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Aliasing & Anti-Aliasing

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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

CS-321Dr. Mark L. Hornick

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What is Aliasing? (DSP Viewpoint)

Low sampling frequency distorts signal

CS-321Dr. Mark L. Hornick

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Aliasing Line Examples

Non-horizontal lines distorted (primarily) Vertical lines ok?

(dimmer? Both CRT & LCD?)

Oblique lines “jagged”

How can we compensate?

CS-321Dr. Mark L. Hornick

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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?

CS-321Dr. Mark L. Hornick

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Supersampling

CS-321Dr. Mark L. Hornick

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A Better Algorithm

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

Intensity: ~3/9 ~8/9

CS-321Dr. Mark L. Hornick

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Weighting Masks

1 1 1

1 1 1

1 1 1

1 2 1

2 4 2

1 2 1

Uniform Center weighted

CS-321Dr. Mark L. Hornick

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ExampleComputer Graphics, Foley et al., Addison-Wesley, 1996, p. 134.

CS-321Dr. Mark L. Hornick

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Line Intensity Effects

Pixels per unit length Fewer for diagonal lines anti-aliasing

compensates

Display device effects CRT’s

Vertical lines appear dimmer than horizontal Why?

How about LCDs?

CS-321Dr. Mark L. Hornick

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Anti-alias example 1

Line drawn in PowerPoint

CS-321Dr. Mark L. Hornick

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Another example

Line drawn in PowerPoint

CS-321Dr. Mark L. Hornick

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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 = (4*Red_fore + 3*Wht_back + 2*Yel_fill)/9

“Clear Type” in Windows (font smoothing)