Color Fundamentals
Fasih ur Rehman
Color Fundamentals
White light consists of a continuous spectrum of colors ranging
from violet to red.
Color Spectrum
Band of visible light is relatively narrow in the band of frequencies
in the electromagnetic spectrum.
Perception
• The colors that humans perceive of an object
are determined by the nature of the light
reflected from the object.
– Green objects reflect light with wavelengths in the 500
to 570 nm, and absorb those at other wavelengths.
• The light is visible to human eyes if its
wavelength is between 380-780 (nm).
• If the light is achromatic, its only attribute is
intensity.
– The term gray level refers to a scalar ranging from
black to white.
Perception (Cont.)
Primary Colors
• The cone cells in human eye can be divided into
three categories, corresponding roughly to red,
green and blue (Figure 6.3).
• Due to these characteristics of the human eye,
colors are seen as variable combinations of the
primary colors red (700 nm), green (546.1 nm),
and blue (435.8 nm).
– Standardized in 1931.
– This standardization does not mean these three
primary colors can generate all spectrum colors.
Secondary Colors
• The primary colors can
be added to produce the
secondary colors of light:
Cyan, Magenta, Yellow.
• The primary colors of
pigments are cyan,
magenta, and yellow,
while the secondary
colors are red, green, and
blue.
More Fundamentals
• The characteristics generally used to distinguish
one color from another are hue, saturation, and
brightness.
– Hue: associated with color as perceived by an observer.
– Saturation: relative purity or the amount of white light
mixed with a hue.
– Brightness: intensity of light.
• Hue and saturation are taken together are called
chromaticity; therefore, a color can be
characterized by its chromaticity and brightness.
Color Diagram
Chromaticity Diagram
• Has superior performance over other color
transforms especially in clustering of color
distribution and estimate of color
difference
• Shows color as a function of x (red) and y
(green)
• Useful for color mixing
• Boundary of the diagram shows fully
saturated
• Equal energy point
Color Models
• The color model (color space or color system) is
to facilitate the specification of colors in some
standards.
• Color model is a specification of a coordinate
system and a subspace within the system where
a color is represented.
– RGB: color monitor
– CMY (cyan, magenta, yellow): color printing
– HSI (hue intensity and saturation): decouple the color
and gray-scale information.
RGB Color Model
• Images represented in the RGB color model consist of
three component images, one for each primary image
Color Planes
The RGB Color Model
• The number of bits used to represent each pixel in RGB
space is called the pixel depth.
• The term full-color image is used to denote a 24-bit RGB
color image.
The CMY Color Model
• Suppose colors in RGB are normalized in [0, 1].
The RGB to CMY conversion is given by
 C  1  R 
 M   1  G 
    
 Y  1  B 
• Instead of adding C,M, and Y to produce black,
a fourth color black is added, giving rise to the
CMYK color model.
The HSI Color Model
• Human describes color in terms of hue,
saturation and brightness.
– Hue: describe the pure color, pure yellow,
orange, green or red.
– Saturation measures the degree to which a
pure color is diluted by white light.
– Brightness is a subjective descriptor difficult to
be measured.
• Comparison:
– The RGB model is ideal for image color
generation.
– The HSI model is an ideal tool for developing
image processing algorithms based on color
descriptions
Conversion (RGB to HSI)
• Consider a color point in the RGB color
cube.
– Intensity: find the intersection on the intensity
axis with a perpendicular plane containing the
color point.
– Saturation: The distance of the color point to
the intensity axis.
• The saturation on the intensity axis is zero.
– Hue: consider the triangle enclosed by white,
black, cyan. The color on this triangle is a
mixture of these three colors
Conceptual Relationship RGB - HSI
• All pointes contained in the plane segment are defined by the
intensity and boundary of the cube have the same hue
Hue Measurement
The HSI Color Model
Conversions
• From RGB to HSI
 
H 
360  
if
BG
if
BG


1 / 2[( R  G)  ( R  B)]
  cos 
2
1/ 2 
[( R  G)  (( R  B)(G  B)] 
1
• S=1-[3/(R+G+B)][min(R, G, B)]
• I=(R+G+B)/3
Conversions (Cont.)
• RG sector (0<H<120)
B = I(1-S)

S cos H 
R  I 1 

o
cos(
60

H
)


G = 3I-(R+B)
Conversions (Cont.)
• GB sector (120≤H<240)
– First, let H = H -120
R=I(1-S)

S cos H 
G  I 1 

o
cos(
60

H
)


B=3I-(R+G)
Conversions (Cont.)
• BR sector (240 ≤ H ≤ 360)
– First, let H = H -240
G = I(1-S)

S cos H 
B  I 1 

o
 cos(60  H ) 
R = 3I-(G+B)
The HSI Color Model