1. Half Toning Dithering RGB CMYK Models
Colors in Graphics
Half Toning
Dithering
RGB
CMYK Models

2. Halftoning
A technique used in newspaper printing Only two intensities are possible, blob of ink and no blob of ink But, the size of the blob can be varied Also, the dither patterns of small dots can be used

3. Halftoning

4. Halftoning – dot size

5. Halftoning – Moire Patterns
Repeated use of same dot pattern for particular shade results in repeated pattern
Perceived as a moire pattern
Instead, randomize halftone pattern

6. Dithering
Halftoning for color images

7. Specifying Color Color perception usually involves three quantities:
Hue: Distinguishes between colors like red, green, blue, etc
Saturation: How far the color is from a gray of equal intensity
Lightness: The perceived intensity of a reflecting object
Sometimes lightness is called brightness if the object is emitting light instead of reflecting it.
In order to use color precisely in computer graphics, we need to be able to specify and measure colors.

8. Combining Colors Additive (RGB) Subtractive (CMYK)
Shining colored lights on a white ball
Subtractive (CMYK)
Mixing paint colors and illuminating with white light

9. HSV Color Space
Computer scientists frequently use an intuitive color space that corresponds to tint, shade, and tone:
Hue - The color we see (red, green, purple)
Saturation - How far is the color from gray (pink is less saturated than red, sky blue is less saturated than royal blue)
Brightness (Luminance) - How bright is the color (how bright are the lights illuminating the object?)

10. HSV Color Model Hue (H) is the angle around the vertical axis
Saturation (S) is a value from 0 to 1 indicating how far from the vertical axis the color lies
Value (V) is the height of the hexcone”

11. HSV Color Model H S V Color 0 1.0 1.0 Red 120 1.0 1.0 Green
Blue
* White
* Gray
* * 0.0 Black ? ? ?
Figure 15.16&15.17 from H&B

12. HSV Color Space A more intuitive color space H = Hue S = Saturation
V = Value (or brightness)
Saturation
Value
Hue

13. Electromagnetic Spectrum
Visible light frequencies range between ...
Red = 4.3 x 1014 hertz (700nm)
Violet = 7.5 x 1014 hertz (400nm)
Figures 15.1 from H&B

14. Visible Light
Hue = dominant frequency (highest peak) Saturation = excitation purity (ratio of highest to rest) Lightness = luminance (area under curve)
White Light
Orange Light
Figures from H&B

15. How well do we see color? What color do we see the best?
Yellow-green at 550 nm
What color do we see the worst?
Blue at 440 nm
Flashback: Colortables (colormaps) for color storage
Which RGB value gets the most bits?
Can perceive color differences of 10 nm at extremes (violet and red) and 2 nm between blue and yellow
Metamers – different energy radiations look like the same color
Color perception also affected by surrounding light and adaptation

16. RGB Color Space (Color Cube)
Define colors with (r, g, b) amounts of red, green, and blue

17. Converting Color Spaces
Converting between color models can also be expressed as such a matrix transform: Note the relative unimportance of blue in computing the Y

18. Perceptually Uniform Color Space
Color space in which Euclidean distance between two colors in space is proportional to the perceived distance
CIE, RGB, not perceptually uniform
Example with RGB
LUV was created to be perceptually uniform

19. The CMY Color Model
Cyan, magenta, and yellow are the complements of red, green, and blue
We can use them as filters to subtract from white
The space is the same as RGB except the origin is white instead of black
This is useful for hardcopy devices like laser printers
If you put cyan ink on the page, no red light is reflected
Add black as option (CMYK) to match equal parts CMY