What is aliasing? Explain the anti-aliasing techniques.
1 Answer

Aliasing :

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.

In the line drawing algorithms, we have seen that all rasterized locations do not match with the true line and we have to select the optimum raster locations to represent a straight line. This problem is severe in low resolution screens. In such screens line appears like a stair-step, as shown in the figure below. This effect is known as aliasing. It is dominant for lines having gentle and sharp slopes.

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The aliasing effect can be reduced by adjusting intensities of the pixels along the line. The process of adjusting intensities of the pixels along the line to minimize the effect of aliasing is called antialiasing.

The aliasing effect can be minimized by increasing resolution of the raster display. By increasing resolution and making it twice the original one, the line passes through twice as many column of pixels and therefore has twice as many jags, but each jag is half as large in x and in y direction.

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As shown in the figure above, line looks better in twice resolution, but this improvement comes at the price of quadrupling the cost of memory, bandwidth of memory and scan-conversion time. Thus increasing resolution is an expensive method for reducing aliasing effect.

With raster system that are capable of displaying more than two intensity levels (colour and gray scale), we can apply antialiasing methods to modify pixel intensities. By appropriately varying the intensities of pixels along the line or object boundaries, we can smooth the edges to lessen the stair-step or the jagged appearance.

Antialiasing methods are basically classified as :-

Supersampling or 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 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.

Area sampling or Prefiltering:-

In this antialiasing method pixel intensity is determined by calculating the areas of overlap of each pixel with the objects to be displayed. Antialiasing by computing area is referred to as Area sampling or Prefiltering. A modification to Bresenham's algorithm was developed by Pitteway and Watkinson. In this algorithm, each pixel is given 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 anti-aliasing is not very prominent, although it still exists. For sampling shapes other than polygons, this can be very computationally intensive.

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