Mumbai University > Computer Engineering > Sem 4> Computer Graphics

Marks: 5 Marks

Year: Dec 2016

Aliasing:

Aliasing is a phenomenon that occurs when sampling a continuous function with insufficient resolution.

In computer graphics, the process by which smooth curves and other lines become jagged because the resolution of the graphics device or file is not high enough to represent a smooth curve. Smoothing and antialiasing techniques can reduce the effect of aliasing.

Anti-aliasing Techniques:

• Prefiltering.

  Prefiltering methods treat a pixel as an area, and compute pixel color based on the overlap of the scene's objects with a pixel's area. These techniques compute the shades of gray based on how much of a pixel's area is covered by a object. For example, a modification to Bresenham's algorithm was developed by Pitteway and Watkinson.

  In this algorithm, each pixel is given an intensity depending on the area of overlap of the pixel and the line. So, due to the blurring effect along the line edges, the effect of antialiasing is not very prominent, although it still exists. Prefiltering thus amounts to sampling the shape of the object very densely within a pixel region. For shapes other than polygons, this can be very computationally intensive.

• Postfiltering.

  Supersampling or postfiltering is the process by which aliasing effects in graphics are reduced by increasing the frequency of the sampling grid and then averaging the results down. This process means calculating a virtual image at a higher spatial resolution than the frame store resolution and then averaging down to the final resolution. It is called postfiltering as the filtering is carried out after sampling.

  Supersampling is basically a three stage process.

  1. A continuous image I(x,y) is sampled at n times the final resolution. The image is calculated at n times the frame resolution. This is a virtual image.

  2. The virtual image is then lowpass filtered

  3. The filtered image is then resampled at the final frame resolution.