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-detectionhardware”. 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.

The wagon wheel illusion that you see in old western movies: Even though the wagon is moving at a rapid rate, the wheel appears to move slowly, in the opposite direction, or not at all. This is because the frame rate of the film in the camera is too slow to get an accurate representation of the wheel's motion: If one spoke in the wheel points straight up every 1/24th of a second, then it will appear to the camera that the wheel is not in motion. If you have ever been to some sort of dance party that had a strobe light and an electric fan running, you've probably seen the same effect IRL.

Aliasing can also be a dangerous pitfall in data collection using digital elements. If my sampler takes 100 readings a second, then it will be able to accurately characterize a 5Hz sine wave, but a sine wave input of 105Hz will end up looking exactly the same to the instrument. I have been told that this sort of thing has crashed rockets in the past, when an unexpectedly high-frequency vibration has caused some feedback mechanism to erroneously compensate.

Moral of the story: Make sure that your bandwidth is plenty high for what you want to see, and stick a low-pass filter on the input to make sure that you don't see anything else. And keep Nyquist's Theorem in mind, too.

Aliasing is an effect that impedes data consistency in computer systems. It occurs when memory is addressed based on previous calculations and there's no check for changes afterwards so that following calculations might be done on outdated data.

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