Computer Vision –
Fundamentals of Human
Vision
Hanyang University
Jong-Il Park
Introduction
 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
Brightness (Perceived Luminance)
 Light ~ radiant energy which, by its action on the
organs of visions, enables them to perform
their function of sight


Spectral energy distribution of the light source
L( λ ), λ = 350nm ~ 780nm
Light received from an object
I ( λ )  ρ( λ )L(λ )
ρ( λ ) : reflectivity or transmissivity of the object
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Human 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 Photoreceptors
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The Human Visual System
공막
홍채
(눈알의)
맥락막
각막
(안구의)
수양액
Shape of lens, rather than the distance between lens and screen, is changed
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The Human Visual System (cont.)
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Eye Physiology
 Luminance (or Intensity)
: independent of luminance of the surrounding object
Relative luminous efficiency function
f ( x, y)   I ( x, y,  )V ( )d
Light distribution

Luminosity Function (Relative Luminous Efficiency
Function)
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Brightness
 Contrast
 Brightness : dependent upon the surroundings
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Brightness adaptation
Dynamic range ~ 1010
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Intensity Discrimination Experiment
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Weber’s Law
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(100,101)
(100,102)
(100,105)
(150,151)
(150,153)
(150,162)
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I
 C  0.02weber fraction 
I
I
For small value of
: small changein intensityis discriminable
I
 good brightnessdiscrimination
rod
cone
typicalweber ratioas a fuctionof intensity
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Brightness(cont.)
 Mach Bands
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MTF of the Visual System
 Measurement of visual system in frequency domain
MTF: Modulation Transfer function
    

  
H (1 ,  2 )  H  (  )  A    exp   
   0  
  0 

  12   22 cycles/degree
 Useful Values for ImageCoding Applications 
A  2.6,   0.0192,   1.1
 0  (0.114) 1  8.772, .
peak frequency: 8 cycles/degree
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Isopreference
k: # of bits/pel  gray-level resolution
N: # of pels  spatial resolution
Preference depends on image!
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Optical illusions
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Moire pattern
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Image Fidelity Criteria
 Goal :
 Image quality measurement
 Performance evaluation of image processing
techniques or systems
 Quantitative Criteria
1
2
 Mean square criterion :  ms 
MN

SNR(signal-to-noise ratio) :

PSNR(peak-to-peak SNR) :
M
N
 u(m, n)  u(m, n)
2
m 1 n 1
2
SNR  10log10 2
 ms
PSNR  10log10
(peak - to - peak value) 2
2
 ms
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Image Fidelity Criteria (cont.)
 Subjective Criteria
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Perception of Intermittent Light
Consider a light that flashes on for a brief duration N times/sec

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.
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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 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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Colorimetry
 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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Color representation
 Hue varies along the circumference
 Saturation varies along the radial distance
} Chromaticity
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Eg. Color representation
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Color absorption spectra
T hereare threedifferenttypesof conesin theretina
with absorptionspectraSi  , i  1,2,3
i (C) : spectral response
T hus,coloredlight,C  will produce
a colorsensation,given by
i (C)  
max
min
Si ( )C( )d ,
i  1,2,3
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Color Vision Model
e1   C  λ S1dλ
e2   C  λ S 2 dλ
e3   C  λ S3 dλ, Si  λ  : spect ralsensit ivity
d1  loge1 
d 2  loge2   loge1 
d 3  loge3   loge1 
 luminance
 chromaticity of a color
Details to be
covered later
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