CS 490: Computer Vision
Color Theory:
Fundamentals
Fall 2015
Dr. Michael J. Reale
Shades of Gray

Up to this point, we’ve concentrated on
grayscale images (even binary images)…
http://1.bp.blogspot.com/-MhAlx4s-OYM/T6Cm1hirfKI/AAAAAAAAChM/kUKseHNjOQY/s1600/normal_the_wizard_of_oz-071.jpg
In Living Color
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So now let’s talk about Color
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Electromagnetic Spectrum

White light  composed of continuous spectrum of color
◦ Sir Isaac Newton first noticed this in 1666 when sunlight passed through a prism

Colors  different wavelengths/frequencies on visible
electromagnetic spectrum
How We See Color

Cones in eyes  primarily responsible for color
perception
Cone Type

Wavelength
Sensitivity
Light Color
Sensitivity
Long cones
Long wavelength
Red light
Medium cones
Medium wavelength Green light
Short cones
Short wavelength
Blue light
Objects reflect or absorb certain wavelengths
◦ Object absorbs all wavelengths  no light reflected
Additive Primary Colors

Three additive primary colors:
◦ Red
◦ Green
◦ Blue

“Additive”  mixtures of light
◦ Examples:
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

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Add red and green light  get yellow light
Add red and blue light  get magenta light
Add green and blue light  get cyan light
Add all three lights  get white light
Display Devices

Display devices (like monitors and TVs) use additive
primary colors
◦ E.g., CRT monitor
 Three electron guns
 Three primary colors
from each phosphor triad
are “mixed” together
◦ E.g., LCDs
 Have three “subpixels”
(red, green, and blue)
CIE Color Standard

In 1931, the CIE (Commission Internationale de
l’Eclairage – the International Commision on
Illumination) designed the following specific wavelength
values to the three primary colors:
◦ Red = 700nm
◦ Green = 546.1 nm
◦ Blue = 435.8 nm

This was before experimental data in 1965 that showed
that the cones absorb:
◦ Red light  peak at 575 nm
◦ Green light  peak at 535 nm
◦ Blue light  peak at 445 nm
Secondary (Complementary) Colors

Three complementary colors:
◦ Cyan
◦ Magenta
◦ Yellow

Also called Subtractive primaries:
◦ Cyan  does not reflect red
◦ Magenta  does not reflect green
◦ Yellow  does not reflect blue

“Subtractive”  mixing pigments  less light reflected
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Add magenta and yellow pigments  reflects red light
Add cyan and yellow pigments  reflects green light
Add cyan and magenta pigments  reflects blue light
Add all three pigments  no light reflected  black (technically)
Color Printers

Color printers use secondary colors
◦ One exception: Cyan + Magenta + Yellow =
very muddy-looking black
◦ Ergo, usually use CMYK (K = black) in
printing
Definitions


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Chromatic  refers to color
Achromatic  void of color
Three basic quantities to describe chromatic light
source:
◦ Radiance – energy flowing from light source
 Usually measured in watts
◦ Luminance – energy that the observer perceives from light
source
 Usually measured in lumens
◦ Brightness – perceived intensity of the light; subjective;
embodies achromatic notion of intensity
 Next to impossible to measure
Characteristics of Color

Three characteristics of each color
◦ Brightness
 Achromatic notion of intensity
◦ Hue
 Dominant wavelength in a mixture of light waves
 Dominant color
◦ Saturation
 Relative purity of color
 Amount of white light mixed with a hue
 Pure spectrum color  fully saturated
 Mix (e.g., pink = red + white)  partially saturated
 White  no saturation
Characteristics of Color (cont.)

Hue and saturation  chromaticity

Color may be characterized by:
◦ Chromaticity
◦ Brightness
Characteristics of Color (cont.)
Hue
Intensity
Saturation