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Optical Flow

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Nicolas Vallejo
Optical Flow
on Aug 13, 2013 at 4:42:23 pm

I have a bunch of footage that I shot on 60fps and conforming to 24fps for slow motion.

My question - out of floor, nearest neighbor, frame blending and optical flow, which produces the best results?
I don't care about storage, rendering time, etc. - just what is the best option.

Also, when I select a clip I conform, it gives me an option in Modify - Conform - Video Quality - Normal/Frame Blending/Optical Flow

What does this option do, exactly?


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Nicholas Zimmerman
Re: Optical Flow
on Aug 14, 2013 at 4:02:32 am

If you are conforming 60P to 24P you don't really have to worry about them. However, they are very important if you try taking your footage any slower.

Those options allow you to adjust the frame interpolation on footage that's been slowed down. Back in FCP7 you'd have to just deal with frame blending (layering each of the frames together with partial transparency). The only way to achieve high quality frame interpolation back then was to use a plugin (BCC Retime, Twixtor, etc.) or an external application (Motion, AE, etc.) We are very lucky to now have this built directly into the NLE.

What an optical flow algorithm does is look at multiple frames and try to figure out what the realistic flow of motion would be. This allows you to slow down footage that was shot 24 down to 20, or 16, or hell, 2 fps. Now it's not magic, and it can easily leave visible artifacts. If you've got a copy of Silver Linings Playbook laying around you should check the tail-gating scene. It's all slowed down significantly with some form of optical flow, and it's pretty noticeable.

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