Outline
• Color perception
– Introduction
– Theories of color perception
• Yong-Helmhotz trichromatic theory
• Hering opponent process theory
• Dual process theory
– Physiological mechanisms for color perception
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.
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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
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Light Spectra
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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
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Sunlight
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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
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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
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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
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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.
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• 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
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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
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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.
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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
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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
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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
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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
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Opponent-Process
stage
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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
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Color Receptors
“Red” cone
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“Green” cone
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“Blue” cone
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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
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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
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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.
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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
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CIE x, y
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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
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• 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
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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.
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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
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Color books
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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
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Constancy
• We observe the color of • Lightness constancy
the light reflected from
– how light is the surface,
independent of the
surfaces
brightness of the
• But we want surface
illuminant
colour
– problem is known as
colour constancy
– issues
• spatial variation in
illumination
• absolute standard
– Human lightness
constancy is very good
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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
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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
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