Car Shows, and Wheel Spin

joshua1

I know this is probably a stupid question, but in car shows, such as top gear etc. when the cars travel at speed, the spokes of the wheel appear to spin in different directions at different speeds...... why is that? Its probably something very simple, but i've never understood it.

Fallingman

I always assumed that it was something to do with the frame-rate of the camera, and the speed of the spokes being such that each new frame, the next spoke looks like its behind where the previous one was, giving the impression that they changed direction.

Either that, or they're going SO fast they broke time.

joshua1

.........that makes so much sense, it could be right........ but I have noticed it with my own eyes too..... I know eyes can only take a certain fps, but I thought that was quite higher than camera fps... But you are smart man, Fallingman.

EclecticGroove

It's just an optical illusion. They aren't actually spinning backwards or anything (unless someone has a set of spinners designed to do that), but due to the way your eye sees the light bouncing off them, that's what it looks like to you.

Fallingman

Hmmm, what speed can your brain process images? Is there a "frame rate"? Quite an interesting thought...

joshua1

im actually pretty sure there is a frame rate for the brain. Just like dogs can't have any TV love, its frame rate being too fast/slow for them to perceive as a fluid image, the eye cannot see things going a certain speed. I have a strange urge to say 30 m per second, but I have no idea where the number was dredged from in my damp and twisted mind.

I know the reverse wheel spin is an optical illusion, but I would like to know what the optical illusion actually is, and fallingman seems to have it pretty well explained, for my miniscule brain.

PirateJon

Google is your friend.

In movies, a wheel spinning onscreen may appear to rotate slowly in the wrong direction. This is because movie cameras capture still images of a scene at a finite rate (usually 24 frames per second) and the brain fills in the gaps between these images by creating the illusion of continuous motion between the similar frames. If the wheel rotates most of the way around between one frame and the next, the most obvious direction of motion for the brain to pick up on is backwards, since this direction suggests the minimal difference between the two frames.

But the "wagon-wheel" phenomenon isn't just limited to Clint Eastwood flicks. People experience the effect in real life, even in continuous light. This cannot be explained by stroboscopic or filmic factors. Two competing theories are currently rolling around the academic journals for acceptance.

One proposes that the visual cortex, much like a movie camera, processes perceptual input in temporal packets, taking a series of snapshots and then creating a continuous scene. Perhaps our brain processes these still images as it does frames in a movie, and our perceptual mistake results from a limited frame rate.

While some form of temporal parcing certainly occurs in the brain, it isn't clear that this is sufficient to explain the wagon-wheel effect in continuous light.

One key experiment shows that two identical, adjacent spinning wheels are reported by subjects as switching direction independantly of each other. According to the movie-camera theory, the two wheels should not behave differently, since the frame rate is the same for everything in the visual field.

This has led some scientists to a theory that explains the effect as a result of perceptual rivalry, which occurs when the brain creates two different interpretations to explain an ambiguous scene.

Those interpretations then vie for attention from higher-order brain processes that determine, ultimately, how we see the world. A similar example of this is the Necker cube, a two-dimensional cube that "pops" back and forth between two three-dimensional visualisations.

Cycophant

For a more visual demonstration (at least of the effect through a camera), watch this helicopter.

Basically it's exactly as described above though. Once the frequency difference between a spinning item like that and the camera's FPS synch up, that's the effect you get.

As an aside, I didn't realize the ball was still out on why it happens in continuous daylight to our own eyes. That's pretty neat...

The Count Of Midget Fisto

Long answer. Suppose the movie camera snaps a frame and the wheel turns just enough in the 1/24th of a second before the camera snaps the next frame so that the next spoke has rotated into the position of the first spoke. Then the camera snaps another frame. The second frame looks identical to the first frame even though the individual spokes are in different positions because all the spokes look alike and they line up with the first-frame positions. At this speed, the wheel doesn't seem to be turning at all in the movie.

If the wheel turns a little faster than this, the spokes seem to slowly move forward. That's because the second spoke, going a little fast, went by the position of the first spoke by the time the camera snapped the second frame. If the wheel turns a little slower, then the spokes appear to move slowly backwards.

That's why wheels can appear to move backwards, then forwards as the cars picks up speed. If the car slows down, the wheels can seem to change direction again and go backwards. It all depends on how fast the camera snaps frames relative to the rotation of the car's tires.

There's also allot of disagreement on exactly why we see this in with our own eyes. Here’s an article from Usatoday explain the two popular explanations

Edit:Beat'n

Midshipman

The question has pretty much been answered several times already, I'd just like to throw in that the engineering/science term for this phenomanom is aliasing. A rotating spoked wheel can be modeled as a series of overlapping sinusoids. Depending on relation between the sampling frequency (framerate in this case) and actual frequency (how fast the wheel is spinning) the sample can show a wide range of aliased frequencies.

Pretty picture from Wikipedia (I wonder if they allow hotlinking).

EggyToast

It mostly occurs because spokes are not a solid color -- just like Midshipman states with it being referred to as aliasing. It's a matter of perception and is the same idea behind using two colors to simulate a third, distinct color, or to show a soft edge on a digital image.

It's also similar to why flipbooks and zoetropes work, as well as why strobe lighting looks hella-strange -- our eyes aren't infinite resolution organs and they don't oscillate between "on" and "off." The 2nd explanation linked to above, in the USAToday article, is also why when you're looking at a moving or vibrating object, and then dart your eyes away or around the picture, the illusion of a moving surface is disrupted. This is easy to test in ceiling fans. If you just look at the center of a ceiling fan, it will appear to whirl around. If you dart your eyes away, it will look more static -- not as blurred. And if you visually latch on to one blade, the fan will appear to go much faster than you first thought.

enderwiggin13

joshua1 wrote: »

Just like dogs can't have any TV love, its frame rate being too fast/slow for them to perceive as a fluid image

Not to derail the thread but...

I've always heard this was the case, but it can't be true for all dogs. I have two, a black lab mix and an Australian sheep dog mix. The lab could care less about the TV, but the sheep dog charges when she sees animals on screen. I have a 46" widescreen, so things on there are fairly large. I know she's not just interested in movement or sound, cause she only chases animals - moving or not - making noise or not.

If I flip past the Dog Whisperer, she goes nuts. I have watched her charge at a dog, sitting still on screen, not making any noise and she'll run straight up to it (usually hitting her nose on the screen - good thing i have the glass faceplate on it)


Also, everyone nailed the answer to the OP.

joshua1

yeah, the question was thoroughly answered. Go team! Thanks for the help.