1. Introduction to Computer Graphics
EEL
Introduction to Computer Graphics
PPT12: Color models

2. Properties of light What is color?
In physical terms, color is electromagnetic radiation within a narrow frequency band
Other frequency bands include
Radio waves, microwaves, infrared waves, and x-rays
Spectral Color
Each frequency value within the visible region corresponds to a distinct spectral color
At the low end is red and at the high end is violet
For a spectral color, its wavelength and frequency are inversely proportional
c = λf
λ is the wavelength
f is the frequency
Color frequency band
Defined in terms of wavelength
780 nm to 380nm

3. Properties of light Light
Oscillating transverse electric and magnetic fields propagating through space
Frequency
The rate of the oscillation between the fields
Period
The time between any two consecutive positions on the wave that have the same amplitude
T = 1/f
Wavelength
The distance the wave travels between the beginning of two consecutive oscillations
c = λf
Color of objects
When light hits an object some of the frequencies are absorbed while others are reflected
The color of the object is the combination of the reflected frequencies
Color of light
The red end of the spectrum is described as having a dominant frequency
The dominant frequency of a light source is called its hue, a.k.a. the color of the light

4. Properties of light Energy distribution of white light
Each frequency component within the visible range contributes approximately equally
Energy distribution of dominant frequency
If one frequency contributes much more than other frequencies
The light will be the hue of that frequency

5. Color models Primary colors
The hues that are chosen as the sources for all other color mixing in the color model
Color gamut
The set of all colors that can be produced from the primary colors
Complementary colors
Two primary colors that produce white
Example: red and cyan
Color mixing and matching functions
Define the amount of each of the primary colors needed to produce any spectral color
Example graph below

6. Color mixing Generating tints, shades, and tones
Starting with a pure color
Mixing with black will produce different shades
Mixing with white will produce different tints
Mixing with both black and white will produce different tones
Artist’s color scheme
The set of colors that the artist uses to
create a piece of art

7. Color models

8. Chromaticity diagram
The image to the right is the chromaticity diagram for the normalized XYZ color space
The x and y coordinates are the x and y values from the equations on the previous slide
The output color is the color at that coordinate location
Points along the curve are the spectral colors
Except the line connecting red and violet
This is known as the purple line and it is not part of the spectrum

9. Chromaticity diagram Color gamut
All of the possible colors that can be mixed from some given colors
In the chromaticity diagram a color gamut
Between two colors is identified as the straight line between them
Between three or more colors is the polygon formed by the points
Complementary colors
C is the point of white light in the diagram
Complementary colors can be seen on the diagram
The line between the colors must pass through C
The respective distance each color is from C on the line
Represents the amount of each color that will be needed to mix white
Color gamut of 3 colors
AA and D are complementary

10. Chromaticity diagram
The dominant wavelength of a color can be determined by drawing the straight line through the sample point and C
The spectral color that the line hits is the dominant wavelength of the sample color
The purity is defined as the relative distance the sample point is from C compared to the distance from C to the dominant wavelength

11. RGB color model Tri-stimulus theory of vision
Our eyes perceive color through the stimulation of three visual pigments in the cones of the retina
One of the pigments is most sensitive to red light
Another is most sensitive to green light
And the last pigment is most sensitive to blue light
By comparing the intensities in a light source, we perceive the color of the light
This theory is the basis for the RGB color model
This is the model used to output color from computer monitors
This model can be represented by the unit cube on the right

12. RGB color model Color cube Used to represent the RGB color model
Examples below
The origin is black
White is opposite of black at coordinate (1,1,1)
Each axis represents one of the primary colors
RGB color gamut
It is clear from the diagram below that the RGB gamut cannot display all colors

13. YIQ color model The YIQ color model
NTSC color encoding for forming a composite video signal
Y component the same as for XYZ color model (luminance)
I and Q give the chromaticity information
I contains orange-cyan color information
Y contains green-magenta color information
Y is stored with higher precision than I and Q because we can detect slight changes in brightness more easily than slight changes in hue
Conversion between RGB and YIQ
Y = R G B
I = R – Y
Q = B – Y
From YIQ to RGB

14. CMY color model CMY color model Very similar to GRB RGB is additive
CMY is subtractive
C is cyan
M is magenta
Y is Yellow
Looking at the CMY color cube to the right
It is clear that it is similar to RGB
Except the origin is now white
(1, 1, 1) is now black
Because it is the subtraction
of all three colors
conversions

15. CMYK color model From CMYK to RGB
The CMY printing process uses four ink dots
Three are the CMY components
The last is a black component
K is the black parameter
The conversions between CMYK and RGB are very similar to those between CMY and RGB
From RGB to CMYK
Use RGB to CMY conversion
Set k = max(R, G, B) and then subtract k from each of C, M, and Y
From CMYK to RGB
Use CMY to RGB conversion
Set k = min(R, G, B)
Subtract k from R, G, and B

16. HSV color model
Sometimes easier to understand concept by using the concepts rather than a set of primary colors
HSV is centered on idea of taking a spectral color and then mixing it with black and white
H parameter represents the hue
S parameter represents the saturation
V parameter represents the value
The interaction between the three parameters can be seen in the image below
H is the rotation about the vertical axis
S is horizontal distance
V is the vertical distance

17. HSV color model Generating tones with hue, value and saturation
Start with a hue
This is the base color
The value of S is the purity of the color
A value of 1 is pure
A value of 0 is grayscale
The value of V is the amount of black added to the color
A value of 1 means no black added
A value of 0 means the color will be black

18. HLS color model
Similar to the HSV color model, HLS is based on intuitive color parameters
Used by the Tektronix Corporation
The color space of the model can be seen in the image below
Has a double-cone shape
H parameter is the hue
L parameter is the lightness
S parameter is the saturation
To get a color in the HLS color model
First, choose a hue
Then increasing L brightens
the color
Decrease L to darken the color
Decrease S to move toward
grayscale

19. References https://color.adobe.com/