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Jacob Moeller

Speeding up slow motion

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Hi all,

 

I was in two minds as to what forum was appropriate for this, but I decided to plunk it here. Hope that's okay.

 

My question is to do with Slow Motion.

 

CAN I: Shoot my footage slowmo (say... 50 frames) and then change my mind in editing and "speed it up" so it's normal speed (the film is 25 fps), and it will be like i shot it normally? (possibly even sync it up with on-set sound, like normal) Or am I oversimplifying things here and/or are there other things to consider whilst shooting, to make this possible. I suppose the footage would still be 50 frames, but will it run seemlessly with the other normal 25 frames footage and look... normal?

 

I've always told myself I knew the answer to this to be "yes" but now that I actually need to do it, I want to check with some pros.

 

We're shooting quite a long montage, with alot of footage, but I know I can't quite gaze the tempo and rhythm of it all, until i'm editing, and therefore would like to leave myself the option, of having some of it in normal speed.

 

I have of course asked my friend google, but I am getting some pretty conflicting answers, from some pretty questionable scources, so I decided to bother you all, afterall.

 

We're shooting on the RED Epic.

 

Edit: And we're editing in AVID, if that helps.

 

Thanks alot

 

/Jacob

Edited by Jacob Moeller

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If you shoot at 50fps with a 180 degree shutter (1/100) and convert it to 25fps, it will look like you shot it with a 90 degree shutter, which isn't bad, and preferable, I think, to using a 360 degree shutter just to get 1/50th at 50fps.

 

Some motion conversion processes can frame blend to smooth out the motion and make a 1/100th shutter speed look more like a 1/50th.

 

Or you could split the difference between choosing 1/100 versus 1/50.

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Some motion conversion processes can frame blend to smooth out the motion and make a 1/100th shutter speed look more like a 1/50th.

 

David - would time re-mapping be involved in this kind of thing?

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I don't know the in's and out's of this in terms of the actual software workflows, especially with Red footage in Avid. But conceptually it's pretty simple and as David points out, you pretty much have two temporally interleaved streams of 1/100s exposures. Up to you which one you want to extract, odd or even :)

 

You could blend every couple of frames into one and achieve a kind of 180deg shutter that way. Keep in mind that the 180deg is composed not of one 'half' exposure, but of two 90deg quarters separated by 90 of non-exposure and summed, something that might look interesting if you were to do a frame by frame analysis. Not sure if Avid would understand that or not, but it could be done easily enough in your own code/console app, once you had spent the time learning how to access, manipulate and save the values of the frames non-destructively - that is, outside of the effect you apply which by nature will be 'destructive' in terms of pure information - of course it's up to you how 'constructive' you see the end the result turning out.

 

I'd hesitate to call it time remapping when the divisors are not only nice integers but are factors of 2. Time-remapping, at least for me, infers the more complex process of frame to frame interpolation.

 

 

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... I'd hesitate to call it time remapping when the divisors are not only nice integers but are factors of 2. Time-remapping, at least for me, infers the more complex process of frame to frame interpolation.

 

When using "some motion conversion processes", as David mentioned, the 50p-to-25p conversion is not a discarding of every second frame. Twixtor, for example, analyzes the 50p the same whether it will then process it to 1000p, 100p, 50p, 33.333p, 25p etc. When Twixtor adds "motion blur" it simulates a longer shutter opening by means of interpolated frames.

 

For example, I recently received 50p footage made by an amateur camera that chose to greatly shorten exposure times because of the high scene illumination; @#$%&! As a result, chosing every other frame yielded 25p with such small effective shutter angle that simple walking action looked choppy at 25 fps. Twixtor, in effect first converted the 50 fps footage to a large N fps. Then it summed groups of N divided by 50 frames, creating an ersatz 50p shot with 1/50 sec exposure. Then, discarding every other frame yielded a 25p as if shot with 1/50 sec exposure. That walking action looked OK at 25 fps.

Edited by Dennis Couzin

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When using "some motion conversion processes", as David mentioned, the 50p-to-25p conversion is not a discarding of every second frame.

Well, it could be... At least, in my mind the option should be there.

 

The reason I've often resorted to coding my image processing applications is because After-Effects et al. are a black box when it comes to some simple things - the algorithms are too presumptuous and 'clever' for my purposes, with dumbed down parameters that can even appear to contradict themselves - although, I agree if someone less interested in this stuff doesn't know what I'm talking about, then maybe it is is perfect for them...

 

Twixtor, for example, analyzes the 50p the same whether it will then process it to 1000p, 100p, 50p, 33.333p, 25p etc. When Twixtor adds "motion blur" it simulates a longer shutter opening by means of interpolated frames.

For example, I recently received 50p footage made by an amateur camera that chose to greatly shorten exposure times because of the high scene illumination; @#$%&! As a result, chosing every other frame yielded 25p with such small effective shutter angle that simple walking action looked choppy at 25 fps. Twixtor, in effect first converted the 50 fps footage to a large N fps. Then it summed groups of N divided by 50 frames, creating an ersatz 50p shot with 1/50 sec exposure. Then, discarding every other frame yielded a 25p as if shot with 1/50 sec exposure. That walking action looked OK at 25 fps.

I think we're in agreement :)

 

ersatz <<=>> "of course it's up to you how 'constructive' you see the end the result turning out."

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After Effects also has a handy tool for this called 'CC Force Motion Blur' It works on a similar principle to Twixtor. If you have remapped footage from 50 to 25 fps and the action looks choppy I've found this will yield good results in all but the most extreme circumstances.

 

Drop it onto the footage set the shutter angle to 180 and up the samples from the default 8 to between 30 and 60 (you may need to go higher if you have very quick motion elements in frame). Be aware that it does take some time to process (the higher the samples, the longer the processing time) so it's probably best to wait until you have edit lock and only apply the effect where necessary rather than to all the rushes.

 

I've used this on both time remapped live action and animation that has been rendered without motion blur to very good effect. :)

 

Also handy for rough and ready extreme motion blur effects, if that's your bag, as it will allow for shutter angles into the thousands.

Edited by James Wallace

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