Aliasing

1
Aliasing Anti-Aliasing
2
Aliasing Anti-Aliasing

• What is aliasing?
• An artifact (loss of detail and false details)
• Caused by discretization (finite resolution)
• A rasterized line is an Alias of the ideal line

3
What is Aliasing? (DSP Viewpoint)
Low sampling frequency distorts signal
4
Aliasing Line Examples

• Non-horizontal lines distorted (primarily)
• Vertical lines ok? (dimmer? Both CRT LCD?)
• Oblique lines jagged
• How can we compensate?

5
Aliasing in Graphic Displays

• Sampling issue
• Not really sampling a line
• But generating misplaced points
• What can we do?
• Increase screen resolution?
• Add extra information another way?
• Manipulate pixel intensity?

6
Supersampling
7
A Better Algorithm

• Assume that lines have normal thickness
• Superimpose the wide line on the subpixels

• Intensity 3/9 8/9

8
Weighting Masks
Uniform
Center weighted
9
Example
Computer Graphics, Foley et al., Addison-Wesley,
1996, p. 134.
10
Line Intensity Effects

• Pixels per unit length
• Fewer for diagonal lines
• anti-aliasing compensates
• Display device effects
• CRTs
• Vertical lines appear dimmer than horizontal
• Why?
• How about LCDs?

11
Anti-alias example 1

• Line drawn in PowerPoint

12
Another example

• Line drawn in PowerPoint

13
Color Effects

• Additional effects can be included
• Color
• R,G, and B intensities
• Background and fill
• Factor in their of coverage
• Ex.
• 9-pixel grid (4 foreground/outline, 3 background,
  2 fill)
• C (4Red_fore 3Wht_back 2Yel_fill)/9
• Clear Type in Windows (font smoothing)