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

2. EE465: Introduction to Digital Image Processing
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
EE465: Introduction to Digital Image Processing

3. EE465: Introduction to Digital Image Processing
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: million, all over the retina surface and sensitive to low levels of illumination
EE465: Introduction to Digital Image Processing

4. 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

5. EE465: Introduction to Digital Image Processing
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)?
EE465: Introduction to Digital Image Processing

6. 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%.
EE465: Introduction to Digital Image Processing

7. EE465: Introduction to Digital Image Processing
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”)
EE465: Introduction to Digital Image Processing

8. EE465: Introduction to Digital Image Processing
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”)
EE465: Introduction to Digital Image Processing

9. 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.
EE465: Introduction to Digital Image Processing

10. 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
Patches p and r happen to have the same luminance, because the lower reflectance of p is counterbalanced by its higher illuminance.
0.9 10
0.9
0.1
EE465: Introduction to Digital Image Processing

11. Photography Illustration
Q1: Do a and b have same lightness (perceived reflectance)?
Q2: Do and b have same brightness (perceived
luminance)? a A b
Q3: Do A and B have different lightness (perceived reflectance)?
Q4: Do A and B have different brightness (perceived
luminance)? B
Answers: YNYN
EE465: Introduction to Digital Image Processing

12. Lightness Constancy Problem
Recall: Image Formation Model
f(x,y)=i(x,y)r(x,y) p q r 9 1 90 10
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
EE465: Introduction to Digital Image Processing

13. Importance of Visual Context
Importance of edges
Importance of corners
EE465: Introduction to Digital Image Processing

14. EE465: Introduction to Digital Image Processing
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).
EE465: Introduction to Digital Image Processing

15. Another Lightness Illusion
You will verify that A and B have exactly the same value in CA3.
EE465: Introduction to Digital Image Processing

16. 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”
EE465: Introduction to Digital Image Processing

17. EE465: Introduction to Digital Image Processing
Mach Bands
EE465: Introduction to Digital Image Processing

18. Brightness Discrimination*
Weber ratio=I/I
EE465: Introduction to Digital Image Processing

19. Simultaneous Contrast
Same luminance but varying brightness (perceived luminance)
EE465: Introduction to Digital Image Processing

20. EE465: Introduction to Digital Image Processing
Optical Illusions
EE465: Introduction to Digital Image Processing