Outline

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
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.

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

4
Light Spectra
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

6
Sunlight
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

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

9
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

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 Ma 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 cant be matched like
  this instead, must write Ma A b Bc 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
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

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 Cant get enough colors from Newton
  actually, color is obtained by mixing lights or
  pigments.

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

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

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

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
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

18
Color Receptors
19
Color Opponent Cells

• Color opponent cells in LGN
• Responses in the LGN of moneys are roughly
  conformed to the pattern predicted by Herings
  opponent process theory
• They are also found in the bipolar and ganglion
  cells in the retina

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

21
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.

22
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 -gt
  some colors can be matched only subtractively.
• CIE XYZ
• Color matching functions are positive
  everywhere, but primaries are imaginary

23
CIE x, y
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 colorscolors that can be mixed
  with C to get white

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.

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

27
Color books
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

29
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

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

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