1. Introduction to Perception and Color
David S. Ebert
Computer Science & Electrical Engineering Department
University of Maryland Baltimore County

2. Perception Basics

3. Perception Basics Light enters eye and is focused on the retina
Retina contains photoreceptors
They send electrical impulses through the neurons to the brain
Rods -used under low illumination (night)
achromatic
very sensitive
higher concentration at periphery

4. Retinal Cones
3 Types:
L-receptors: sensitive to long wavelengths of light (red)
M-receptors: sensitive to medium wavelengths of light (green)
S-receptors: sensitive to short wavelengths of light (blue)

5. Receptive Fields Millions of receptors in the retina
Only ~1 million optic nerve fibers
Receptors are grouped into fields and the output of the field is sent to the brain
So vision reacts at the level of receptive areas (fields), not individual receptors

6. Mach Bands and Receptor Fields

7. Another Intensity Illusion

8. Cones & Rods vs. Luminance
Eye can detect light variations on a range of 1:100,000,000,000,000 cd/cm2
Rods most sensitive at low illumination levels
Become saturated between moonlight and low room light illumination
Cones start functioning in the upper range of rod sensitivity

9. Color Vision Model

10. Trichromatic Color Theory vs Opponent Color Theory
Three primary color receptors in the eye
Opponent Color Theory:
Certain phenomena can not be explained by trichromatic color
Why don’t we see reddish greens or yellowish blues ?

11. Opponent Color Theory (cont.)
Neural processing of trichromatic values yields 2 major opponent color classes
Spectrally opponent processes (channels)
Red/Green Channel: difference of the red and green cone signals
Yellow/Blue Channel: difference of the blue cones and the sum of red+green

12. Opponent Color Theory (cont.)
Luminance : Spectrally non-opponent process (black vs. white)
based on input from all cones
Greater sensitivity to luminance than chrominance
Greatest bandwidth for conveying information
Chrominance sensitivity decreases with low luminance

13. Color Spaces
RGB
YIQ
HSV
CIE Perceptually Linear Color Spaces

14. RGB Color Space

15. YIQ – TV broadcasting Recording of RGB for
transmission efficiency and
downward compatibility with B&W TV.
Recoded signal is transmitted using NTSC standard
Y = Luminance
(only one shown in B&W TV)
I = Chromaticity

16. HSV Color Space User Oriented H = hue S = saturation V = Value
measured by angle around verticle axis with red at 8º, green at 120º, etc. . .
S = saturation
ranging from 0 to 1
V = Value

17. Perceptually Linear Color Spaces
CIE created 2 perceptually linear (uniform) color spaces:
CIE lab and CIE L*u*v*
CIE L*u*v*
L = lightness
u* = red-green
v* =blue-yellow
Not commonly used- Why?
Ignorance
Complexity of conversion
Need cube root - expensive
Need tristimulus coordinates for monitor

18. CIE Chromaticity Diagram

19. What’s Wrong with RGB?
Interpolation in RGB will not give correct perceptual information
RGB linear color maps will give incorrect information
What about vector interpolation of RGB?
Is there something better and quick?
Approximations based on table look-ups

20. Possible Alternative for RGB Interpolation
Use Opponent Color Model
R-G
B-Y
Luminance
Use Hue, Saturation, and Value
Can you understandably display three variables using these techniques?

21. More Color Issues Viewing on a CRT: Non-linear device
Luminance = voltagegamma
Small range of luminance values compared to visual sensitivity

22. Other Visual Perception Factors
Depth
Curvature
Orientation
Transparency
Texture
Motion

23. Pre-attentive Processing
Hardwired processing without conscious processing
At-a-glance processing
Processed in parallel
Less than 200 milliseconds
Accurate
Independent of the display size

24. Which Attributes are Pre-attentive?
Chromaticity (2 dimensions)
Luminance (1)
Curvature (1?)
Orientation (1?)
Texture (3+)
Motion (4)
Opacity?

25. Pre-attentive Bandwidth
Very little discrimination pre- attentively
2 -3 bits per channel
Color: studies show seven color pre- attentively
Can we successfully combine several pre-attentive channels?
Sometimes
Interference can occur

26. Good References Pages Colin Ware: The Joy of Visual Perception:
The Joy of Visual Perception:
Chris Healey

27. Conclusion Illumination, and Shading Affect Your Resulting Images
Can improve perception of information or decrease it
Visual Perception must be taken into account in choosing colors, illumination, and shading techniques
Visual system is very sensitive to certain characteristics, not very sensitive to others
Take advantage of this!