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#87162 - 11/18/02 05:01 AM that seems to be rotating backwards effect
belligerentyouth Offline
old hand

Registered: 12/20/00
Posts: 1055
Loc: Berlin
This one may go down in the books of the bleeding obvious, but here goes ...
What is the name for visual effect that occurs when some circular marked object (e.g. a car wheel) spins quickly to make it appear as if the wheel (or the markings on it) is/are rotating backwards but far more slowly? And in what relation is the slower backwards motion to the 'true' forward motion, btw.

#87163 - 11/18/02 05:09 AM Re: that seems to be rotating backwards effect
Wordwind Offline
Carpal Tunnel

Registered: 09/30/01
Posts: 6296
Loc: Piedmont Region of Virginia, U...
I don't know the term and the mathematical relationship, but that slower movement backwards can be mermerizing. I'll look forward to finding out the answers.

#87164 - 11/18/02 05:49 AM Re: that seems to be rotating backwards effect
Faldage Offline
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Registered: 12/01/00
Posts: 13803
It's an example of sampling error. You'll see it typically in movies where you have a sampling rate of 24 samples per second. The rotational speed at which the wheel appears to be going backwards is going to depend on that sampling rate and on the number of things on the wheel (typically spokes on the wheel of a wagon or stage coach) that give you the detail to see the apparent rotation. If the wheel has six spokes and goes around almost one time or almost 5/6th or almost 1-1/6th of a full rotation or variations on this (e.g., almost 3-1/2 times) it will appear that it has gone backwards just that little bit less than whatever multiple of 1/6th of a rotation it did in reality. Watch the wheels as they are picking up speed. The sampling rate in TV is thirty samples per second.

Confused yet?

#87165 - 11/18/02 06:11 AM retrograde?
AnnaStrophic Offline
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Registered: 03/15/00
Posts: 6511
Loc: lower upstate New York
... or does that only apply to planets?

#87166 - 11/18/02 06:21 AM Re: retrograde?
Buffalo Shrdlu Offline
Carpal Tunnel

Registered: 06/24/02
Posts: 7210
Loc: Vermont
what a great question!
doesn't this happen in real life, too, though? (with respect to Fald's reply) seems to me I've seen it while traveling. do our eyes "see" at a certain rate?

as far as a term goes: if there isn't one already, let's make one. how about:

... or does that only apply to planets?

don't forget music.

formerly known as etaoin...

#87167 - 11/18/02 06:34 AM Re: retrograde?
Faldage Offline
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Registered: 12/01/00
Posts: 13803
doesn't this happen in real life, too, though?

You'll see this sort of thing if there is something causing a strobe effect. If you wave your hands, fingers splayed, in front of a TV screen you see something like it or if your main light source is a fluorescent light, which strobes at 60 times per second you'll see this sort of thing with rotating objects. Another source of this sort of strobing can be from something like fence pickets moving between you and the wheel.

Our eyes see pretty much continuously but there is a persistence of vision that lets us smooth out the strobing of movie and TV images and fluorescent lights. It becomes obvious only when we see the apparent retrograde motion of things like rotating wheels.

#87168 - 11/18/02 07:44 AM Re: that seems to be rotating backwards effect
TEd Remington Offline
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Registered: 07/17/00
Posts: 3467
Loc: Marion NC

Since there's a word for everything else, it stands to reason there's a word for the optical illusion you described. Later today I am going to call the U of Colorado psych department and ask them if they have a name for it.

Someone below said the human eye sees more or less continuously. Somewhere deep in the recesses of my mind is a memory of a professor's telling us in class that the eye does actually sample a certain number of times a second. This was in conjunction with an explanation of how motion pictures look as though there is true "fluid" motion.

I do know that when I am driving or riding in a car I can sometimes get a stop motion of the wheel on a car beside me if I glance over at it quickly. Never a good enough picture to tell how many spokes, but enough information to be able to say that the wheel is spoked as opposed to a disc. I've always thought that I had to have taken that little snapshot in something around a 20th of a second to be able to get that much detail.

If I find out anything I'll post back.



#87169 - 11/18/02 08:10 PM rotating backwards
wofahulicodoc Offline
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Registered: 08/06/01
Posts: 6943
Loc: Worcester, MA
I think it's one manifestation of what is nowadays technically called "aliasing," pronounced as in "forming an alias."

It happens when two wave phenomena are slightly out of phase (or, more likely, some integral multiple of one is slightly out of phase with the other), giving the result that one moves slightly ahead of or behind the other. That gives the illusion that a third object is present, with different properties/frequencies.

Any kind of wave will do: light (as in this thread), sound (think of tuning a stringed instrument using "beats"), radio waves. (I suspect anything based on interferometry works on this principle, but now I'm getting a little off base.)

Aliasing in medicine is seen in ultrasound imaging and must be tuned out, or at least compensated for.

#87170 - 11/18/02 08:22 PM Re: stroboscope
wwh Offline
Carpal Tunnel

Registered: 01/18/01
Posts: 13858
Back about 1934 or 35, at my older brother's graduation from MIT, Vannevar Bush had
a display of the stroboscope, which could produce extremely frequent very powerful
flashes, the ratie of which could be changed to show and airplane propeller apparently
slowing down and stopping, so that any cracks started to develop in propeller could
be seen. Having seen this, when I see in movies on TV the wheels appearing to be
going backwards when the car is going forward, I think of it as a stroboscopic effect
I don't know what an engineer would call it.

Here is a URL that seems to agree with me:

#87171 - 11/18/02 08:31 PM Re: stroboscope
Buffalo Shrdlu Offline
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Registered: 06/24/02
Posts: 7210
Loc: Vermont
good thought, Bill! makes me think of Harold Edgerton and his strobe photos. cool stuff.

used to use a strobotuner(every band room used to have one) to tune the old trombone...

formerly known as etaoin...

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