CSE399b Computer Vision

Spring 2006

Jianbo Shi

Color

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Light: consisting of EM waves

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Reflectance and Light– Materials may reflect different wavelengths of light differently

Color, the physics– As previously noted different wavelengths/frequencies of EM radiation

correspond to different colors

– Different light sources have different spectral characteristics

Why is the sky blue, and red looks hot

1) air: intensity of radiation scatters by fourth power of the frequency

2) 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

3) We refer to the color temperature of a source by comparing its spectral character to the character of a block body radiator

Grassmans Law

– Trichromacy in human perception• Any colored light can be matched by a weighted

combination of three primary light sources

Additive and Subtractive Color

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/(X+Y+Z), Y/(X+Y+Z)

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CIE color space

• CIE XYZ

• CIE LAB

(x,y) = (X/(X+Y+Z), Y/(X+Y+Z)

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

Vincent van Gogh Painting Sunflowers, By Paul Gauguin, 1888, Rijksmuseum, Amsterdam

• 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

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 = R+G+B

RG= R-G

Y = R +G; BY = B-(R+G)

Perception of color and lightness

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