

⎦

⎥⎥⎥= rX rY

rZ

⎡

⎣

⎢⎢⎢

⎤

⎦

⎥⎥⎥

s

⎡

⎣

⎢⎢⎢

⎤

⎦

⎥⎥⎥.

SpectralresponsivityofXYZ

V = MXYZs. V ∈!3, MXYZ ∈!

3×λ , s∈!λ( )

9/19/17

4

MinH.Kim(KAIST) CS681:ComputationalColorimetry

ColorReproductionasLinearAlgebra

• ThespectrumthatisproducedbythedisplayforthecolorsignalsR,G,Bis:

• Againthediscreteformcanbewrittenasamatrix:

• or,

Sa (λ) = Rsr (λ)+Gsg (λ)+ Bsb (λ).

sa

⎡

⎣

⎢⎢⎢

⎤

⎦

⎥⎥⎥=

sR sG sB

⎡

⎣

⎢⎢⎢

⎤

⎦

⎥⎥⎥

RGB

⎡

⎣

⎢⎢⎢

⎤

⎦

⎥⎥⎥.

sa = MRGBC.SpectraofRGBphosphors

sa ∈!

λ , MRGB ∈!λ×3, s∈!3( )

MinH.Kim(KAIST) CS681:ComputationalColorimetry

ColorReproductionasLinearAlgebra

• Whatcolordoweseewhenwelookatthedisplay?

• FeedC(R,G,B) todisplay• Displayproducessa• Eyeslookatsa andproduceV

V = MXYZMRGBC.

XYZ

⎡

⎣

⎢⎢⎢

⎤

⎦

⎥⎥⎥=

rX ⋅ sR rX ⋅ sG rX ⋅ sBrY ⋅ sR rY ⋅ sG rY ⋅ sBrZ ⋅ sR rZ ⋅ sG rZ ⋅ sB

⎡

⎣

⎢⎢⎢

⎤

⎦

⎥⎥⎥

RGB

⎡

⎣

⎢⎢⎢

⎤

⎦

⎥⎥⎥.

XYZ

saC

V

MXYZ ∈!

3×λ , MRGB ∈!λ×3( )

9/19/17

5

MinH.Kim(KAIST) CS681:ComputationalColorimetry

ColorReproductionasLinearAlgebra

• Goalofreproduction:visualresponsetos andsa isthesame:

• Substitutingintheexpressionforsa ,

MXYZ s = MXYZ sa .

MXYZ s = MXYZMRGBC.

C = (MXYZMRGB )−1MXYZ s.

Colorreproductionmodelfordisplay XYZ sa≈s

s

saC

V

MinH.Kim(KAIST) CS681:ComputationalColorimetry

ColorReproductionasLinearAlgebra

XYZ

Spanofeye’sspectralresponsefunctions

Spanofdisplay’sprimaries

Visualresponsetosandsa

Spectrums

Spectrumsa C

V

9/19/17

6

MinH.Kim(KAIST) CS681:ComputationalColorimetry

Wherearethecolortransforms?

• Nowadays,ineverydigitalimagingdevices:– TV,digitalcameras,camcorders,inkjetprinters,laserprinters,LCDdisplays,etc…

• Otherwise…

≠

MinH.Kim(KAIST) CS681:ComputationalColorimetry

COLORMANAGEMENTSYSTEM

12

9/19/17

7

MinH.Kim(KAIST) CS681:ComputationalColorimetry

WhatisICCprofile?• InternationalColorConsortium(foundedin1993)

• TheICCspecificationallowsformatchingofcolorwhenthecolorsaremovedbetweenapplicationsandoperatingsystems,orfromonedevicetotheotherone.

• TheICCdefinestheformat,butnotdefinealgorithmsorprocessingdetails.

MinH.Kim(KAIST) CS681:ComputationalColorimetry

TheneedforColorManagement

• Imagingworkflowsinvolvemultiplesamplingandrenderingsystems.

• Therenderingsystemmaynotbeknownatthetimeofsampling.

9/19/17

8

MinH.Kim(KAIST) CS681:ComputationalColorimetry

TheneedforColorManagement

• Inatraditionalreproductionworkflow,eachpairofdevicesrequiresaseparatetransformationinordertoensurethatthedesiredcolorcanbereproduced.

MinH.Kim(KAIST) CS681:ComputationalColorimetry

TheneedforColorManagement

• Incolormanagement,foreachdevicethereisatransformationbetweendevicecodevaluestoreferencecolorspace.

9/19/17

9

MinH.Kim(KAIST) CS681:ComputationalColorimetry

Keyelementsofacolormanagementsystem• Profiles– Definecharacterizationmodelforagivenmedium–Mayincludeotherelementsofimagecolorreproductiontransform,suchasgamutmappingandchromaticadaptation

– Profilecreatorsarerequired

• CIEcolorimetryspecification– Providesdevice-independentsideofprofiles

MinH.Kim(KAIST) CS681:ComputationalColorimetry

ColorManagementFramework

• TheICCcolormanagementframeworkincludes:

– Profiles– Acolormatchingmodule(CMM)

– Theapplicationthatinitiatesthetransformandcallstheprofiles;anoperatingsystemmodulethatcanstoreandfetchtheprofiles.

3rd partyCMM

3rd partyCMM

Application

Graphicslibrary

Imaginglibrary

DefaultCMM

Colormanagementframeworkinterface

Profiles

9/19/17

10

MinH.Kim(KAIST) CS681:ComputationalColorimetry

ICCcolorprocessingmodels

• ProfileConnectionSpace(PCS),e.g.CIEXYZ,CIELCH• Thecolorprocessingmodelforamatrix-shaperprofiles• Matrix-shaperprofilesarefasttoimplement,butareonly

validwherethereisnomixingbetweenchannels• Thismakesthemacceptablefor:• CRTdisplays,colorspaceprofiles(occasionally)scanners

MinH.Kim(KAIST) CS681:ComputationalColorimetry

ColorTarget(Input)

• Cameraandscanner

!

9/19/17

11

MinH.Kim(KAIST) CS681:ComputationalColorimetry

ICCcolorprocessingmodels

• TheprocessingmodelforaCLUT• Acolorlookuptableisthefastestwayofimplementingmorecomplextransformations

• CMYprinter

MinH.Kim(KAIST) CS681:ComputationalColorimetry

ColorTarget(Output)• CMYKPrinters

9/19/17

12

MinH.Kim(KAIST) CS681:ComputationalColorimetry

IssuesinColorManagement• Wheretoputthesmarts– theprofileortheCMM?• Gamutmapping• Colorappearance• Multispectralimaging• Controlprocedures• Needtosimplifyandmakemoreconsistenttheprofilecreationandcolortransformationworkflows