Since the resolution of a screen is limited, we can see jaggies (or staircases) on a line drawn using the algorithms described before. This undesirable effect is due to the all-or-nothing approach to scan conversion. This phenomenon is called aliasing.
To reducing aliasing, there are some antialiasing methods.
1. Increasing Resolution
By increasing the resolution, the appearance of the lines will be improved but the jaggies effect would not be removed. Also, this will require more memory and increase scan conversion time.
A. Varying Intensity of Lines as a Function of Slope
Since the densities of the pixels along lines with different slopes are different, slope line intensities are adjusted according to horizontal and vertical lines. Horizontal and vertical lines with minimum intensity and 45 degree lines are given max intensity. The number of pixels turned on for a 45 degree line is the same as that for a horizontal but the length of this 45 degree line is 2 times longer, so its intensity has to be increased by 2 times.
A pixel is a spot covering a small area of the screen. Lines have a width equal to that of a pixel. We may think that any line as a rectangle covering a portion of a grid, which may represents a screen, as shown in the figure below.
Then only the squares (or pixels) covered by line will be turned on. To perform antialiasing, the intensity of each square depends on how much the square is covered by the line. If a square is totally inside the line, it will have 100% of intensity, otherwise, it will only have an intensity proportional to the area covered.
The problem of this method is that it will complicate the scan conversion algorithm a lot.
3. Pixel Phasing
Intensities of line edges are adjusted by micro-partitioning of the electron beam. Individual pixel can be shifted by a fraction of a pixel diameter.