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Optical Flow Stuttering

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Justin Mrkva
Optical Flow Stuttering
on Mar 9, 2013 at 1:50:56 am

I've been using Optical Flow in a complex project where I was editing using Proxy media. The flowed footage looked perfectly fine.

However, after switching to Original media (it happens to be H.264 from an iPhone) the optical flowed segments gained a distinct stutter, where it seems as if it's interleaving portions at regular intervals and playing them in alternating directions...

The problem persists even when exporting via FCPX's export.

An example is available at



- you can see it's actually alternating going forwards and backwards in certain segments.

The way to fix this is to render to Optimized media; somehow it seems there's a problem with the iPhone's footage in Optical Flow.

Does anyone have any ideas on if there's a way around this? Fortunately the fix is simple but I have very limited space for Optimized media; there's no way I can transcode it all Optimized but on the other hand it's easy to miss transcoding one that I ended up using Optical Flow on.


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Jeremy Garchow
Re: Optical Flow Stuttering
on Mar 9, 2013 at 5:34:21 pm

As long as you are editing horror and suspense, I'd chalk this up to a happy accident.

Sweet effect. ;)

What particular DSLR are you using?

I think you have the solution, optimize only the clips you need to.


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Dave Gage
Re: Optical Flow Stuttering
on Mar 9, 2013 at 8:13:25 pm

[Jeremy Garchow] "As long as you are editing horror and suspense, I'd chalk this up to a happy accident.

Sweet effect. ;)"


I agree with Jeremy, I love that unintentional effect! Very Cool. Of course, it would nice to get the workflow fixed so it does work the way you want it to.

Dave


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Justin Mrkva
Re: Optical Flow Stuttering
on Mar 10, 2013 at 4:31:17 am

It's not a DSLR - it was shot hand-held on an iPhone 4s. :)

And yeah, I agree, I'll just optimize the clips I need to. I'm probably going to be getting my RAID system back up and running, so at that point going optimized shouldn't be a space problem.

While I'm at it, what kind of drives do you use for mobile editing? I have an OWC Mercury Elite Pro Dual Mini with twin 7200 RPM 750GB drives, RAID-0 striped, but on the Retina MacBook Pro (brand new!) the Apple FireWire 800 adapter doesn't provide enough wattage to power it. I've got a USB3-eSATA adapter, but unfortunately I need a wall wart to power the drive itself since the adapter's FW connection can't. Any thoughts? I don't want to spend any significant money right now; I'm waiting for OWC to update their lineup with some Thunderbolt RAID enclosures or something that I can power from the rMBP.


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Jeremy Garchow
Re: Optical Flow Stuttering
on Mar 10, 2013 at 4:39:33 pm

[Justin Mrkva] "It's not a DSLR - it was shot hand-held on an iPhone 4s"

Duh. You mentioned that, apologies. I don't know why I asked that.

In the fcpx browser, what's the frame rate of your iPhone clips.


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Justin Mrkva
Re: Optical Flow Stuttering
on Mar 11, 2013 at 8:46:28 pm

It's 30fps, and so is the project. I did find that apparently iPhone video can actually have variable frame rates. Not sure what the details or the implications are, but I suspect that has something to do with the Optical Flow issues prior to Optimized conversion.

I'm in the middle of writing a new application for video processing, so I'm actually dealing with frame rate handling right now. I'll try running that video through once I get that straightened out and see what actually happens with the timecode. :D


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Dave Gage
Re: Optical Flow Stuttering
on Mar 11, 2013 at 11:00:41 pm

[Justin Mrkva] " I did find that apparently iPhone video can actually have variable frame rates."

I have found the same thing with my MBP. It will do 30fps, but not if there is not enough light.

Dave


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Virgil Scott
Re: Optical Flow Stuttering
on Mar 13, 2013 at 5:53:03 am

Justin-

There are a few things potentially causing this.

1) Since you say the proxy media is not displaying this, then I would guess that long GOP format clips (h.264) can behave this way when optical flow retiming is applied on the proxy media (Pro Res 422 1/4 res scaled to full frame- a non long group of pictures codec) and then you switch to the original media, the the interpolation markers that optical flow retiming is looking for aren't in exactly the same places....

and

2) iPhone frame rate is variable. BOOM.


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