What are these visual phenomena?

December 21, 2011 12:16 PM

I have the perfect question for the hive mind: Does anyone know the name for and reasoning behind the following visual phenomena: 1 The patterns you see when two fences (or any repeating "grid" of objects) pass each other (or appear to pass each other due to parallax) slightly out of phase. I was told the name for this and it bugs me that I've forgotten it. 2 The odd wobbling about you see in LED clocks when eating or (even more pronounced if you can do it) when "rolling" the letter R. The name is what I'm most interested in RE 1, and the reasoning for 2!
posted by cdenman to science & nature (21 answers total)

It's called an interference pattern.
posted by Jairus at 12:16 PM on December 21, 2011

Jairus has got it, more specifically it's a moiré pattern.
posted by RichardP at 12:22 PM on December 21, 2011 [1 favorite]

1 is a moiré pattern.
posted by mr_roboto at 12:22 PM on December 21, 2011

moire pattern
posted by Oli D. at 12:22 PM on December 21, 2011

And the reason for 2 is pretty much the same - LED displays (usually) aren't on continuously, but multiplexed (switched on and off very quickly). The wobbling is due to the interference pattern between the switching on and off and the slight movement of your eyes when you roll / buzz the letter R (I've never seen the effect during eating!)
posted by Pinback at 12:23 PM on December 21, 2011 [2 favorites]

This is why propeller blades and wheels seem to spin "backwards" as they gain speed: the human brain only processes about 10-12 FPS, so as the objects rotate through speeds which are multiples of that frequency, the individual blades or spokes seem to slow down, speed up, and even change directions as the two frequencies move in and out of phase. It's also why the same thing happens in video images, only there it's the speed of the camera which is usually in play.
posted by valkyryn at 12:31 PM on December 21, 2011 [4 favorites]

Wow! Brilliant answer. Seems to be very well known but (I say defensively) hard to google without knowing what it's about.

The comment about frame rates was really fascinating. I had no idea the human brain had a limit on processing rate.

Thanks folks :-)
posted by cdenman at 12:34 PM on December 21, 2011

♪ When two screens interfere making patterns quite queer, that's-a moiré ♬

I wouldn't call the LED clock or the propeller effects as moiré effects - they're time-based where moiré patterns are necessarily spatial.

For the fence, the moiré pattern is the illusion of varying fence density when in fact the slats (or wires or whatever) are evenly spaced the whole way through, on both fences. This is particularly noticeable when you're moving, but it happens statically.

For the clock, I would call the vibration of the numbers an interference pattern between the vibration of your head and the pulsing of the numbers in the clock. This applies to moiré patterns as well.

For situations where these kinds of things are caused by the rate of sampling, Aliasing is the proper term to use. That includes stuff like CRTs flickering on videos, stuff looking blurry/blocky, and some moiré patterns in digital media.
posted by aubilenon at 12:44 PM on December 21, 2011 [4 favorites]

I didn't write that in the right order. The broadest term is "interference patterns" which encompasses all these situations where new periodic signals (patterns) arise out of the interplay of two (or more) other periodic signals. I think that's the only one that actually includes all of the effects described here.
posted by aubilenon at 12:47 PM on December 21, 2011

"I had no idea the human brain had a limit on processing rate."

Well, it doesn't really, at least not so much in terms of FPS as valkyryn put it. The human visual system is sensitive to things like luminance, chrominance, and movement - but the sensitivity to the rate of change of each of these is different, and differs depending where it occurs in the visual field (e.g. we've very good at detecting changes in luma and chroma in our central vision, but shithouse at detecting it in our peripheral vision; similarly, our ability to detect differences in chroma is highly dependent upon the rate of change).

People recognise the "wagon-wheel effect" as an example primarily because we're used to seeing it in movies and TV, and assume it happens in real life much more often that it does. When it does, it's usually due to other stroboscopic effects anyway (such as illumination by 50/60/100/120Hz lighting). Actual examples of it happening due to processing speed limitations are quite rare.

Wikipiedia has a passable writeup here.
posted by Pinback at 12:50 PM on December 21, 2011 [1 favorite]

This is why propeller blades and wheels seem to spin "backwards" as they gain speed: the human brain only processes about 10-12 FPS, so as the objects rotate through speeds which are multiples of that frequency, the individual blades or spokes seem to slow down, speed up, and even change directions as the two frequencies move in and out of phase.

This is absolute nonsense. This is not how human vision works.
posted by mr_roboto at 1:17 PM on December 21, 2011 [2 favorites]

The comment about frame rates was really fascinating. I had no idea the human brain had a limit on processing rate.

Depending on the amount of motion in shot, you can see the difference between 60 frames per second and 100 frames per second, (and with ideal subject matter and movement in shot, I'd put down money I could pass a double-blind on the difference between 100 and 150, and higher)

10-12 fps is actually used as a rule of thumb by animators as the slowest speed at which your brain will begin to translate frames into a moving image. This is the origin of the misconception that you can only see that many frames per second. In actual fact, much much more can be seen, and higher framerates make an enormous difference.
posted by -harlequin- at 1:31 PM on December 21, 2011

> This is absolute nonsense. This is not how human vision works

Yeah, this will only happen when there's a second pattern to interfere, like the framerate of video or at night, under streetlights that are actually flickering very fast.
posted by Tom-B at 1:37 PM on December 21, 2011 [1 favorite]

A similar phenomena happens with sounds, it's called Beat Frequency.
posted by Confess, Fletch at 1:54 PM on December 21, 2011

The two-fences moire is caused by the different visual frequencies of the two fences--even though the fences are constructed identically, the farther-away fence 'appears' smaller, so it makes a stripey moire with the nearer fence.

For dancing LED displays, look at seven-segment LED numbers in a dark room. You see numbers. Now do a saccade that crosses the display. You see secret code. The segments of each number are lit one-at-a-time, generally 60 times a second. The saccade is fast enough to spread the image across your retina.

The 'integration' time of human vision depends on brightness--the time increases in dimmer scenes. This is the basis of the Pulfrich effect(Youtube, and you need a dark filter--eg, sunglasses).

But I would not agree with Mr Robato's characterization of 'absolute nonsense'. The explanation was not absolute nonsense, it was just wrong.
posted by hexatron at 1:59 PM on December 21, 2011

Time-based interference is often called strobing.
posted by flabdablet at 7:56 PM on December 21, 2011

This is why propeller blades and wheels seem to spin "backwards" as they gain speed

Thirding that this does not happen unless there is a fast pulsed light source around, like a 60 Hz streetlight or a tens-of-frames-per-second video camera. Watch propellers etc. spin up using your eyes in bright sunlight and you won't ever see the "backwards" artifact.
posted by fantabulous timewaster at 11:54 PM on December 21, 2011 [2 favorites]

Time-based interference (strobing) is why many safety codes prevent the use of fluorescent tubes with power tools, since they can appear to be stationary when they're running.
This bothers some people about the changeover to compact fluorescent lightbulbs instead of incandescent ones.
posted by blue_wardrobe at 12:54 PM on December 22, 2011

I had no idea the human brain had a limit on processing rate.

Get a copy of Mind Hacks. It's basically the lab manual for a psychology class you didn't take in college and gives you lots of experiments you can do in your own home to reveal all the kluges that your brain does to (among other things) give you the illusion of real time full color vision.

Uh, I've seen a sort of pulsed quasi-stationary effect on truck wheels (when the truck and my car were very much in motion) in broad daylight and found myself wonder how the hell it was happening sans an artificial light source.
posted by Kid Charlemagne at 2:33 PM on December 22, 2011 [1 favorite]

a sort of pulsed quasi-stationary effect on truck wheels

I've seen a few variants of this. One involves a glancing view along the side of the truck, and wheelnuts that stand a long way proud of the surface of the wheel, so that on any given wheel I'm looking through the nearest wheelnuts to see the farthest ones, resulting in a kind of Moire stroboscope.

The other is more subtle, and involves resting my tired head on the door pillar while looking at a truck through my window, so that body vibration in the car directly couples to my skull. Sometimes it happens that some component of that vibration is close enough to a harmonic of the truck wheel rotation rate to cause strobe effects.
posted by flabdablet at 6:01 PM on December 22, 2011

The vibration one sounds like a possibility. Up to now I kind of wrote it off as having something to do with the way my brain edits saccadic movements out of my perception but if that sounds hand wavy, that's because it is.

Experiment time!!!
posted by Kid Charlemagne at 12:42 PM on January 2, 2012