Computer Vision – Computer Vision – Fundamentals of Human Fundamentals of Human VisionVision

Hanyang University

Jong-Il Park

            

Division of Electrical and Computer Engineering, Hanyang University

IntroductionIntroduction

Understanding of Mechanism of Human Vision

To construct the measures of image fidelity & intelligibility

To design and evaluate image processing algorithms and imaging systems

            

Division of Electrical and Computer Engineering, Hanyang University

Light ~ radiant energy which, by its action on theorgans of visions, enables them to perform

their function of sight Spectral energy distribution of the light source

L( ), = 350nm ~ 780nm Light received from an object

: reflectivity or transmissivity of the object

λ

) λ ρ(

)λ)L(λρ()λ ( I

Brightness (Perceived Luminance) Brightness (Perceived Luminance)

λ

            

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Human EyeHuman Eye

visible range : 350 nm < wavelength < 780 nm photoreceptors of the retina

rods : about 75-150 millions cones : about 6.5 millions (Color Vision)

scotopic vision : rods (dark environment) mesopic vision : rods + cones (middle range) photopic vision : cones (bright environment)

rods are more sensitive to light than the cones

            

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Distribution of PhotoreceptorsDistribution of Photoreceptors

            

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Division of Electrical and Computer Engineering, Hanyang University

The Human Visual SystemThe Human Visual System

홍채

각막

( 안구의 )

수양액

공막

( 눈알의 )

맥락막

Shape of lens, rather than the distance between lens and screen, is changed

            

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The Human Visual System (cont.)The Human Visual System (cont.)

            

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Luminance (or Intensity)

: independent of luminance of the surrounding object

Luminosity Function (Relative Luminous Efficiency Function)

dVyxIyxf )(),,(),(

Eye PhysiologyEye Physiology

Light distribution

Relative luminous efficiency function

            

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Contrast Brightness : dependent upon the surroundings

BrightnessBrightness

            

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Brightness adaptationBrightness adaptation

Dynamic range ~ 1010

            

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Intensity Discrimination ExperimentIntensity Discrimination Experiment

            

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Weber’s LawWeber’s Law

            

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(100,101) (100,102) (100,105)

(150,151) (150,153) (150,162)

            

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fractionweber 02.0

CI

I

tiondiscrimina brightness good

blediscrimina isintensity in change small : of valuesmallFor

I

I

intensity offuction a as ratio weber typical

cone

rod

            

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Brightness(cont.)Brightness(cont.)

Mach Bands

            

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MTF of the Visual SystemMTF of the Visual System

Measurement of visual system in frequency domain

MTF: Modulation Transfer function

reecycles/deg

exp)(),(

22

21

0021

AHH

reecycles/deg 8 :frequency peak . ,772.8)114.0(

1.1 ,0192.0 ,6.2 nsApplicatio Coding Imagefor Values Useful

10

A

            

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IsopreferenceIsopreference

k: # of bits/pel gray-level resolutionN: # of pels spatial resolution

Preference depends on image!

            

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Optical illusionsOptical illusionsOptical illusionsOptical illusions

            

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Moire patternMoire patternMoire patternMoire pattern

            

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Goal : Image quality measurement Performance evaluation of image processing

techniques or systems

Quantitative Criteria Mean square criterion :

SNR(signal-to-noise ratio) :

PSNR(peak-to-peak SNR) :

M

m

N

nms nmunmu

MN 1 1

22 ),(),(1

2

2

10log10ms

SNR

2

2

10

)peak value-to-peak(log10

ms

PSNR

Image Fidelity CriteriaImage Fidelity Criteria

            

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Image Fidelity Criteria (cont.)Image Fidelity Criteria (cont.)

Subjective Criteria

            

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Perception of Intermittent LightPerception of Intermittent Light

Perception depends on its frequency ( N cycles/sec) small N : Flashes appear separated in time increase N : unsteady flicker, unpleasant increase N further : Continuous light perception

Fusion frequency : Frequency at which an observer begins perceiving light flashes as continuous light

Critical Fusion frequency (CFF) : about 50 ~ 60 Hz.

Consider a light that flashes on for a brief duration N times/sec

            

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Perception of Intermittent Light (cont.)Perception of Intermittent Light (cont.)

Higher fusion frequency for larger size and larger intensity of the flickering object very dim, small light : A few cycles/sec very bright, large light : Over 100 cycles/sec

Examples of intermittent light fluorescent light : Over 100 times/sec motion picture : 24 frames/sec with 1 frame shown

twice TV monitor : 30 frames/sec, 2fields/frame

60 fields/sec (NTSC system)

            

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Empirical ObservationEmpirical Observation

Sharper images look better

Same noise in uniform background region is more visible than noise in edge areas (spatial masking)

Same noise in dark areas is more visible than noise in bright areas

Same amount of artificial noise appears worse than natural looking noise

            

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ColorimetryColorimetry

The perceptual attributes of color (HIS system) Intensity : the amount of light, perceived luminance

ex) distinction between dark grey and light grey hue : the color as described by wave length

ex) distinction between red and yellow saturation : the amount of color that is present

ex) distinction between red and pink the vividness of color

Three primaries : Red, Green, Blue (RGB)

            

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Division of Electrical and Computer Engineering, Hanyang University

Hue varies along the circumference Saturation varies along the radial distance

tyChromatici }

Color representationColor representation

            

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Eg. Color representationEg. Color representation

            

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Color absorption spectraColor absorption spectra

max

min

1,2,3 , )()()(

idCSC ii

response spectral:)(Ci

3,2,1,S spectra absorptionwith

retina in the cones of typesdifferent threeare There

i i

bygiven sensation,color a

produce willC light, colored Thus,

            

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Color Vision ModelColor Vision Model

133

122

11

33

22

11

loglog

loglog

log

ysensitivit spectral :,

eed

eed

ed

λSdλSλCe

dλSλCe

dλSλCe

i

luminance

color a ofty chromaticiDetails to be covered later