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Aliasing and Antialiasing techniques CGVR
- 1. Aliasing and Anti Aliasing • Displayed primitives generated by the raster algorithms discussed in earlier Chapter have a jagged, or stairstep, appearance because the sampling process digitizes coordinate pints on an object to discrete integer pixel positions. • This distortion of information due to low-frequency sampling (undersampling) is called aliasing. • We can improve the appearance of displayed raster lines by applying antialiasing methods that compensate for the undersampling process. An example of the effects of undersampling is shown in Fig. • To avoid losing information from such periodic objects, we need to set the sampling frequency to at least twice that of the highest frequency occurring in the object, referred to as the Nyquist sampling frequency (or Nyquist sampling rate) fs:
- 2. • Fig :
- 3. • One way to increase sampling rate with raster systems is simply to display objects at higher resolution. • But even at the highest resolution possible with current technology, the jaggies will be apparent to some extent. There is a limit to how big we can make the frame buffer and still maintain the refresh rate at 30 to 60 frames per second. • And to represent objects accurately with continuous parameters, we need arbitrarily small sampling intervals. • Therefore, unless hardware technology is developed to handle arbitrarily large frame buffers, increased screen resolution is not a complete solution to the aliasing problem.
- 4. Anti Aliasing Techniques • Post – Filtering : This technique of sampling object characteristics at a high resolution and displaying the results at a lower resolution is called supersampling (or postfiltering, since the general method involves computing intensities ,it subpixel grid positions, then combining the results to obtain the pixel intensities). Displayed pixel positions are spots of light covering a finite area of the screen, and not infinitesimal mathematical points. Yet in the line and fill-area algorlrhms we have discussed, the intcnsity of each pixel is determined by the location of a single point on the object boundary. By supersanipling, we obtain intensity information from multiple points that contribute to the o-erall intensitv of a pixel.
- 5. • Pre- Filtering: Antialiasing by computing overlap areas is referred to as area sampling (or prefiltering, since the intensity of the pixel as a whole is determined without calculating subpixel intensities). Pixel overlap areas are obtained by determining where object boundaries intersect ind individual pixel boundaries.
- 6. • Pixel Phasing : Raster objects can also be antialiased by shifting; the display location of pixel areas. This technique , called pixel phasing, is applied by "micropositioning" the electron beam In relation to object geometry