On 10/22/07, Martin Peach martin.peach@sympatico.ca wrote:
Mathieu Bouchard wrote:
A very simple way to explain aliased frequencies would be: spin a bicycle wheel. When you accelerate it beyond a certain point, it will begin to look like it's going backwards instead. This is because the wheel speed, together with the repetitiveness of the wheel's appearance, have crossed the Nyquist frequency of your eye.
That won't work in sunlight for example.
Haven't you ever seen it? (in sunlight that is)
You need a flashing light source like a fluorescent tube. It's the Nyquist frequency of the light source that causes the aliasing, not of your eyes, unless you try it while blinking rapidly. The wagon wheels in western movies appear to turn backwards when the spokes are moving faster than the frame rate of the movie.
Martin
Vision doesn't work exactly like a camera. There's no sampling that occurs--the visual signals are more/less continuous except for momentary "refresh" periods brought about by eye motion and nystagmus. I thought about it today, and I'm not entirely sure why it happens (perhaps there is a certain maximum speed which can be perceived--this is equivalent to saying there is a finite bandwidth of motion perception). I don't really know. I need to defer to *real* experts, here. but I agree, the description is more precise when talking about a camera effect...
Chuck