EE465: Introduction to Digital Image Processing 1Waterfall by M.C Escher

Image Perception

Drawing hands by M.C Escher

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Human Eye Structure

Three membranes enclose the eye:Cornea and sclera, Choroid, Retina

ciliarybody

irisdiaphragm

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 rodsCone: 6-7 million in each eye, central part of

retina (fovea) and highly sensitive to colorRod: 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?

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Image Formation in the Eye

Focal length: 14-17mm Length of tree image2.55mm

For distant objects (>3m), lens exhibits the least refractive power (flattened)

For nearby objects (<1m), lens is most strongly refractive (curved)

http://en.wikipedia.org/wiki/Amblyopia

Q: What if the image is focused in one eye but not the other (i.e., lazy eye)?

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Lightness Perception: Objective QuantitiesLuminance 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)<∞

0<i(x,y)<∞

0<r(x,y)<1 reflectance

Illumination/shading

luminance – proportional to energyradiated by a physical source

(“intrinsic images”)

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Example

r(x,y) i(x,y)f(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.

Patches p and r happen to have the same luminance, because the lower reflectance of p is counterbalanced by its higher illuminance.

0.1

0.90.9

0.1

90

10

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Photography Illustration

A

B

a

b

Q1: Do a and b have same lightness (perceived reflectance)? Q2: Do and b have same brightness (perceivedluminance)?

Q3: Do A and B have different lightness (perceived reflectance)? Q4: Do A and B have different brightness (perceivedluminance)?

Answers: YNYN

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Lightness Constancy Problem

f(x,y)=i(x,y)r(x,y)Recall: Image Formation Model

“Illuminance and reflectance images are not arbitrary functions. They are constrained by statistical properties of the world.” - Land and McCann

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)

pqr

919

901010

0.10.10.9

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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 cannotoperate over such a highdynamic range simultaneously,But accomplish such largevariation by changesin its overall sensitivity, a phenomenon called “brightnessadaptation”

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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