Image Perception
Drawing hands by M.C Escher
Waterfall by M.C Escher
EE465: Introduction to Digital Image Processing
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Human Eye Structure
Three membranes enclose the eye:
Cornea and sclera, Choroid, Retina
ciliary
body
iris
diaphragm
Pupil size: 2-8mm
Eye color: melanin (pigment) in iris
http://en.wikipedia.org/wiki/Evolution_of_the_eye
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Retina
When the eye is properly focused, light
from an outside object is imaged on the
retina
Two classes of receptors are located over
the surface of retina: cones and rods
Cone: 6-7 million in each eye, central part of
retina (fovea) and highly sensitive to color
Rod: 75-150 million, all over the retina surface
and sensitive to low levels of illumination
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Rods and Cones in Retina
Q: Can you see a traffic light turn green while looking away from it?
EE465: Introduction to Digital Image Processing
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Image Formation in the Eye
Focal length: 14-17mm
Length of tree image2.55mm
For distant objects (>3m), lens exhibits the least refractive power (flattened)
For nearby objects (<1m), lens is most strongly refractive (curved)
Q: What if the image is focused in one eye but not the other (i.e., lazy eye)?
http://en.wikipedia.org/wiki/Amblyopia
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Lightness Perception: Objective
Quantities
 Luminance is the amount of visible light that
comes to the eye from a surface.
 Illuminance is the amount of light incident on a
surface.
 Reflectance (also called albedo) is the
proportion of incident light that is reflected from a
surface.
varies from 0% to 100% where 0% is ideal black and
100% is ideal white. In practice, typical black paint is
about 5% and typical white paint about 85%.
http://www-bcs.mit.edu/people/adelson
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Image Formation Model
f(x,y)=i(x,y)r(x,y)
0<f(x,y)<∞
luminance – proportional to energy
radiated by a physical source
0<i(x,y)<∞
Illumination/shading
0<r(x,y)<1
reflectance
(“intrinsic images”)
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Example
f(x,y)
r(x,y)
i(x,y)
Q: How to separate r(x,y) and i(x,y) from f(x,y)? (Google “intrinsic images”)
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Lightness Perception: Subjective
Quantities
Lightness is the perceived reflectance of a
surface. It represents the visual system's
attempt to extract reflectance based on the
luminance in the scene.
Brightness is the perceived intensity of
light coming from the image itself, rather
than any property of the portrayed scene.
Brightness is sometimes defined as
perceived luminance.
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Checker-block Illustration
Patches p and q have the
same reflectance,
but different luminances.
Patches q and r have different
reflectances and different
luminances; they share the
same illuminance.
90
0.1
0.9
0.9
0.1
10
Patches p and r happen to
have the same luminance,
because the lower
reflectance of p is
counterbalanced by its
higher illuminance.
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Photography Illustration
A
a
b
Q1: Do a and b have same
lightness (perceived
reflectance)?
Q2: Do and b have same
brightness (perceived
luminance)?
Q3: Do A and B have different
lightness (perceived
reflectance)?
Q4: Do A and B have different
brightness (perceived
luminance)?
B
Answers: YNYN
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Lightness Constancy Problem
Recall: Image Formation Model
f(x,y)=i(x,y)r(x,y)
p
q
r
9
1
9
90
10
10
0.1
0.1
0.9
If E(x,y) and R(x,y) are arbitrary functions, then for any E(x,y) there
exists an R(x,y) that produces the observed image.
The problem appears impossible, but humans do it pretty well.
How do we do it? (not completely known yet, only partial explanation)
“Illuminance and reflectance images are not arbitrary functions.
They are constrained by statistical properties of the world.”
- Land and McCann
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Importance of Visual Context
Importance of edges
Importance of corners
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Lightness Illusion
If we cover the right side of the figure and view the
left side, it appears that the stripes are due to paint
(reflectance). If we cover the left side and view the
right, it appears that the stripes are due to different
lighting on the stair steps (illumination).
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Another Lightness Illusion
You will verify that A and B have exactly the same value in CA3.
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Brightness Adaptation*
Human visual system cannot
operate over such a high
dynamic range simultaneously,
But accomplish such large
variation by changes
in its overall sensitivity, a
phenomenon called “brightness
adaptation”
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Mach Bands
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Brightness Discrimination*
Weber ratio=I/I
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Simultaneous Contrast
Same luminance but varying brightness (perceived luminance)
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Optical Illusions
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