Color Standards
• A pixel color is represented as a point in 3-D
space. Axis may be labeled as independent
colors such as R, G, B or may use other
independent indicators such as Hue,
lightness, saturation.
• RBG, HSV, HSL are the most popular.
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RGB
• RGB (Red green blue)
– For CRT display (TV, computer monitor)
– Additive combination of r g and b
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HSV
• HSV (Hue Saturation Value)
Hue is effectively a measure of the wavelength of the
main color. It has a value between 0-255 (0-360o,
Red = 0o, Green=120o and Blue=240o.)
Hue can be calculated from RGB values as follows:
Rh = R - min (R, G, B)
Gh = G - min (R, G, B)
Bh = B - min (R, G, B)
At least on of these values is 0. Hue value is in
between.
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HSV
For example if Rh = 0
Hue angle = (240xBh + 120xGh ) / (Bh + Gh)
If two are zero, then hue is the angle corresponding to
the third non-zero color.
If three are zero, then there is no color hue. The
monitor will display a gray level (between black and
white).
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Saturation
• Saturation is the amount of pure hue in the final color
If Saturation = 0, final color is without hue (
white light only)
If Saturation = 255, no white light in final color
Saturation = (max(R,G,B) - min (R,G,B)) / max(R,G,B)
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Value
• Value (brightness) is a measure of the intensity of
the brightest component and given by
Value = max(R,G,B)
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The HSV Model
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HSV Example
Given R=100, G=200, B= 40, convert this RGB
color model to HSV.
Rh = 100 - min(100,200,40) = 60
Gh = 200 - min (100,200,40) = 160
Bh = 40 - min (100,200,40) = 0
H = (60*0 + 160*(120*256/360))/(60+160)
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HSV Example (Continued)
Saturation
= (max(R,G,B) - min (R,G,B)) / max(R,G,B)
= (max(100,200,40)-min(100,200,40))/
max(100,200,40)
= (200-40)/200
= 0.8 = 80%
Saturation = 80*256/100 = 204
Value = max(100,200,40) = 200
= 200*100/256 = 78%
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HLS
HLS (Hue Lightness Saturation)
• Similar to HSV except that the hue angle start at Blue = 0o, and
the model is double cone with a lightness axis going from L=0
(black) to L=1 (white)
• For HLS, hue is calculated the same way as for HSV model
except Blue = 0 and lightness and saturation are given by:
• Lightness = (max(R,G,B) - min(R,G,B))/2
(max+min)/(max-min)
if L <= 0.5
(max-min)/(2-max-min)
otherwise
• Saturation =
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HLS
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CMY & CMYK
• CMY (Cyan Magenta Yellow) เป็ นระบบสีที่พฒ
ั นาขึ ้นมาใช้
สาหรับการพิมพ์ภาพสีโดยมีสีหลักคือสี Cyan Magenta และ
Yellow ซึง่ เรี ยกว่า Subtractive primaries Color (สีแดง
เขียวและน ้าเงิน เรี ยกว่า Additive primaries Color) ระบบสี
แบบ CMY สามารถหาได้ โดยการนาเอาสีในระบบ RGB ลบกับสีขาว
ดังนี ้คือ
C  1 R
M 1  G
Y  1 B
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CMY & CMYK
• ระบบสี CMY จะนาไปใช้ สาหรับการพิมพ์ภาพสีแต่ยงั ไม่ดีเท่าที่ควร
เนื่องจากไม่ยงั ไม่สามารถสร้ างสีดาได้ อย่างถูกต้ อง ดังนันจึ
้ งมีการใช้ ระบบ
CMYK แทนโดย K แทนสีดา
K  min( C , M ,Y )
C CK
M M K
Y Y K
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The Human Vision
• Better than any camera yet developed
• Eye has 2 classes of discrete light receptors;
Cones and rods
• 6-7 million cones, sensitive to bright light
• >= 75 Million rods, sensitive to light intensity
but not color
• eye color perceptions
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Color Additive
• Problem with CRT
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