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Aliasing in Real Life
Old 01-23-2018, 01:39 PM   #1
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Aliasing in Real Life

In the movies, wagon wheels seem to rotate backwards because we only see discrete frames.

Yesterday, I was fishing here:



I got the same effect as trucks drove by and I saw their wheels through the gaps in the bridge railing.
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Old 01-23-2018, 01:46 PM   #2
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Originally Posted by TromboneAl View Post
In the movies, wagon wheels seem to rotate backwards because we only see discrete frames.

Yesterday, I was fishing here:



I got the same effect as trucks drove by and I saw their wheels through the gaps in the bridge railing.
Yep, the slots in the bridge have a similar sampling effect as movie recording.
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Old 01-23-2018, 01:49 PM   #3
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Check this, very stunning effect ~ 3 min in:



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Old 01-23-2018, 02:02 PM   #4
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Check this, very stunning effect ~ 3 min in:
I'm the furthest thing from a techie...but that was pretty neat! Thanks.
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Old 01-23-2018, 02:15 PM   #5
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Love Destinís channel.

But here is the LBYM version:

https://youtu.be/nP1elMR5qjc
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Old 01-23-2018, 02:21 PM   #6
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Old 01-23-2018, 02:31 PM   #7
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That's also the reason for the old time photos of race cars with the elongated wheels at high speeds. Many cameras then used a vertical focal plane shutter, which began the exposure by wiping downward (upward in the scene). This effect was also often exaggerated in children's cartoons of the time.

So the shutter slit opens and travels upward, while the wheels are moving forward (left to right in the photo). As the wheels move while the shutter is open, the effect is to elongate them.

Photo credit:

By Bundesarchiv, Bild 183-1991-1209-503 / CC-BY-SA 3.0, CC BY-SA 3.0 de, https://commons.wikimedia.org/w/index.php?curid=5348237
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Old 01-23-2018, 02:45 PM   #8
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In still photos some digicams make straight fan blades appear curved if the fan is rotating.
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Old 01-23-2018, 02:50 PM   #9
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I like helicopters better:

https://youtu.be/1AT_HKV70o4
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Old 01-23-2018, 03:53 PM   #10
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It's not necessary to use an external sampling device (e.g. a movie camera) to observe this phenomenon - the naked human eye is sufficient. When I was in college, I had an old fan that would start up slowly. I was surprised to observe aliasing in the fan blades as the blades spun up. I crudely estimated the sampling rate of the human visual system at about 30 Hz, which seems to be in line with modern estimates.
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Old 01-23-2018, 04:04 PM   #11
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It's not necessary to use an external sampling device (e.g. a movie camera) to observe this phenomenon - the naked human eye is sufficient.
If you saw this with your naked eye, chances are it was in a room with flourescent lights, which flicker due to AC line frequency. The human eye does not 'sample', per se. There is a maximal rate at which it does not sense 'flicker', which is around 30hz - this is probably what you are thinking of. This is why you don't notice the flourescents flickering, and why movies tend to have frame rates around 30fps.
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Old 01-23-2018, 04:15 PM   #12
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If you saw this with your naked eye, chances are it was in a room with flourescent lights, which flicker due to AC line frequency. The human eye does not 'sample', per se. There is a maximal rate at which it does not sense 'flicker', which is around 30hz - this is probably what you are thinking of. This is why you don't notice the flourescents flickering, and why movies tend to have frame rates around 30fps.
I had incandescent bulbs. There is an easy experiment to test the sampling hypothesis: construct a fan with a single blade, and ramp up the rotation rate while watching for aliasing under different lighting conditions. Of course, with only a single blade you'll need a strong mount because the load is highly imbalanced. Let me know how the experiment turns out. 😎
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Old 01-23-2018, 04:33 PM   #13
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I had incandescent bulbs. There is an easy experiment to test the sampling hypothesis: construct a fan with a single blade, and ramp up the rotation rate while watching for aliasing under different lighting conditions. Of course, with only a single blade you'll need a strong mount because the load is highly imbalanced. Let me know how the experiment turns out. 😎
I was surprised to read recently that incandescent bulbs do have flicker. It's fairly low, but measurable, and observable with the right conditions - something like 5-10%? So the filament actually does cool down a bit between half-wave peaks, ~ 8 msec.

I assume in this video, it's the difference between the 60 Hz and the camera frame rate?



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Old 01-23-2018, 05:12 PM   #14
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If you saw this with your naked eye, chances are it was in a room with flourescent lights, which flicker due to AC line frequency. The human eye does not 'sample', per se. There is a maximal rate at which it does not sense 'flicker', which is around 30hz - this is probably what you are thinking of. This is why you don't notice the flourescents flickering, and why movies tend to have frame rates around 30fps.
Oh yea. The beads can move, all by themselves!
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Old 01-23-2018, 07:45 PM   #15
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The new LED lights are causing all kinds of interesting effects from sampling of things in motion to odd interference patterns.
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Old 01-23-2018, 08:36 PM   #16
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If I put a clean shiny cookie sheet on the stove with the the fan/light hood LEDs on, I can get motion sickness if I watch for a minute or so. So I don't.
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Old 01-24-2018, 03:57 PM   #17
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Both the incandescent and fluorescent bulbs flicker, and apparently so do the LEDs. I learned about this several decades ago when I bought an optical tachometer for the glow engines I used on model airplanes. The way to check the calibration was to aim it at a light bulb or fluorescent bulb/tube.

Used outside, they check RPM by pointing it at the spinning propeller and it counts the flicker of the blades going by and dividing by the appropriate number of blades. There's a switch on the housing to set that.
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Old 01-24-2018, 06:39 PM   #18
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Both the incandescent and fluorescent bulbs flicker, and apparently so do the LEDs. ....
Incandescents have a very low flicker, due to thermal inertia. Fluorescents with the old-style ballast definitely flicker, full on/off 120x per second. But the modern electronic ballasts switch at a much higher frequency, but it depends how good of a job they do smoothing the line power. Same with LEDs, the cheapest style flicker full on/off, but if they have a switching supply, it's similar to the electronic ballast in FL.

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Old 01-25-2018, 01:55 PM   #19
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Last time I checked, if you look at an LED clock and clear your throat or make grumbling sounds, the digits will jiggle up and down. I'm guessing your retina is jiggling, and catching the digit illumination at different times for different digits.
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Old 01-25-2018, 02:09 PM   #20
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Last time I checked, if you look at an LED clock and clear your throat or make grumbling sounds, the digits will jiggle up and down. I'm guessing your retina is jiggling, and catching the digit illumination at different times for different digits.
Right. And there may be only a single segment on at a time. They might sequence through digits, or segments, or even segments of the digits.

A high speed camera might see, at 11:11:
Code:
All at once:

|   |     |  |
|   |  :  |  |  or

One digit at a time,

             |
       :     |  or

all of one segment at a time, 

|   |     |  |
       :        or

a single segment at a time,

             |
       :
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