1. 1 Aliasing & Anti-Aliasing

2. 2 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. CS-321 Dr. Mark L. Hornick 3 What is Aliasing? (DSP Viewpoint) Low sampling frequency distorts signal

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

5. CS-321 Dr. Mark L. Hornick 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. CS-321 Dr. Mark L. Hornick 6 Supersampling

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

8. CS-321 Dr. Mark L. Hornick 8 Weighting Masks 111 111 111 121 242 121 UniformCenter weighted

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

10. CS-321 Dr. Mark L. Hornick 10 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?

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

12. CS-321 Dr. Mark L. Hornick 12 Another example Line drawn in PowerPoint

13. CS-321 Dr. Mark L. Hornick 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 = (4*Red_fore + 3*Wht_back + 2*Yel_fill)/9 “Clear Type” in Windows (font smoothing)