Spatial aliasing is the kind of aliasing that most Computer Graphics geeks are used to. It is the Stair-step effect you get when you try to draw a curved or diagonal line with pixels. Human beings have an especially hard time with aliased images because our brains are so good at picking them out. This is because our visual system includes a large amount of edge-detection “hardware”. In the old days (and even today) it was very useful to be able to pick out the shape of potential predators/prey in the distance. It is not as useful when you’re staring at a CRT for twelve hours a day.
To combat the visually jarring effect of aliasing, we exploit the human visual system in a different way. While we notice edges too well, we can handle quite a bit of noise. Trading noise for jaggedness is anti-aliasing.
Another lesser known type of aliasing is temporal aliasing. Did you ever wonder why it is that movies can be shown at ~24 frames per second without any noticeable problems, while computer games don’t look so hot at the same frame rate? This is especially true for computer animations that feature objects moving very quickly. Objects moving too quickly in a computer animation that is not temporally anti-aliased will appear very jumpy or as though they are under a strobe light. The reason this happens is that while movie cameras only take 24 shots per second, the camera lens stays open for 1/50 of a second. This makes objects that are moving rapidly have motion blur. You can see this effect by waving your hand in front of your face very rapidly. (If you can see this effect while slowly moving your hand, you may be encountering a different effect)
Computer animations don’t have a lens, so they don’t benefit from natural motion blurring. However, it is not too hard to add motion blur into an animation. The increase in realism is extraordinary. A good example of temporally anti-aliased computer animation is Star Trek II: The Wrath of Khan. The genesis effect scene is incredibly realistic looking because Lucas Digital Ltd. added in just the right amount of motion blur.