03 Colors

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Color Perception 
and 
Representation
Lecture 3
1107190 - Introdução à Computação Gráfica – Turma 01
Prof. Christian Azambuja Pagot
CI / UFPB
Universidade Federal da Paraíba
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Colors
Image by Johan Feyaerts
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Electromagnetic Spectrum
Visible spectrum
Penubag (Wikipedia)
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Human Eye
● Rods
– Monochrome vision.
– Low-light intensity levels.
● Cones
– Color vision.
– Higher-light intensity levels.
– Normally, there are 3 types 
(whose peak responses at 
440nm (~blue), 545nm 
(~green) and 580nm (~red)..
Rhcastilhos (Wikipedia)
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First Experiments with Colors
Reference color Adjustable color
Primaries
(fixed chromacity and variable 
brightness)
User
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CIE RGB Color Space (1931)
Marco Polo (Wikipedia)
Wavelength
In
te
ns
ity
 le
ve
l
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CIE XYZ Color Space
Acdx (Wikipedia)
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CIE Chromaticity Diagram
Chromatiticy Diagrams (link)
http://graphics.stanford.edu/courses/cs178/applets/threedgamut.html
Stanford Computer Graphics Laboratory
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CIE xy Chromaticity Diagram
PAR (Wikipedia)
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Achromatic Light
● No sensations 
related to colors.
● Sensing of energy 
intensity.
● Visually represented 
through gray levels.
● Gray levels within 
the range [0,1].
Image by Christian A. Pagot
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Color Combination Models
● Subtractive (pigments).
● Additive (light).
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Subtractive Color Models
● RYB
– Primaries:
● Red.
● Yellow.
● Blue.
Sakurambo (Wikipedia)
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Subtractive Color Models
● CMYK
– Primaries:
● Cyan.
● Magenta.
● Yellow.
● Key (detalis, contrast).
– Most ink jet printers.
Hewlett-Packard
Mike Horvath (Wikipedia)
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● RGB
Additive Color Models
BenRG (Wikipedia)
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Additive Color Models
● RGB encoding in computers:
– The most common is to use 8 bits per component: 
R8G8B8.
– Other formats are also used (from OpenGL 
specification):
● R3G3B2.
● R4G4B4.
● R12G12B12.
● R16G16B16.
● Etc.
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Gamma Functions
● Gamma Correction
● Gamma Compression
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Gamma Correction
● Compensates for the non-linear relationship 
between the input signal voltage and the 
luminance at the screen.
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Gamma Correction
V
i
 = input voltage.
L = luminance at the screen.
L
Gamma for Video Display
(γ≈2.5 )L∝V i
γ
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Gamma Correction
V
i
 = input voltage.
L = luminance at the screen.
V
c
 = gamma corrected input voltage.
L
Gamma for Video Display
(γ≈2.5 )L∝V i
γ
(γ≈ 12.5=0.4)V c=V i1γ
L=V c
γ=V i
( 1γ )
γ
=V i
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Gamma Compression
● Human vision is more sensible to lower 
intensity values.
● Gamma compression compensates this by 
allocating more bits for the representation of 
lower intensity values.
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Gamma Compression
L = luminance.
E = eye response.E
L
Gamma for Video Display
E∝Lγ (γ≈ 12.2=0.45)
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Gamma Compression
Pc∝Pi
γ (γ≈ 12.2=0.45)
P
i
 = Original pixel value.
P
c
 = Gamma compressed pixel value.
Gamma for Video Display
Pi
Pc
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Gamma Compression
Pc
L
L∝Pc
γ (γ≈2.2 )
P
c
 = Gamma compressed pixel value.
L = Luminance.
Gamma for Video Display
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Other Color Spaces
● HSL (Hue, Saturation and Lightness)
– Good for color pickers.
– Can be converted to RGB.
SharkD (Wikipedia)
Christian A. Pagot
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