Why do rims in motion spin backwards when seen on tv?

[rh71]

 

[ndee]

because monitors flicker sometimes on TV!

 

[Ikonomi]

Because all cars you see on TV are equipped with Sprewells.

 

[WinkOsmosis]

Spinners.

 

[Heisenberg]

Is this a serious question?

 

[rh71]

yes, it is. I've been wondering about this for the longest time.

 

[neutralizer]

Hmm... I've wondered about that too.

 

[Eli]

It doesen't have to be on TV.... you see the same thing with anything spinning that has lines of contrast..

I do not know the explination, though.

 

[Ikonomi]

Actually, the wheels appear to spin backwards because they are making rotations at a rate that's different from the framerate of the camera that filmed it. Assume that the commercial was filmed at 24 frames per second. If the wheels on the car were spinning 24 times each second, they would appear to stand still, as every time the camera records a frame, the wheels would be in the same position they were in the last frame.

Make sense?

It's the same reason monitors flicker, but in the case of monitors, it's because the refresh rate is different from the frame rate, so you catch the monitor screen between refreshes.

Sorry I don't know the numbers for the backward wheel motion, but it's all about what position the wheel is in when the frame is recorded.

 

[Heisenberg]

It's because of the frame rate of the film. As the wheels start they'll look normal, then as they speed up you can see them stop as they "catch up" to the frame rate, and then start to spin backwards.

 

[ndee]

guys, your sarcasm-o-meters are broken.

 

[rh71]

Let's think about this... it's true it doesn't have to be on tv only... I've seen where fans spin the "correct" way at first but as it goes even faster, it appears to spin the other way with the naked eye. Do we see stuff at fps ? Something about us not being able to discern faster than 24fps (on a tv) anyway ?

 

[WinkOsmosis]

Originally posted by: rh71
Let's think about this... it's true it doesn't have to be on tv only... I've seen where fans spin the "correct" way at first but as it goes even faster, it appears to spin the other way with the naked eye. Do we see stuff at fps ? Something about us not being able to discern faster than 24fps (on a tv) anyway ?

That's because of flourescent bulbs. They flicker at 60hz. Some of the bulbs they use for street lights seem to give the same effect. I think it's mercury discharge or something.

 

[bradruth]

Originally posted by: rh71
Let's think about this... it's true it doesn't have to be on tv only... I've seen where fans spin the "correct" way at first but as it goes even faster, it appears to spin the other way with the naked eye. Do we see stuff at fps ? Something about us not being able to discern faster than 24fps (on a tv) anyway ?

I think the human eye can distinguish up to ~60fps.

 

[beer]

The technical explanation also ties into the reason why CDs are sampled at 44.1 KHz. Whenver a signal is sampled in discete time (aka video reccording equipment which samples at 30 fps, or a CD at 44.1 KHz), it must be sampled at strictly greater than twice the frequency (CDs go up to 20 KHz, for instance) or aliasing occurs. The motion of the wheel can be represented as a unit circle with one revolution = 2 pi radians. You do the math from this point, it's pretty trivial.

http://www.vishay.com/brands/measurements_group/guide/ta/dsp/dspa.htm <-- first google result

 

[BDawg]

Guys, this phenomenon doesn't just occur on TV. Sometimes when you pass a car on the highway, depending on thier wheel type, it looks like the wheel is spinning backwards.

It seems to happen whenever something spinning hits a certain rotation per second.

 

[Ikonomi]

Originally posted by: BDawg
Guys, this phenomenon doesn't just occur on TV. Sometimes when you pass a car on the highway, depending on thier wheel type, it looks like the wheel is spinning backwards.

It seems to happen whenever something spinning hits a certain rotation per second.

I've honestly never seen this phenomenon in real life, but Wink's fluorescent lights make sense. Like they create a 60 Hz strobe effect. But... no, never seen it on the highway.

Beer's post/link are informative.

 

[LordMorpheus]

its a framerate thing. The video is taken at such a framerate that in one frame, the spokes are in a certain position, and at the next frame, the next spoke advances to just behind where the spoke in front of it was in the first frame, thus creating the ILLUSION of MOTION (get the reference?) of the wheel going backwards. Sometimes you see the car going just the right speed so the wheel looks like its stopped. Thats pretty cool.

 

[BillGates]

Originally posted by: BDawg
Guys, this phenomenon doesn't just occur on TV. Sometimes when you pass a car on the highway, depending on thier wheel type, it looks like the wheel is spinning backwards.

It seems to happen whenever something spinning hits a certain rotation per second.

I agree - I used to see it at a certain speed on my mountain bike tires all the time.

 

[sharkeeper]

Stroboscopic Effect.

Cheers!

 

[beer]

BDawg, that is again because your eyes don't truly see everything in continuous-time, your eye can only see so many things per second so it is a discrete-time model.

What I said is the final explanation as to why this happens, and anyone four weeks into DSP 101 can tell you this.

 

[PoPPeR]

yeah i've seen it happen in real life before. Don't know if anyone noticed, but doesn't this happen in video games too? Of course that would mean they did it intentionally, meaning that it does happen in real life. iirc, if you look at the side of a car during a game like Project Gotham Racing or something, the wheels start by spinning forward, but because the cars are accelerating so fast they quickly slow down then start going backwards.

 

[gururu]

I've seen the same thing in real life. Nothing to do with framerate. If that were the case, then what explanation is there for why the wheels don't seem to go faster forward as opposed to backward?
The answer is simply that it is an optical illusion. Its like spinning a pinwheel, and seing the lines move inward. Or a kaleidoscope. Our optical senses are not good at contrasting moving horizontal and vertical lines. Its information overload. Our mind then tells us that it is doing something that it truly is not. AN optical illusion.

 

[beer]

Originally posted by: gururu
I've seen the same thing in real life. Nothing to do with framerate. If that were the case, then what explanation is there for why the wheels don't seem to go faster forward as opposed to backward?
The answer is simply that it is an optical illusion. Its like spinning a pinwheel, and seing the lines move inward. Or a kaleidoscope. Our optical senses are not good at contrasting moving horizontal and vertical lines. Its information overload. Our mind then tells us that it is doing something that it truly is not. AN optical illusion.

No, sorry, wrong again. While it is an illusion, yes, it has everything to do with 'framerate' of what your eye can see. The human eye cannot detect every bit of motion it sees, and therefore, actually is a discrete system. If something is moving faster than your eye can 'sample' it at, when your brain puts it back together it spins backward because of the frequency issue addressed in my first post in this thread.

 

[ohtwell]

I have no idea! I think it's so people like you will have something to ask on AT!


: ) Amanda