FCP X: Understanding Optical Flow

Posted on by Larry

Optical flow, and it’s less sophisticated cousin, Frame Blending, are techniques that Final Cut Pro X uses to smooth playback of extremely slow-motion clips.

The problem with slow-motion created using software (as opposed to that shot by the camera) is that the slower the video plays back, the more jerky the images appear. This is caused by not having enough frames to smooth out the movement. For normal or fast video, this isn’t a problem. But for clips running at 20% of normal speed, or less, it’s a big problem.

This week’s online training (#59 – Changing Clip Speed in FCP X) shows how to create speed changes in Final Cut Pro X. The video below is an excerpt from this webinar and illustrates what optical flow is, when to use it, and how to apply it.

NOTE: This is an excerpt from a recent webinar entitled: Changing Clip Speed in Final Cut Pro X. You can buy the complete webinar now and download it from our store or, for only $5.00 more, get all my training in my new subscription membership.

NOTE: Be sure to view the video at full screen so you can see the subtle changes that frame blending and optical flow add to the playback.

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12 Responses to FCP X: Understanding Optical Flow

  1. […] A useful article on Apples new algorithm for slow motion (optical flow) can be found here […]

  2. Erik Lindahl says:

    I would very much disagree with the fact “optical flow was designed for speeds below 15%”. In most cases, even in the example you show here, it degrade the picture quality enormously making is close to useless. I’d rather say it’s completely up to the shot when this should be used and how good it works but I rarely recommend a client to push slow-motion further down than 50% in post. In extremely good cases perhaps 25% – and even then a lot of shots start to enter the “time warp effect” when using optical flow.

    It’s all about the shot and what works and not, you can’t really put a number to it and mostly retiming in post gives odd side-effects.

  3. The optical flow files are quite large. When one deletes either project or event renders, are the Optical Flow files also deleted?

    • Larry Jordan says:


      Yes. Since speed changes are applied in the Timeline, the Optical Flow analysis files are stored in the Project folder. When you delete the Project, the analysis files are deleted with it.


  4. Thanks Larry for the info, but what I meant was if you delete the project ‘render files’ , not the project, will the optical analysis files also be deleted.

    • Larry Jordan says:


      Hmmm… I was wrong. Your question forced me to go back and test this.

      Optical flow analysis files are stored in Event Folder > Analysis Files > Optical Flow. This means they are available to any project.

      Deleting Project Render files does not delete analysis files.

      Sorry for my earlier, incorrect information.


  5. Paul says:

    Hi Larry, just a quick one, I’ve got clips shot at 120fps, what is the best way to get these into FCP X? Without losing frames I mean, as I believe it can only import and handle up to 60fps?

  6. Mudh says:

    Larry a lot many of us are facing a problem in understanding the basic concept of fcpx ….everything has to be kept in control all the time….clips keep falling apart,snapping and getting disturbed ….kindly write a small write up about how one should approach fcpx , how should one just replace concepts to be able to do exactly how they worked earlier….because by changing the interface….same needs of editing won’t change….plz tell us how to think to use this interface….and if you have written something like this ….kindly let me know the link….Thanks.

  7. Paul says:

    Hey Larry,

    It seemed to have greyed out the option for optical flow. the codec of the clip is avchd. what do i have to do to use the optical flow?

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