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7.3 การประมวลผลภาพสี
Color Image Processing
• Color is a perceptual manifestation
of light which in turn is an
electromagnetic signal.
• Color is a sensation produced by the
human eye and nervous system.
Human Visual System
Image formation
•Cornea
•Lens
Exposure
Control
Detection
Processing
•Iris/pupil
•Photoreceptor
sensitivity
•Retina
•Rods
•Cones
•Brain
Human Eye
Ciliary Muscle
Sclera
Iris
Pupil
Ear side (Temporal)
Vitreous Humor
Eyelens
Fovea
Retina
Optic Nerve
Cornea
Nose side (Nasal)
Aqueous Humor
Suspensory ligament

Choroid
Human eye is a complete imaging system.
Overwhelming experimental evidence
tells us that the perception of a color is
related to the strength of three signals
which are transmitted along the optic
nerve to the brain.
7.3.1 Color Models
Color Model
A "color model" is any method of
associating names or numbers with
colors.
The importance of a color model is
that it allows communication about
what color should be produced.
Examples include
• CMYK dot densities as reproduced
by an offset press,
• RGB voltages sent to the electron
guns of a cathode ray tube display,
• CIE LAB numbers as measured by
a "colorimeter", or
• Pantone color numbers as seen on
the "swatch books" published by that
company.
Color Model
• The international Commission on Illumination
(CIE, for Commison Internationale de
l’Eclairage) developed a device-independent color
model based on human perception.
• Color representation consists in a reduction of
the spectral color space to a finite-dimensional
space.
1) The RGB (CMYK) Color
Model
• RGB and its subset CMY form the
most basic and well-known color
model.
• This model bears closest
resemblance to how we perceive color.
• It also corresponds to the principles
of additive and subtractive colors.
RGB
Red, green, and blue are the primary stimuli for human color
perception and are the primary additive colors.
The relationship between the colors can be seen in this illustration:
The secondary colors of RGB, cyan,
magenta, and yellow, are formed by the
mixture of two of the primaries and the
exclusion of the third.
Red and green combine to make yellow,
green and blue make cyan, blue and red
make magenta.
RGB(CYMK) Model
2) The HSV Color Model
The HSV (Hue, Saturation, and
Value) color model is more intuitive
than the RGB color model.
The user specifies a color (hue)
and then adds white or black.
There are 3 color parameters:
Hue, Saturation, and Value.
Changing the saturation
parameter corresponds to adding or
subtracting white and changing the
value parameter corresponds to
adding or subtracting black.
The HSB/HLS Color Model
In HSB, these are hue, saturation,
and brightness;
in HLS, they are defined by hue,
lightness, and saturation.
Hue defines the color itself, for
example, red in distinction to blue or yellow.
The values for the hue axis run from 0–
360° beginning and ending with red and
running through green, blue and all
intermediary colors like greenish-blue,
orange, purple, etc.
Saturation indicates the degree to
which the hue differs from a neutral
gray. The values run from 0%, which is
no color saturation, to 100%, which is
the fullest saturation of a given hue at a
given percentage of illumination.
Lightness indicates the level of
illumination. The values run as
percentages; 0% appears black (no
light) while 100% is full illumination,
which washes out the color (it appears
white). In this respect, the lightness axis
is similar to Munsell's value axis. Colors
at percentages less than 50% appear
darker while colors at greater than 50%
appear lighter.
The Munsell Color System
Munsell modeled his system as an orb
around whose equator runs a band of colors.
The axis of the orb is a scale of neutral gray
values with white as the north pole and black as
the south pole.
Extending horizontally from the axis at
each gray value is a gradation of color
progressing from neutral gray to full
saturation.
With these three defining aspects, any
of thousands of colors could be fully
described.
Munsell named these aspects, or
qualities, Hue, Value, and Chroma.
Hue
Munsell defined hue as "the quality by
which we distinguish one color from
another."
He selected five principle colors: red,
yellow, green, blue, and purple; and five
intermediate colors: yellow-red, greenyellow, blue-green, purple-blue, and redpurple; and he arranged these in a wheel
measured off in 100 compass points:
Value
Value was defined by Munsell defined value
as "the quality by which we distinguish a
light color from a dark one."
Value is a neutral axis that refers to the grey
level of the color.
This ranges from white to black.
Chroma
Chroma is the quality that distinguishes
the difference from a pure hue to a gray
shade.
The chroma axis extends from the value
axis at a right angle and the amount of
chroma is noted after the value designation.
7.3.2 Color descriptors
1. Color histograms
สามารถนิยามได้ในรู ปแบบของ tuple
<C, M, {  Ci}, {H(  Ci)}>
โดย C คือ colar space,
Ci เป็ น color quantization ของเซลล์ที่ I,
M เป็ นจานวนเซลล์ และ H(Ci) = Ni/N โดย Ni เป็ นจานวน
จุดภาพทั้งหมดในภาพนั้น
2. Color feature descriptor
3. Color names
4. Color-induced sensations