Visual Effect - Wheels rotating backward at speed...

Of course, happens on all wheels with evenly spaced spokes when sampled into video frames (a good example happens with rotating wagon wheels in an old TV western) or when illuminated by a lamp running on 50Hz/60Hz power -- see "aliasing" at wikipedia.

 

Happens with all wheels; known as a kinephantom.

 

It's similar to the old western movies with the stagecoach wheels appearing to turn backwards. In that case, it had to do with the film frames per second synchronizing with the spinning spokes in just such a way that they appeared to be going in reverse. As a kid (yes, I'm that old), I recall looking out the window of an airliner as the props spun up to speed after the engines were started and seeing them appear to spin in reverse for a brief moment. I think it's a similar effect to the frames per second of the film. The brain retains images from the eye for a fraction of a second, and takes a moment (when the RPM are just right) to synch to the fast spinning prop (or spokes), and for that moment synchs up in such a way that it sees the spokes briefly going in reverse. I'm sure that someone can explain this better than I, but I think I have the gist of it. 5-spoke wheels probably have just the right "frequency" to generate the illusion.

 

Wikipedia.... doesn't have an article... on Kinephantom...

End of the internet?

 

huh. i was pretty sure (actually, quite positive) it had nothing to do with the sampling rate of the video, but rather sampling rate of our eyes. can you speak to that? you can see it in real life also, you know.

 

Yeah, I've seen it in real life and in movies and such.

 

I thought MySpace was the end of the Internet - but I guess I was wrong about that, too. Dictionary.com doesn't have anything on kinephantom, but a quick Google search - which isn't much help. But the search brings up something called the Pulfrich effect - which seems like it could mean the same thing. This, of course, can be found on Wikipedia - but the entry of Pulfrich effect makes no mention of wheels.

 

LOL!

 

Like others have said the effect is similar to that which is seen on film. It comes from sampling or measuring the position of the wheel periodically.

An example can be made with a "one spoke" wheel. Or a rotating circle with a mark at one point on the edge. If it rotated at say 10 hz (10 times per second) and observed it at 10 hz it wouldn't look to be rotating at all. Each time we looked at the wheel the mark would be in the same place. If the wheel is slowed slightly, than each time we look at the wheel, the mark will have made almost 1 rotation around. Thus the wheel will look to be rotating very slowly backwards. As it slows more, the wheel will look to be speeding up in the backwards direction.

If the wheel is a 5 spoke wheel, then we would see the wheel "stop" each time that its frequency is at a multiple of our observing frequency divided by 5 (10/5 = 2). So at 10, 12, 14... 20 hz etc it would look like the wheel is stopped.

The effect is easily calculated when the observing frequency is known: for instance with film, 24 hz, or HDTV at 60 hz. For the human eye the frequency can vary. From person to person and I've even read that different areas of your eye can see at different frequencies. One figure I have seen is 20 hz for the human eye.

Since 20/5 is 4, the wheel would appear to be stationary at 12, 16, 20, 24, 28 hz... So a five spoke wheel would look to be moving backwards if rotating between 10 and 12 hz, which is between 46.4 and 55.6 mph. There are of course other speeds where this would also occur (27 - 38 mph for example).

On HDTV at 60 hz it would occur at different ranges. As the car accelerated up to ~27 mph the wheel would look to be turning forward then it would start to rotate backwards until the car hit 54 mph.

edit- forgot to mention that this leads to the Nyquist frequency. Basically you have to sample twice as fast as the thing being measured cycles in order to truly Know how fast it is cycling. For a 5 spoke wheel this is complicated by the fact that the wheel looks the same if it travels just 1/5 of a rotation. You therefore need to sample 10 times as fast as the wheel is spinning to see its true frequency.

 

Here's the same illusion with helicopter rotors. Pretty cool.

http://www.youtube.com/watch?v=jQDjJRYmeWg

 

You can't see it in real life unless the rotating object is lit by an artificial light which is responding to the 60 cycle ac power. For example a car wheel under the street lights at night. Your eye doesn't have a "sampling rate" that would cause that effect.

 

then why does film (at 24 fps) not look like stop motion? why does the wheel look like a blur in real life and not "discrete" (sorry, don't know how to put it in words, i think you know what i mean)?

i'd swear i've seen this effect in daylight; i could be mistaken.

 

Get a strobe light and point it at a spinning ceiling fan, adjust strobe rate for different fan movement [speed/direction]

 

That would work too. Someone has too much time on their hands.

 

Vision is a function of the brain, not the eyes. The input signal is compared to past experience. So film looks like continuous motion. Because the brain sees what it expects to see.

If you see wheels going backwards by sunlight, maybe you've been watching too much TV.

 

The wheels don't even need to be actually spinning!
Image Unavailable, Please Login

 

OUCH! (this may be the one time when a thread is NOT improved by pictures..........;--)


Tritone

 

whoa

 

I can lie in bed and if I blink quick enough I can make the ceiling fan go backwards. Though everyone would love to know that.

 

Is this before or after you dropped the tab of acid?

 

I seem to recall something from Psych102 called The Phi Effect[or Phenomenon]. Had to do with a visual illusion or distortion...my brain was having waves back then,too.
Didn't relate to Figi's[PhiGams] even tho those boys were a mess...
cheers,
RE

 

I've always explained this phenomena with a clock analogy. If you took a picture of a clock's second hand every 60 seconds it would appear to be stopped in place. If you took the picture every 59 seconds it would appear to be running backwards, if you took the picture every 61 seconds it would appear to be running forwards.

And to reinforce how much sampling rate you need to discover the reality of the seconds hand, if you took the picture every 30 seconds you could tell that the hand was indeed moving, but you couldn't tell which direction.

With film/video the sampling rate is fixed and the objects change frequency, but the effect is exactly the same.

 

Must be a child of the 60's.