1. Outline Color perception Introduction Theories of color perception
Yong-Helmhotz trichromatic theory
Hering opponent process theory
Dual process theory
Physiological mechanisms for color perception

2. Visual Perception Modeling
Color Perception
The sensation of color is caused by the brain.
One way to get it is the response of the eye to the presence/absence of light at various wavelengths.
11/14/2018
Visual Perception Modeling

3. Physical description of light
Photon
A tiny packet of vibrating electro-magnetic energy
Characterized by the wavelength of its vibration
The photons we experience in visible light cover just a small portion of the electro-magnetic spectrum
11/14/2018
Visual Perception Modeling

4. Visual Perception Modeling
Light Spectra
11/14/2018
Visual Perception Modeling

5. Physical description of light
Photon
A tiny packet of vibrating electro-magnetic energy
Characterized by the wavelength of its vibration
The photons we experience in visible light cover just a small portion of the electro-magnetic spectrum
Physical description of light
Number of photons it contains at each wavelength
11/14/2018
Visual Perception Modeling

6. Visual Perception Modeling
Sunlight
11/14/2018
Visual Perception Modeling

7. Psychological description of color
Color space
The subjective experience of surface colors can be described in terms of three dimensions
Hue
Saturation
Lightness
Color space is the three-dimensional coordinate system in which each color experience is represented by a point
11/14/2018
Visual Perception Modeling

8. Psychophysical correspondence
Mapping between physical descriptions and psychological ones
Mean wavelength determines hue
Spectral area determines lightness
The total number of photons
Variance determines saturation
11/14/2018
Visual Perception Modeling

9. Visual Perception Modeling
Basic Phenomena
Light mixture
Only a small portion of the colors correspond to monochromatic lights
Two or more colors must be combined in order to non-spectral colors and de-saturated colors
By experience, it is possible to match almost all colors using only three primary sources
11/14/2018
Visual Perception Modeling

10. Additive and subtractive matching
Choose colors A, B, C such that no two can be mixed to match the third - Primaries.
Many colors can be represented as a mixture of A, B, C write M=a A + b B + c C
This is additive matching.
Gives a color description system - two people who agree on A, B, C need only supply (a, b, c) to describe a color.
Some colors can’t be matched like this: instead, must write M+a A = b B+c C
This is subtractive matching.
Interpret this as (-a, b, c)
Problem for building monitors: Choose R, G, B such that positive linear combinations match a large set of colors
11/14/2018
Visual Perception Modeling

11. Visual Perception Modeling
Some Phenomena
Color blindness
People who cannot discriminate among all the colors
There are several distinct varieties of color blindness
Color afterimages
Mach bands
Chromatic adaptation
11/14/2018
Visual Perception Modeling

12. Early theories and confusion
Widespread: We see things by rays fired out of the eyes, hitting surfaces; colors come from different kinds of rays.
Newton: Color is obtained by differential refraction of white light.
Artists: Can’t get enough colors from Newton; actually, color is obtained by mixing lights or pigments.
11/14/2018
Visual Perception Modeling

13. Young-Helmholtz Trichromatic Theory
Propose by Young-Helmholtz
There are three types of color receptors in the human eye
They were hypothesized to produce the psychologically primary color sensations of red, green, and blue
All other colors were explained as combinations of these primaries
11/14/2018
Visual Perception Modeling

14. Opponent Process Theory
Trichromatic theory cannot explain very well some facts and observations
Yellow is the additive mixture of red and green primaries but the subjective experience does not appear to be that way
Color experiences are always lost in certain pairs
11/14/2018
Visual Perception Modeling

15. Opponent Process Theory – cont.
Opponent process theory by Hering
There are four chromatic primaries rather than three
They are structured in pairs of polar opposites
Red/green
Blue/yellow
Black/write
11/14/2018
Visual Perception Modeling

16. Dual Process Theory
For many decades there were heated debates between two warring factions – Helmholtz vs. Hering
In 1957, Hurvich and Jameson proposed a dual processing theory
Trichromatic theory
Opponent-Process stage
11/14/2018
Visual Perception Modeling

17. Physiological Mechanisms
Three cone systems
There are three types of cones in the normal trichromat retina, each of which contains a different light-absorbing pigment
Absorption spectra of these three cone types are determined using different techniques
11/14/2018
Visual Perception Modeling

18. Visual Perception Modeling
Color Receptors
“Red” cone
“Green” cone
“Blue” cone
11/14/2018
Visual Perception Modeling

19. Visual Perception Modeling
Color Opponent Cells
Color opponent cells in LGN
Responses in the LGN of moneys are roughly conformed to the pattern predicted by Hering’s opponent process theory
They are also found in the bipolar and ganglion cells in the retina
11/14/2018
Visual Perception Modeling

20. Theory for Color Perception
Trichromatic theory is perhaps the only theory that explains many perceptual phenomena nicely and in a unified framework
People have been trying to find theories in the other areas of perception
Texture perception
11/14/2018
Visual Perception Modeling

21. Visual Perception Modeling
Representing Colors
Accurate color reproduction is commercially valuable - e.g. Kodak yellow, painting a house.
Of the order of 10 color names are widely recognized by English speakers - other languages have fewer/more, but not much more.
Color reproduction problems increased by prevalence of digital imaging – e.g. digital libraries of art.
Choosing pixel values to reproduce/evoke experiences, e.g. an architectural model.
Consistency in user interfaces, monitor-printer consistency, etc.
11/14/2018
Visual Perception Modeling

22. Visual Perception Modeling
Color spaces
Linear color spaces describe colors as linear combinations of primaries
Choice of primaries
choice of color matching functions
choice of color space
RGB
primaries are monochromatic, energies are 645.2nm, 526.3nm, 444.4nm
Color matching functions have negative parts -> some colors can be matched only subtractively.
CIE XYZ
Color matching functions are positive everywhere, but primaries are imaginary
11/14/2018
Visual Perception Modeling

23. Visual Perception Modeling
CIE x, y
11/14/2018
Visual Perception Modeling

24. Qualitative features of CIE x, y
Linearity implies that colors obtainable by mixing lights with colors A, B lie on line segment with endpoints at A and B
Monochromatic colors (spectral colors) run along the “Spectral Locus”
Dominant wavelength = Spectral color that can be mixed with white to match
Purity = (distance from C to spectral locus)/(distance from white to spectral locus)
Wavelength and purity can be used to specify color.
Complementary colors=colors that can be mixed with C to get white
11/14/2018
Visual Perception Modeling

25. More linear color spaces
Monitor RGB: primaries are monitor phosphor colors, primaries and color matching functions vary from monitor to monitor - careful!
YIQ: mainly used in television, Y is (approximately) intensity, I, Q are chromatic properties. Linear color space; hence there is a matrix M that transforms XYZ coords to YIQ coords. I and Q can be transmitted with low bandwidth.
11/14/2018
Visual Perception Modeling

26. Non-linear color spaces
HSV: Hue, Saturation, Value are non-linear functions of XYZ.
because hue relations are naturally expressed in a circle
Uniform: equal (small!) steps give the same perceived color changes.
Munsell: describes surfaces, rather than lights - less relevant for graphics. Surfaces must be viewed under fixed comparison light
11/14/2018
Visual Perception Modeling

27. Visual Perception Modeling
Color books
11/14/2018
Visual Perception Modeling

28. Device independent color imaging
Problem: ensure that colors on a display, printer, etc. give the same experience that a viewer would have seeing relevant light spectra
Difficulty: limited gamuts of most output devices
Strategy: exploit a model of human experience
11/14/2018
Visual Perception Modeling

29. Visual Perception Modeling
Constancy
We observe the color of the light reflected from surfaces
But we want surface colour
problem is known as colour constancy
Lightness constancy
how light is the surface, independent of the brightness of the illuminant
issues
spatial variation in illumination
absolute standard
Human lightness constancy is very good
11/14/2018
Visual Perception Modeling

30. Finding Skin Using Image Color
It is very useful to find human skin in images
Gesture-based user interfaces
Content-based retrieval
Ignore some pictures
A computational model of skin
11/14/2018
Visual Perception Modeling

31. Surface Color from Image Color
The color of light in an image is determined by two factors
Spectral reflectance of the surface
Spectral radiance of the light falling on that surface
Color of light falling surfaces can vary very widely
Image color can be a bad representation of the color of surfaces being viewed
Color constancy in human perception
11/14/2018
Visual Perception Modeling