Interesting Video Effect

[Larry Jacks]

It's funny sometimes to see how video cameras capture images. In old movies, you'd sometimes see things like wagon wheels that appeared to be spinning backwards. This was largely caused by the camera shutter acting somewhat like a strobe. Video cameras seem to do this a lot with things like airplane propellors. I've seen a lot of videos where it appears that the propellor is spinning very slowly.

This video (http://www.alexisparkinn.com/photogallery/Videos/2007-5-21-ufochopper.wmv) is the most extreme example that I've seen. It shows a Russian Mi-24 Hind helicopter. The main rotor appears to be completely stopped and the tail rotor appears to be barely turning. While it's easy to understand what I'm seeing, it still looks really wierd.

[Fazor]

I know you've been toseeked, as we discussed this some time ago in [CT section, IIRC?]. I want to say it was in the Police FLIR thread, but I'm not sure. Wherever it was, it was off topic from the thread.

Anyway, I don't think we ever reached a consensous on whether or not it was an unintentional cooincidence (shutter speed just happened to exactly match rotation speed) or if it was clever editing. I think the later, but have no real evidence to support that. Interesting video though.

[Nicolas]

Don't the eyes do a similar thing, I mean can't you look at wagon wheels in real life and they appear to spin backwards? I know that sometimes, from inside a plane the engines appear to turn real slow or not at all. That's why they have the curl painted on them.

[Fazor]

Don't the eyes do a similar thing, I mean can't you look at wagon wheels in real life and they appear to spin backwards? I know that sometimes, from inside a plane the engines appear to turn real slow or not at all. That's why they have the curl painted on them.

Yes, for the same reason. We can only process so many images per second (34, IIRC).

[Nicolas]

Is that the black to black time (more or less) of our retina elements or something like that?

[Fazor]

I don't know what you mean by that, but that's the number of images that can be recieved, transimtited, and decyphered by our brain in any given second, or what *I* would call our Optic Reflex time. (Don't ask what the actual term is, or the physiology behind it. Or ask, just don't dirrect towards me ;)).

[Jeff Root]

Nicolas, Fazor,

The stopped/slowed/backwards effect dos not occur without some
kind of strobe mechanism. When looking at a spinning aircraft propeller
in real life, all you will see is a faint disk. A blur. It might look like
the blades have completely disappeared, but without some kind of
strobe, they will never appear to be stopped, slowed, or moving
backward. You will never see individual blades. Only a blur.

In making 2001: A Space Odyssey, the effects people found that
at a frame rate of about 60 fps, moving images become amazingly
lifelike. This frame rate was used for a short film shown in the
British Columbia building at the 1986 World's Fair in Vancouver.
It looked pretty good. 18 fps is the minimum rate generally used
to simulate natural motion, but the eye can easily detect the
difference. 24 fps is standard for film, and 30 fps is standard for
video in the USA. Those rates give adequate simulation of
continuous motion in the center of the field of vision. Peripheral
vision sometimes reveals the shuttering effect of individual frames
even when the image looks continuous to central vision.

-- Jeff, in Minneapolis

[Kelfazin]

Nicolas, Fazor,

The stopped/slowed/backwards effect dos not occur without some
kind of strobe mechanism. When looking at a spinning aircraft propeller
in real life, all you will see is a faint disk. A blur. It might look like
the blades have completely disappeared, but without some kind of
strobe, they will never appear to be stopped, slowed, or moving
backward. You will never see individual blades. Only a blur.

I mostly disagree with this. I can drive down the street and, if a car next to me is travelling at the right speed, it appears to be moving backwards. Now I don't see individual spokes on the wheels, that's true, but apparant very slow or backwards motion I have seen.

[Nicolas]

I think we're both right. :) Your eyes cannot get stroboscopic effects such as perfectly outlined, still rotor blades or turntable strobo indicators. However, they can show a blurry image which appears to turn backwards or doesn't appear to turn at all. At least, that's what I've experienced in reality. And as a still blurry engine appears dangerously harmless, they paint the curl on it, as an extra indication. No clear curl = DANGER!!

@fazor: what I meant to ask was which of these was the critical factor. Brain processing or the receptors in the retina.

[Fazor]

I spend too much time on my bed staring up at the ceiling fan (seriously), and while it doesn't appear to move backwards, you do see the blades appearing to move slower than they are (with the blured images of the blades in the surrounding space). What's fun is i can watch that, then focus on one blade and follow it in a circular motion with my vision and the blur goes away (usually can't do that for more than a couple seconds, eyes get tired trying to move that fast). Or you can blink rapidly and it exaggerates the effect discribed in the OP.

I agree it requires a strop to see it without the blur, but for something to appear to rotate slower/backwards (with blur) that's not necessarily true (generally, spokes for instance stand out at certian rotational angles due to reflecting the stationary light source differently, so in some positions they stand out more than others, allowing you to see these particular "frames" as more than just a blur. At least, that's my guess).

[Jeff Root]

If a rotating object such as a fan or airplane propeller is reflecting
light to your eyes only at one point in its rotation, then you may
see that reflection as an unblurred image of the blades appearing
to be motionless. The fan or propeller would typically appear to
have only a single blade, but you would actually be seeing all the
blades in succession at the same location.

In that kind of situation, what you see will usually just be a bright
highlight on the fan or propeller blade, not the whole blade.

Kelfazin, the speed of the car makes no difference, as long as the
wheels are turning fast enough that you aren't seeing individual
spokes, and the car is moving slowly enough that you are able to
follow the forward motion of the wheel. Reflections off the shiny
spokes will appear in the same location if the wheel is spinning in
place, or will move in one direction or the other if the wheel is
moving past you, changing the angle from the light to the spoke
to your eye.

I might call this "self-strobing".

Of course, another source of strobing is the flickering light from
a fuorescent lamp or TV picture tube. My computer monitor must
use very fast-decaying phosphors because it makes very sharp
silhouettes of my fingers as I wave my hand in front of it.

-- Jeff, in Minneapolis

[Kelfazin]

By speed of the car, I meant the speed of the wheel's rotation. The image may be a blur of spokes, but it still looks like the wheel is rotating in reverse. The car the wheel belongs to can be in perfect lock-step beside me and the phenomenon will still occur. I have noticed this at freeway speeds.

[EricM407]

Yes, for the same reason. We can only process so many images per second (34, IIRC).

I think you could do a simple test to see if you can process more images per second than this: Move your mouse pointer very quickly from one side of the screen to the other. It helps if you have good contrast between the background and the pointer.

The screen is (probably) refreshing at 60 Hz, and you should be able to see the individual point-samples of the mouse spaced 1/60th second apart rather than a continuous blur. The faster you move the mouse, the more widely spaced the samples are from each other. I think that even at a much higher frequency we'd still be able to see them.

[Peter Wilson]

Yes, for the same reason. We can only process so many images per second (34, IIRC).

The rate of processing is much slower, something like 14 frames-per-second. But even at 14 fps, we can detect much faster flickering:

18 fps is the minimum rate generally used
to simulate natural motion, but the eye can easily detect the
difference. 24 fps is standard for film, and 30 fps is standard for
video in the USA...

In cinema projectors, the shutter blinks twice for each frame, so flicks actually flicker at 48 hz. Film has always run at 24 fps, but at some point (long ago) they added the double-shutter trick, and flicks became less flickery, but the flick-term is still around.

[Captain Kidd]

By speed of the car, I meant the speed of the wheel's rotation. The image may be a blur of spokes, but it still looks like the wheel is rotating in reverse. The car the wheel belongs to can be in perfect lock-step beside me and the phenomenon will still occur. I have noticed this at freeway speeds.It's amazing what you notice on a long, boring commute isn't it? I've noted the same thing. I can't see the individual spokes, but they have the impression of rotating backwards.