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Abstract

RGB Color Model CMY Color Model YIQ Color Model HSV Color Model
Computer Graphics
Color Theory
Topics
RGB Color Model
CMY Color Model
YIQ Color Model
HSV Color Model
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Colors
Visible band
Each frequency corresponds to a distinct color
Low-frequency end (4.3 x 1014 Hz): Red
High-frequency end (7.5 x 1014 Hz): Violet
Wavelength = v/f, where v=300,000km/sec
Low frequency High frequency
red orange yellow green blue violet
Long wavelength Short wavelength
700nm 400nm
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RGB Color Model
Used in light emitting devices
Color CRT monitors
Additive
Result = individual contributions of each primary color
added together
C= rR + gG + bB, where r, g, b[0, 1]
R= (1, 0, 0)
G= (0, 1, 0)
B= (0, 0, 1)
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RGB Color Model
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RGB Color Model
Color Cube
R+ G= (1, 0, 0) + (0, 1, 0) = (1, 1, 0) = Y
R+ B= (1, 0, 0) + (0, 0, 1) = (1, 0, 1) = M
B+ G= (0, 0, 1) + (0, 1, 0) = (0, 1, 1) = C
R+ G+ B= (1, 1, 1) = W
1 W= (0, 0, 0) = BLK
Grays = (x, x, x), where x(0, 1)
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Color Cube
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CMY Color Model
CMY: Complements of RGB
Used in light absorbing devices
Hardcopy output devices
Subtractive
Color specified by what is subtracted from white light
Cyan absorbs red, magenta absorbs green, and yellow
absorbs blue
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CMY Color Model
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CMY Color Model
W= (0, 0, 0) B= (1, 1, 1)
Conversion from RGB to CMY
Conversion from CMY to RGB
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B
G
R
Y
M
C
1
Y
M
C
B
G
R
1
CMYK Color Model
Motivations
Do we get black if paint cyan, magenta and yellow on a white paper?
Which cartridge is more expensive?
CMYK model
K= greatest gray that can be extracted
Given C, M, and Y
K= min(C, M, Y)
C= CK
M= MK
Y= YK
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Try some examples…
YIQ Color Model
Used in U.S. commercial color-TV broadcasting
Recoding of RGB for transmission efficiency
Backward compatible with black-and-white TV
Transmitted using NTSC (National Television System Committee) standard
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YIQ Color Model
YIQ
Y: luminance
I, Q: chromaticity
Only Yshown in black-and-white TV
RGB YIQ
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B
G
R
Q
I
Y
311052802120321027505960114058702990
...
...
...
YIQ Color Model
Human’s visual properties
More sensitive to changes in luminance than in hue or saturation
more bits should be used to represent Ythan I and Q
Limited color sensation to objects covering extremely small part of our
field of view
One, rather than two color dimensions would be adequate
Ior Qcan have a lower bandwidth than the others
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YIQ Color Model
NTSC encoding of YIQ into broadcast signal
Uses human’s visual system properties to maximize
information transmitted in a fixed bandwidth
Y: 4MHz
I: 1.5MHz
Q: 0.6MHz
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Intuitive Color Concepts
Terminology
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Perceptual Term Colorimetry Comments
hue dominated
wavelength
to distinguish
colors
saturation excitation
purity
e.g., red and
pink
Lightness
(reflecting objects)
luminance
Brightness (self-
luminous objects)
luminance e.g., Sun, CRT
Intuitive Color Concepts
Tint: white pigment added to pure pigment
saturation reduced
Shade: black pigment added to pure pigment
lightness reduced
Tone: consequence of adding both white and black pigments to pure pigments
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tints
shades
pure color
white
black
grays
tones
Intuitive Color Concepts
Tints, shades, and tones different colors of same hue
are produced
Grays
= black pigments + white pigments
Graphics packages that provide color palettes to users
often employ two or more color models
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HSV Color Model
HSV = Hue, Saturation, and Value
A.k.a. HSB, where B is Brightness
RGB, CMY, and YIQ: hardware-oriented
HSV and HLS: user-oriented
Cylinder coordinate system
Space: hexcone
hexagon is obtained from the color cube in isometric projection
(h, s, v), where h [0, 360) and s, v[0, 1]
hue: angle round the hexagon
saturation: distance from the center
value: axis through the center
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HSV Color Model
Color Cube Hexcone
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HSV Color Model
W= (-, 0, 1)
B= (-, 0, 0)
R= (0, 1, 1)
Y= (60, 1, 1)
:
M= (300, 1, 1)
Adding white pigments S
Adding black pigments V
Creating tones Sand V
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HSV Color Model
True color system: 16 million colors
Q: Do we need that many?
Human eyes can distinguish
128 hues
130 tints (saturation levels)
23 shades of yellow colors, 16 of blue colors
128 x 130 x 23 = 82720 colors
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HLS Color Model
HLS: Hue, Lightness, and Saturation
Cylinder coordinate system
Space: double cone
base is from the hexagon as in HSV
(h, l, s), where h [0, 360) and s, v[0, 1]
hue: angle round the base
lightness: axis through the center
saturation: distance from the center
W= (-, 0, 1)
B= (-, 0, 0)
R= (0, 0.5, 1), Y= (60, 0.5, 1), …
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