This presentation is based on Color fundamentals and Color models.
~ Introduction to Colors
~ Color in Image Processing
~ Color Fundamentals
~ Color Models
~ RGB Model
~ CMY Model
~ CMYK Model
~ HSI Model
~ HSI and RGB
~ RGB To HSI
~ HSI To RGB
2. INTRODUCTION
Color is the aspect of things that is caused by differing qualities of light
being reflected or emitted by them.
The characteristics used to distinguish one color from another are:
Hue – refers to primary and secondary colors (Yellow, Red, Blue, and Green)
Brightness – refers to amount of intensity
Saturation – the degree of purity of a hue.
3. COLOR IN IMAGE PROCESSING
Motivation to use colors in image processing:
Humans can perceive thousands of shades of color as opposed to only
about two dozen shades of gray
Color is a powerful descriptor that greatly simplifies object
segmentation and identification
4. COLOR IN IMAGE PROCESSING
Color image processing is divided into two major areas
Full-color processing – images are acquired and processed in full color
Pseudo-color processing – images are by nature grayscale and are
converted to color images for visualization purposes
5. COLOR FUNDAMENTALS
In 1666, Isaac Newton discovered
that when a beam of sunlight passes
through a glass prism, the emerging
beam of light is split into a spectrum
of colors ranging from violet at one
end to red at the other.
6. COLOR FUNDAMENTALS
Three basic quantities are used to describe the quality of a chromatic light
source:
Radiance is the total amount of energy that flows from the light source -
measured in watts (W).
Luminance gives a measure of the amount of energy an observer
perceives from a light source - measured in lumens (lm)
Brightness is a subjective descriptor that is practically unmeasurable
CONTD…
7. Hue and saturation taken together are called chromaticity
A color may be characterized by its brightness and chromaticity.
The amounts of red, green and blue needed to form any color are called
the tristimulus values and are denoted, X,Y and Z.
A color is then specified by its trichromatic coefficients which means
x + y + z = 1
COLOR FUNDAMENTALS
CONTD…
8. COLOR FUNDAMENTALS
Primary colors of light
red(R)
green(G)
blue(B)
Secondary colors of light
cyan(green + blue)
magenta(red + blue)
yellow(red + green)
CONTD…
Mixing the three primaries,
or a secondary with its
opposite primary color,
produces white light.
9. COLOR FUNDAMENTALS
CONTD…
Primary colors of pigment
cyan(C)
magenta(M)
yellow(Y)
Secondary colors of pigment
red(R)
green(G)
blue(B)
Mixing the three pigment
primaries, or a secondary
with its opposite primary
color, produces black.
10. COLOR MODELS
The color model aims to facilitate the specifications of colors in some
standard way.
In Digital Image Processing, commonly used models are:
RGB
CMY
CMYK
HSI
11. COLOR MODELS
RGB
Each color appears in its primary colors red,
green, and blue
Based on Cartesian coordinate system
All color values R, G and B have been
normalized in the range [0, 1]
We can represent each of R, G and B from 0
to 255
CONTD…
12. COLOR MODELS
RGB
Each RGB color image consists of three
component images, one for each primary
color
CONTD…
13. COLOR MODELS
RGB
The three images are combined to
produce color image
Number of bits used to represent each
pixel is referred to as pixel depth
24-bit image is often referred as full-
color image
CONTD…
14. COLOR MODELS
Applications of RGB
Used in the representation and
display of images in electronic
systems like computers and
televisions.
Used in conventional photography as
well.
Image scanner which scans images
and converts it to a digital image
mostly supports RGB color.
Used in web graphics.
CONTD…
15. CMY
This model is made of secondary colors
of light - Cyan, Magenta and Yellow
It is a subtractive model appropriate to
absorption of colors - white is at (0.0,
0.0, 0.0) and black is at (1.0, 1.0, 1.0)
Any CMY color when passed through
white light will not reflect the color from
which the combination is made.
COLOR MODELS
CONTD…
17. Relation between CMY and RGB
CMY coordinates are the complements of RGB values.
The conversion formula is
COLOR MODELS
CONTD…
18. Applications of CMY
It is used in color printing as it uses
colored inks.
It is used in most commercial printing like
magazines, books, etc.
COLOR MODELS
CONTD…
19. CMYK
CMYK color space is a variation on the CMY model
It adds black (Cyan, Magenta, Yellow, and blacK)
Closes the gap between theory and practice.
When equal components of cyan, magenta, and yellow inks are mixed, the
result is usually a dark brown, not black.
Adding black ink solves problem.
COLOR MODELS
CONTD…
CONTD…
21. HSI
Stands for Hue, Saturation, Intensity
Human perceptual description of colors
Decouples the intensity component from
the color carrying information (hue and
saturation)
Ideal tool for developing color image
processing algorithms
COLOR MODELS
CONTD…
22. COLOR MODELS
CONTD…
HUE SATURATION INTENSITY
Component that
describes a pure color
(pure yellow, orange or
red)
Component represents the
measure of the degree to
which color is mixed with
white color.
0 degree – Red
120 degree – Green
240 degree – Blue
60 degree – Yellow
300 degree – Magenta
Component refers
to grey level.
Range is [0, 1]
0 means white
1 means black
24. HSI AND RGB
Hue, Saturation and Intensity values can be
obtained from the RGB color cube.
Angle from the red(R) axis gives the Hue
Length of the vector is the saturation
Position of the plane on the vertical axis gives
intensity
25. CONVERSION FROM RGB TO HSI
We can convert any RGB point to a corresponding point in the HSI model
by the geometrical formulas:
The hue H is given by where
The saturation S is given by
The intensity I is given by
