Perception and Measurement of Light, Color, and Appearance.

Perception and Perception and Measurement ofMeasurement ofLight, Color, and Light, Color, and

AppearanceAppearance

ProblemsProblems

• How do cameras measure light and How do cameras measure light and color?color?– RadiometryRadiometry

• How do humans perceive light and How do humans perceive light and color?color?– PhotometryPhotometry

• How do monitors display light and How do monitors display light and color?color?

IntensityIntensity

• Perception of intensity is nonlinearPerception of intensity is nonlinear

Amount of lightAmount of light

PerceivedPerceivedbrightnessbrightness

Modeling Nonlinear Intensity Modeling Nonlinear Intensity ResponseResponse

• Brightness (Brightness (BB) usually modeled as a ) usually modeled as a logarithm or power law of intensity (logarithm or power law of intensity (II))

• Exact curve varies with ambient light,Exact curve varies with ambient light,adaptation of eyeadaptation of eye

3/1

log

IB

IkB

3/1

log

IB

IkB

II

BB

CRT ResponseCRT Response

• Power law for Intensity (Power law for Intensity (II) vs.) vs.applied voltage (applied voltage (VV))

• Other displays (e.g. LCDs) contain Other displays (e.g. LCDs) contain electronics to emulate this lawelectronics to emulate this law

5.2

VI

5.2

VI

Digression: Monitor KnobsDigression: Monitor Knobs

• ““Brightness” knob is offsetBrightness” knob is offset

• ““Contrast” knob is scaleContrast” knob is scale

• Yes, the names Yes, the names areare misleading… misleading…

)( brightnessVcontrastI )( brightnessVcontrastI

CamerasCameras

• Original cameras based on Vidicon obey Original cameras based on Vidicon obey power law for Voltage (V) vs. Intensity power law for Voltage (V) vs. Intensity (I):(I):

• Vidicon + CRT = almost linear!Vidicon + CRT = almost linear!

45.0

IV

45.0

IV

CCD CamerasCCD Cameras

• Camera gamma codified in NTSC Camera gamma codified in NTSC standardstandard

• CCDs have linear response to incident CCDs have linear response to incident lightlight

• Electronics to apply required power lawElectronics to apply required power law

• So, pictures from most cameras So, pictures from most cameras (including digital still cameras) will (including digital still cameras) will have have = 0.45= 0.45

Consequences for VisionConsequences for Vision

• Output of most cameras is not linearOutput of most cameras is not linear

• Know what it is! (Sometimes system Know what it is! (Sometimes system automagically applies “gamma automagically applies “gamma correction”)correction”)

• Necessary to correct raw pixel values for:Necessary to correct raw pixel values for:– Reflectance measurementsReflectance measurements– Shape from shadingShape from shading– Photometric stereoPhotometric stereo– Recognition under variable lightingRecognition under variable lighting

Consequences for VisionConsequences for Vision

• What about e.g. edge detection?What about e.g. edge detection?– Often want “perceptually significant” Often want “perceptually significant”

edgesedges– Standard nonlinear signal close to Standard nonlinear signal close to

(inverse of) human response(inverse of) human response– Using nonlinear signal often the “right Using nonlinear signal often the “right

thing”thing”

Contrast SensitivityContrast Sensitivity

• Contrast sensitivity for humans about Contrast sensitivity for humans about 100:1100:1

• 8-bit image (barely) adequate if using 8-bit image (barely) adequate if using perceptual (nonlinear) mappingperceptual (nonlinear) mapping

• Frequency dependent: contrast Frequency dependent: contrast sensitivity lower for high and very sensitivity lower for high and very low frequencieslow frequencies

Contrast SensitivityContrast Sensitivity

• Campbell-Robson contrast sensitivity Campbell-Robson contrast sensitivity chartchart

Bits per Pixel – Scanned Bits per Pixel – Scanned PicturesPictures

8 bits / pixel / color8 bits / pixel / color 6 bits / pixel / color6 bits / pixel / color

Marc Levoy / Hanna-BarberaMarc Levoy / Hanna-Barbera

Bits per Pixel – Scanned Bits per Pixel – Scanned Pictures (cont.)Pictures (cont.)



Bits per Pixel – Line DrawingsBits per Pixel – Line Drawings



Bits per Pixel – Line Drawings Bits per Pixel – Line Drawings (cont.)(cont.)



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” More sensitive in low light: “scotopic”

visionvision– 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

ColorColor

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

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

ColorspacesColorspaces

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

• RGBRGB– Official standard: R = 645.16 nm,Official standard: R = 645.16 nm,

G = 526.32 nm, B = 444.44 nmG = 526.32 nm, B = 444.44 nm– Most monitors are some approximation Most monitors are some approximation

to thisto this

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 Saturated greens would require

negative rednegative red

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

CIE Chromaticity DiagramCIE Chromaticity Diagram

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 Other color components typically

allocated less bandwidth than Yallocated less bandwidth than Y

Perceptually-Uniform Perceptually-Uniform ColorspacesColorspaces

• Most colorspaces Most colorspaces not perceptually not perceptually uniformuniform

• 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 Perceptually-Uniform ColorspacesColorspaces

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

Perception and Measurement of Light, Color, and Appearance.

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