FRS 123: Technology inFRS 123: Technology inArt and Cultural HeritageArt and Cultural Heritage

ColorColor

ColorColor

• Two types of receptors: rods and Two types of receptors: rods and conescones

Rods and conesRods and cones Cones in foveaCones in fovea

Rods and ConesRods and Cones

• RodsRods– More sensitive in low light: “scotopic” visionMore sensitive in low light: “scotopic” vision

– More dense near peripheryMore dense near periphery

• ConesCones– Only function with higher light levels:Only function with higher light levels:

“photopic” vision“photopic” vision

– Densely packed at center of eye: foveaDensely packed at center of eye: fovea

– Different types of cones Different types of cones color vision color vision

Color PerceptionColor Perception

• 3 types of cones: L, M, S3 types of cones: L, M, S

Tristimulus theory of color

S

LM

Tristimulus ColorTristimulus Color

• Any distribution of light can be summarized Any distribution of light can be summarized by its effect on 3 types of conesby its effect on 3 types of cones

• Therefore, human perception of color is aTherefore, human perception of color is a3-dimensional space3-dimensional space

• MetamerismMetamerism: different spectra, same : different spectra, same responseresponse

• Color blindness: fewer than 3 types of conesColor blindness: fewer than 3 types of cones– Most commonly L cone = M coneMost commonly L cone = M cone

Color ModelsColor Models

• RGBRGB

• CMYCMY

• HSVHSV

• XYZXYZ

• ……etcetc

Color ModelsColor Models

• Different ways of parameterizing 3D Different ways of parameterizing 3D spacespace

• RGBRGB– Official standard: Official standard:

R = 645.16 nm, G = 526.32 nm, B = 444.44 R = 645.16 nm, G = 526.32 nm, B = 444.44 nmnm

– Most monitors are some approximation to Most monitors are some approximation to thisthis

RGB Color ModelRGB Color Model

R G B R G B Color Color 0.00.0 0.00.0 0.00.0 BlackBlack1.01.0 0.00.0 0.00.0 RedRed0.00.0 1.01.0 0.00.0 GreenGreen0.00.0 0.00.0 1.01.0 BlueBlue1.01.0 1.01.0 0.00.0 YellowYellow1.01.0 0.00.0 1.01.0 MagentaMagenta0.00.0 1.01.0 1.01.0 CyanCyan1.01.0 1.01.0 1.01.0 WhiteWhite0.50.5 0.00.0 0.00.0 ??1.01.0 0.50.5 0.50.5 ??1.01.0 0.50.5 0.00.0 ??0.50.5 0.30.3 0.10.1 ??

Colors are additiveColors are additive

CMY Color ModelCMY Color Model

C C M Y M Y Color Color 0.00.0 0.00.0 0.00.0 WhiteWhite1.01.0 0.00.0 0.00.0 CyanCyan0.00.0 1.01.0 0.00.0 MagentaMagenta0.00.0 0.00.0 1.01.0 YellowYellow1.01.0 1.01.0 0.00.0 BlueBlue1.01.0 0.00.0 1.01.0 GreenGreen0.00.0 1.01.0 1.01.0 RedRed1.01.0 1.01.0 1.01.0 BlackBlack0.50.5 0.00.0 0.00.0 ??1.01.0 0.50.5 0.50.5 ??1.01.0 0.50.5 0.00.0 ??

Colors are subtractiveColors are subtractive

HSV Color ModelHSV Color Model

H S V Color 0 1.0 1.0 Red120 1.0 1.0 Green240 1.0 1.0 Blue * 0.0 1.0 White * 0.0 0.5 Gray * * 0.0 Black 60 1.0 1.0 ?270 0.5 1.0 ?270 0.0 0.7 ?

XYZ ColorspaceXYZ Colorspace

• RGB can’t represent all pure RGB can’t represent all pure wavelengths with positive valueswavelengths with positive values– Saturated greens would require negative Saturated greens would require negative

redred

• XYZ colorspace is a linear transform of XYZ colorspace is a linear transform of RGB so that all pure wavelengths have RGB so that all pure wavelengths have positive valuespositive values

CIE Chromaticity DiagramCIE Chromaticity Diagram

CIE Chromaticity DiagramCIE Chromaticity Diagram

(White)

CIE Chromaticity DiagramCIE Chromaticity Diagram

CompareCompareColor Color

GamutsGamuts

IdentifyIdentifyComplementaryComplementary

ColorsColors

DetermineDetermineDominant WavelengthDominant Wavelength

and Purityand Purity

RGB Color Gamut for Typical RGB Color Gamut for Typical MonitorMonitor

Colorspaces for TelevisionColorspaces for Television

• Differences in brightness more Differences in brightness more important than differences in colorimportant than differences in color

• YCYCrrCCbb, YUV, YIQ colorspaces = linear , YUV, YIQ colorspaces = linear

transforms of RGBtransforms of RGB– Lightness: Y=0.299R+0.587G+0.114BLightness: Y=0.299R+0.587G+0.114B

– Other color components typically allocated Other color components typically allocated less bandwidth than Yless bandwidth than Y

Perceptually-Uniform ColorspacesPerceptually-Uniform Colorspaces

• Most colorspaces not Most colorspaces not perceptually uniformperceptually uniform

• MacAdam ellipses: MacAdam ellipses: color within each color within each ellipse appears ellipse appears constant (shown constant (shown here 10X size)here 10X size)

Perceptually-Uniform ColorspacesPerceptually-Uniform Colorspaces

• u’v’ spaceu’v’ space

• Not perfect, but better than XYZNot perfect, but better than XYZ

ZYX

Yv

ZYX

Xu

315

9'

315

4'

L*a*b* Color SpaceL*a*b* Color Space

• Another choice: L*a*b*Another choice: L*a*b*

3/13/1

3/13/1

3/1

200*

500*

16116*

nn

nn

n

Z

Z

Y

Yb

Y

Y

X

Xa

Y

YL

L*a*b* Color SpaceL*a*b* Color Space

• Often used for color comparison when Often used for color comparison when “perceptual” differences matter“perceptual” differences matter

SummarySummary

• Perception and representation of Perception and representation of – Intensity, frequency, colorIntensity, frequency, color

• ColorColor– Tristimulus theory of colorTristimulus theory of color

– CIE Chromaticity DiagramCIE Chromaticity Diagram

– Different color modelsDifferent color models