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Revealing Invisible Motion in Video
http://aquatictechtank.net/viewtopic.php?f=22&t=162
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Author:  mistergreen [ Wed Feb 27, 2013 2:48 pm ]
Post subject:  Revealing Invisible Motion in Video



I wonder what it can reveal in our tanks; the concentration of CO2 around the tank?

Author:  Darkcobra [ Tue Mar 05, 2013 3:07 pm ]
Post subject:  Re: Revealing Invisible Motion in Video

Saw that on Hackaday a while back. Blew my mind when I watched the video!

Had to read the research paper to see how the heck it worked. I think I understand it well enough I could code a replication, at least a primitive one.

It only works with things that oscillate, and the approximate frequency must be known in advance.

Consider a tuned circuit. You apply the resonant frequency, it builds up and is amplified. Here they use an equivalent digital filter, with a separate one attached to every pixel in the video.

If that baby's heart were to abruptly stop (horror!), the video would actually show it continuing to beat, with the strength gradually dropping off. And if it were to restart, it would take time to register that too.

All the same rules that apply to tuned circuits and filters apply here. The more amplification is required and/or the lower the frequency, the slower the filter responds. Multiple filters can be attached to each pixel to detect a wider range of frequencies, with each filter set to a different center frequency. Better frequency resolution can be achieved by using more filters, and increasing the selectivity of each.

Another caveat - it only works on fairly stationary objects. If moving too much, none of the filters attached to any pixel will have time to respond.

A lot of folks were wondering if it could be used as a lie detector for people on TV, in particular for presidential debates. I don't think it would work, all TV is digital and compressed now, and compression tends to remove the fine temporal details needed. A shame analog TV isn't around anymore!

Can't think of any way to use it to detect CO2 levels. Even if a camera could somehow pick up a hint of CO2 concentrations, this technique would only amplify oscillations, and the day-to-day rise/fall of CO2 wouldn't be particularly useful.

Author:  mistergreen [ Wed Mar 06, 2013 12:08 pm ]
Post subject:  Re: Revealing Invisible Motion in Video

Darkcobra wrote:
Can't think of any way to use it to detect CO2 levels. Even if a camera could somehow pick up a hint of CO2 concentrations, this technique would only amplify oscillations, and the day-to-day rise/fall of CO2 wouldn't be particularly useful.


True. But it might reveal water circulation like the heat coming off the candle.
The code is publicly available. I'm sure it'll be an iphone or android app eventually.

Author:  Darkcobra [ Wed Mar 06, 2013 1:37 pm ]
Post subject:  Re: Revealing Invisible Motion in Video

I've heard of using food dye to see circulation in a tank. This could reveal more detail in the flow pattern, with less dye. Good idea!

Author:  mistergreen [ Wed Mar 06, 2013 4:22 pm ]
Post subject:  Re: Revealing Invisible Motion in Video

People use dye in the tank? Nice way to change the color of your fish lol.

Here's the code and more vids.
http://people.csail.mit.edu/mrub/vidmag/

I would give it a try making it into an iphone app but I wouldn't have the time.

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