Aliasing AntiAliasing 1 Aliasing AntiAliasing l What is

Aliasing & Anti-Aliasing 1

Aliasing & Anti-Aliasing l What is aliasing? l l l An artifact (loss of detail and false details) Caused by discretization (finite resolution) A rasterized line is an Alias of the ideal line 2

What is Aliasing? (DSP Viewpoint) Low sampling frequency distorts signal CS-321 Dr. Mark L. Hornick 3

Aliasing Line Examples l Non-horizontal lines distorted (primarily) l l l Vertical lines ok? (dimmer? Both CRT & LCD? ) Oblique lines “jagged” How can we compensate? CS-321 Dr. Mark L. Hornick 4

Aliasing in Graphic Displays l Sampling issue l l l Not really “sampling” a line But generating “misplaced” points What can we do? l l Increase screen resolution? Add extra information another way? l Manipulate pixel intensity? CS-321 Dr. Mark L. Hornick 5

Supersampling CS-321 Dr. Mark L. Hornick 6

A Better Algorithm l Assume that lines have normal thickness l Superimpose the “wide” line on the subpixels Intensity: ~3/9 ~8/9 CS-321 Dr. Mark L. Hornick 7

Weighting Masks 1 1 1 1 2 4 2 1 1 2 1 Uniform Center weighted CS-321 Dr. Mark L. Hornick 8

Example Computer Graphics, Foley et al. , Addison-Wesley, 1996, p. 134. CS-321 Dr. Mark L. Hornick 9

Line Intensity Effects l Pixels per unit length l l l Fewer for diagonal lines anti-aliasing compensates Display device effects l CRT’s l Vertical lines appear dimmer than horizontal § l CS-321 Dr. Mark L. Hornick Why? How about LCDs? 10

Anti-alias example 1 l Line drawn in Power. Point CS-321 Dr. Mark L. Hornick 11

Another example l Line drawn in Power. Point CS-321 Dr. Mark L. Hornick 12

Color Effects l Additional effects can be included l Color l l R, G, and B intensities Background and fill l l Factor in their % of coverage Ex. § § l 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) CS-321 Dr. Mark L. Hornick 13