Optical Flow glitch?

Hi guys!

I've made a decent amount of movies with Final Cut Pro X in the last year.
I love using Optical Flow. I film pretty much everything with my SJ4000 action camera (similar to GoPro).

I just tried filming with my iPhone 5C (so 1080p and 30fps) a clip, and put optical flow on it.
The result looks really bad...
You can check this out here :
videobam.com/HBOHx
(sorry about all the ads on that site)

Any idea why it started doing this suddenly?
My steps to get the Optical Flow effect :

- Click on clip, and slow it down to around 20%
- I go in "Image quality" menu and then select "Optical Flow"

That's it...it usually works well...but it doesn't seem to be working anymore

Thanks a lot!

---

EDIT :

I just tried with another .MOV random file on my computer, and it worked great...so it seems like iPhone videos are making a glitch with Optical Flow. any ideas why?

Hey Mark.

The iPhone sometimes records with a variable framerate, depending on lighting conditions for example. If your footage is consistently lit the variable FPS doesn't come into play. The only time it varies is to compensate for underlit footage. Lower FPS allows a lower shutter speed, and thus more light. You can see this when you open the native video file in MediaInfo. If the frame rate has varied during the recording you will get something like this:

Bit rate : 24.1 Mbps
Width : 1 920 pixels
Height : 1 080 pixels
Display aspect ratio : 16:9
Frame rate mode : Variable
Frame rate : 29.970 fps
Minimum frame rate : 28.571 fps
Maximum frame rate : 31.579 fps

This frame rate variation is bad for optical flow calculations. The solution is to optimize your clip before you apply optical flow retiming to it. Optimizing the clip to ProRes will make the frame rate constant.

- Ronny

Thanks for the infos Ronny.
Very interesting.

I have tried to simply convert the iPhone 5C video .MOV file to a .MP4 format...
And the optical flow started working a lot better. Looks like there's an issue in the movie encoding with it comes directly from the iPhone.

Do you think it is the only solution? Convert the file?

Thanks again

Yes, when it happens it is the only solution. Optical Flow needs a consistent frame rate to precisely calculate the interpolating frames. Converting the video often solves this issue. In FCP X this is just a matter of optimizing the clip. Of course you can avoid this problem from the start by shooting with your iPhone in good lighting conditions. And still then I think you will get smoother Optical Flow when you convert the video to ProRes first.

- Ronny

Great! Thanks alot for your help.