Computer Graphics
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Achromatic and Colored Light
11.1 Achromatic Light
11.1.1 Gamma Correction
 Fact 1: Our eyes are sensitive to ratios of intensity rather
than to absolute values of intensity.
 Lightbulb: 1020; 100110; 100200
 Exponential increase in intensity results in linear
increase of the sensation of intensity.
 Fact 2: Most software computes intensity linearly, e.g.
smooth shading linernly interpolates vertex reflection
intensity into pixel intensity values.
 Such images will appear very dark.
 Need to convert the intensity value using an
exponential factor:
I = K V
Where  is a hardware dependent constant.
 This way of increasing the intensity by the gama
exponent is called Gamma Correction.
 UA’s medallion is too dark due to the lack of
gamma correction. Microsoft photo editor supports
gamma correction (Image->Ballence->Gamma).
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Computer Graphics
11.1.2 Halfton Approximation: Dithering
 Combine pixels to represent more intensities.
 Trade off between spatial resolution and
intensity/color resolution
11.2 Chromatic Color Models
Three channels of intensity.
 RGB Model:
For color monitors.
Additive colors, adding to black.
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Computer Graphics
 CMY Model:
For printers.
Subtractive, subtract from white.
1
0

 
1,
0
,
CMY: White  
Black  
1
0
 R  1  C 
 C 1  R 
     
    
G   1   M 
M  1   G 
     
 B  1  Y 
 Y  1  B 
and
 CMYK Model:
For four-color printing process.
Black: K=min(C, M, Y);
C=C-K; M=M-K; Y=Y-K;
 YIQ Model:
For television. Compatible with B/W TV
Y: Luminance; I and Q: Chromaticity;
Y  0.299 0.587
 R
0114
.
  
 
I

0
.
596

0
.
275

0
.
321
  
 G 
Q 0.212  0.523 0.311   B 
 HSV Model:
For user interaction: hue, saturation, value
(tint, color, brightness)
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Computer Graphics
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Rainbow Colors:
S=1; V=1; H=[0,360].
Convertion Procedures:
RGB_To_HSV
HSV_To_RGB
 CIE Model:
 Standard
 Primaries: X, Y, Z
 Can represent more colors
than other models
 Complicated to display
X 
R
 
 

Y
M
 
G 
 Z 
 B 
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