Some Important Properties of Human Vision
Some Important Properties of Human Vision
CS 450: Introduction to Digital Signal and Image Processing
Bryan Morse
BYU Computer Science
Some Important Properties of Human Vision
Introduction
Human Vision
Some properties of human vision that affect image perception:
I
Linear and non-linear parts
I
Nonlinear (approx. logarithmic) encoding of input
I
Adaptation
I
Relative-contrast encoding
I
Varying sensitivity to spatial frequecies
I
Generally treats brightness and color separately
Some Important Properties of Human Vision
Psychophysics
How Do We Know?
Two main ways to measure perception:
I
Physiological
Measure neurological responses
(the neurons tell us what they see)
I
Psychophysical
Noninvasive perceptual experiments
(the person tells us what they see)
Some Important Properties of Human Vision
Psychophysics
Psychophysics: Discrimination Experiments
I
Many vision experiments involve comparisons
I
“Two alternative, forced choice” experiments:
I
I
is it there? (yes/no)
which is brighter, farther apart, etc.
(top/bottom, left/right, etc.)
I
Random guessing without bias: 50% correct
I
Pick some percentage above which the they must get it right:
often 75% (half the time they “see it”, half the time they guess).
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Vary the experimental parameter to determine the threshold T
above which the observer reaches this level of confidence.
I
This is called the just noticable difference (j.n.d.).
I
Sensitivity: 1/T
Some Important Properties of Human Vision
Brightness Perception
Weber’s Law
I
Many visual properties obey Weber’s Law
I
For intensity discrimination:
I
I
=c
for some constant c
I
In other words, the j.n.d.
the intensity itself
I for intensity is proportional to
I
Also applies to distance judgements, spatial frequency
discrimination, and many others
Some Important Properties of Human Vision
Brightness Perception
Weber’s Law and Logarithmic Encoding
I
Logarithmic encoding of brightness:
log(I +
I)
log I = log
⇣
I+ I
I
⌘
= log (1 + c)
= constant
I
Logarithmic encoding produces Weber’s Law
I
The human intensity sensitivity function isn’t exactly
logarithmic, but it’s close enough to be a useful model
Some Important Properties of Human Vision
Brightness Perception
Adaptation
I
Our eyes have an incredible ability to adapt to lighting
conditions
I
Total j.n.d. steps for the eye is about 1000
I
Total j.n.d. steps for fixed adaptation is about 200
Some Important Properties of Human Vision
Brightness Perception
Contrast Encoding
I
The response of the eye to light isn’t absolute—it’s relative
to the surrounding intensities
I
Caused by neurological on-center/off-surround areas of
response (“receptive fields”)
I
This causes the Mach effect at strong intensity transitions
I
Even our color perception seems to be based on relative
differences (to a point)
Some Important Properties of Human Vision
Brightness Perception
The Mach Effect
Each of these regions has uniform intensity but varying perceived
brightness—rapid changes in brightness are accentuated.
Some Important Properties of Human Vision
Spatial Frequency Perception
Perception of Different Spatial Frequencies
Experimental Structure
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Stimulus: sinusoidal grating with frequency f and
amplitude A
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Vary A in a 2AFC and find the j.n.d. for each frequency f
I
Plot the sensitivity as a function of frequency f :
the contrast sensitivity function (CSF)
1
0.8
0.6
0.4
0.2
0
0
0.2
0.4
0.6
0.8
1
Some Important Properties of Human Vision
Spatial Frequency Perception
Contrast Sensitivity Function
The CSF is the transfer function for the human visual system.
Implications:
I The eye is less sensitive to extremely gradual changes
I The eye is fairly sensitive to more rapid changes
I The eye is decreasingly sensitive to yet higher spatial
frequencies
Some Important Properties of Human Vision
Color Perception
Color Perception
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Rods:
I
I
I
I
brightness only
peripheral vision
low lighting conditions (scotopic)
Cones:
I
I
I
color
central vision (fovea)
bright lighting conditions (photopic)
Some Important Properties of Human Vision
Color Perception
Brightness vs. Color
I
The human visual system seems to treat brightness and
color separately
I
Physically separate pathways in the visual cortex (brain)
I
Some crossover, but weak
I
Perception of shape and form seems to be based on
brightness, not color
I
Much more sensitive to changes in brightness than to
changes in color
Some Important Properties of Human Vision
Display Principles
Displays
When building visual displays you have to consider properties
of vision:
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Exponential encoding for perceptual linearization
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Be careful of Mach effects
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Consider adaptation
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Make it bright!
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Consider the human CSF
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Be careful with color
Copyright A.Kitaoka 2003
Is what you see really what you ve got?
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