Question: WHAT IS SAMPLING??? to do with car wheels ?

John Welford answered on 5 Jul 2012:

Sorry I didn’t manage to answer this properly in chat today. You were asking about why car wheels sometimes look like they’re turning backwards?

What I was trying to explain was that it is usually due to you seeing individual pictures of the wheel, rather than a continuous movement. It is called the “wagon-wheel” effect.

For example, if we think about the way video recordings work. The recording is basically a set of individual pictures taken at points in time. If we flick through the pictures very quickly it fools our brain into thinking that there is continuous movement. This is the same way a flick book works.

The frequency at which the video is recording pictures is called the “frame rate” or “sampling frequency”. Anything over about 12 frames-per-second the brain sees as a moving image. TV broadcasts are around 20 to 50 frames-per-second.

So next, for simplicity, lets image a wheel with only one spoke (or a disk with a line drawn on it if you prefer). If it is rotating slowly compared to the frame rate of the camera we will see it at lots of different positions as it turns. But what if we speed it up?

As the rotating rate of the wheel gets close to the frame rate of the camera we start to see the wheel jumping quite a long way between pictures, and some weird stuff starts to happen. When the wheel is turning at exactly the same rate as the camera is taking pictures, it will be in the same position for every picture – so it will look like it is stationary!

From here you might be able to imagine how it could start to look like it is turning backwards?

If the wheel has more than one spoke then it doesn’t need to be turning as fast for the effect the happen, but the overall mechanism is the same.

You can also see this effect in real life if you have strobe light flashing very quickly. Sometimes it even happens with fluorescent house lights, as they can flicker at mains frequency (about 50 times per second).

The wagon-wheel effect can happen even in continuous light (although it is much less common). Scientists think this must be to do with the way the brain processes images from the eyes, although different theories for this are still being investigated.

I hope that makes sense and answers your question!? (you can see how it was hard to describe quickly in the chat session!)