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Making long exposure video from normal video


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#1 Mei Lewis

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Posted 01 May 2012 - 03:29 AM

I want to make videos where each frame looks like a long exposure photograph. I want to see if I can get the "light trails in the dark "effect that's easy in stills using a long shutter time.

I figure if I shoot video at 1/30s shutter time at 30 frames per second the shutter is always open, and I should be able to get a different 1 second long exposure every 30th of a second by combing frames like this:

new frame 1 = original frames 1 to 30
new frame 2 = original frames 2 to 31
new frame 3 = original frames 3 to 32
...
new frame x = original frames x to (x+29)
...


I planned to combine frames in after effects by overlaying multiple copies offset 1/30s and reducing the opacity of the upper frames. I think the opacity of the bottom layer should be 100%, the next one up 50%, then next 25%, the next 12.5% and so on so that each layer contributes equally to the final image.

Problem is the higher frames require very low opacities, the 30th should be 1.86265 x 10^(-7)%, and that doesn't work even with the project set to 32 bits per pixel.

Does anyone know how to do what I'm asking?
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#2 Chris Millar

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Posted 01 May 2012 - 03:54 AM

yep, had a play with this but in photoshop (using scripts, I think they call them 'actions'). I did run into similar issues.

I think I ended up doing something like taking two images then adding them 50/50- flattening them then taking the result and adding it 50/50 to the next in the series - then repeating that and so on. In your case 30 or is it 29 times Posted Image

That way you only use %50 - but there was some fiddling I think with the layer mode (the math behind the scenes - e.g. overlay vs. multiply vs. normal vs. darken and so on).

I thought AE had a filter that pretty much works the same way anyway - motion blur or something ? (I don't think its controls are so direct however)


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#3 Chris Millar

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Posted 01 May 2012 - 03:58 AM

To answer your question a bit more concretely - learning a little computer programming, maybe using something that does a lot of the work for you like matlab - you can do all this in code down to whatever precision you choose.
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#4 Mei Lewis

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Posted 01 May 2012 - 08:43 AM

Thanks Chris.

Learning enough matlab and other various bits of coding to connect it to something that can work on video files would take a biiiiiit more time than I have right now. I'm only trying to get this effect for fun, nothing serious.

AE has motion blur but the filter closest to what I want seems to be one called "wide time". The results aren't great though, I think maybe because of the limited precision being used.


I thought of doing similar to your suggestion of mixing down, but in after effects using precomposition.I/'m going to shoot something at 4fps and see can I get that to work before I got up to 30.
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#5 Chris Millar

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Posted 01 May 2012 - 03:46 PM

Good plan.

maybe make some testing frames - simple block animations of 255/255/255 vs. 0/0/0 - something you can check the outputs of when you're done.
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#6 Travis Gray

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Posted 01 May 2012 - 04:06 PM

Crazy question, but, any reason to need to do it with video? Could you just do a time-lapse with an SLR?
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#7 Chris Millar

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Posted 01 May 2012 - 10:52 PM

Crazy question, but, any reason to need to do it with video? Could you just do a time-lapse with an SLR?


Thing is that he's overlapping the exposures so each frame shares 29/30ths of the preceding (or following) frame.


You'd need a set up that could tap in and out of the data stream at will while keeping 30 taps concurrent.


It's pretty much having a 10800deg shutter angle.
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#8 Mei Lewis

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Posted 03 December 2013 - 01:28 PM

Just got an email about this thread, prompting me to reply. Not sure why I didn't before.

 

My maths for the layer opacities in the first post is wrong, don't know what I was thinking, they should go down as:

1/1, 1/2, 1/3, 1/4, 1/5...1/25 for the 25 layers, and that's easily doable.

 

Here's a test with some more notes in the comments:

That has an effective shutter angle of 9000degrees.

 

If anyone's interested I can zip up and upload the entire source for that video, it's a Premiere pro edit.


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