CSE399b Computer Vision

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CSE399b Computer Vision
Color

• Spring 2006
• Jianbo Shi

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Light consisting of EM waves
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Reflectance and Light

• Materials may reflect different wavelengths of
  light differently

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Color, the physics

• As previously noted different wavelengths/frequenc
  ies of EM radiation correspond to different
  colors
• Different light sources have different spectral
  characteristics

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Why is the sky blue, and red looks hot

• air intensity of radiation scatters by fourth
  power of the frequency
• Long frequency(short wavelength) scatters more
  short ones,
• light of a long wavelength can travel very much
  further before being scattered than light of a
  short wavelength
• when the sun is high in the sky, blue light is
  scattered out of the ray from the sun to the
  earth
• We refer to the color temperature of a source by
  comparing its spectral character to the character
  of a block body radiator

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Grassmans Law

• Trichromacy in human perception
• Any colored light can be matched by a weighted
  combination of three primary light sources

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Additive and Subtractive Color
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Human Color Perception

• The color sensitive elements in the HVS come with
  three spectral responses which helps to explain
  trichromacy

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Light consisting of EM waves
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CIE color space

• CIE XYZ

(x,y) (X/(XYZ), Y/(XYZ)
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CIE color space

• CIE XYZ
• CIE LAB

(x,y) (X/(XYZ), Y/(XYZ)
L 116(Y/Yn)1/3 - 16 A 500(X/Xn)1/3 -
(Y/Yn)1/3 B 200(Y/Yn)1/3 - (Z/Zn)1/3
Difference in this space is more close to human
color space
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Vincent van Gogh Painting Sunflowers, By Paul
Gauguin, 1888, Rijksmuseum, Amsterdam
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• Color Contrast The visual system compares colors
• Contrast that results from spatial comparison is
  called simultaneous contrast, while contrast that
  results from temporal comparison is called
  successive contrast.

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How to measure the opponent color theory?

• We will start with low wavelength, and add color
  of yellow to cancel out the color of blue,
• The amount of yellow we add tells us how blue
  that wavelength is percieved
• At 500nm, the color appears to be Green. No
  addition is needed
• 600nm, we need to add blue to cancel out the
  color of yellow
• The ratio of blue and yellow we need to add tells
  us the tuning curve for the B-Y perceptor

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Color-opponent theory

• The black-white or achromatic channel sums
  signals from R, G and B cones.
• The red-green color-opponent channel is excited
  by signals from R cones and inhibited by signals
  from G cones.
• The blue-yellow color-opponent channel is excited
  by signals from B cones and is inhibited by a
  combination of signals from R and G cones.

A RGB
RG R-G
Y R G BY B-(RG)
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Perception of color and lightness