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Optical Flow Retiming - Other Options?

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Ross Hendrickson
Optical Flow Retiming - Other Options?
on Aug 16, 2010 at 3:51:54 am

Hello All,

I have some 720p60 footage from a 7D that has been conformed to 24fps, and transcoded to ProRes422HQ.

The clip is a locked off shot of a guy running across the frame on a sidewalk with houses very close in the background. The subject is running rather fast and enters & exits the composition in about 50 frames.

My hope was that optical flow would allow me to slow down this clip and get a relatively smooth playback and some intelligent motion detection.

What I've found is that the background "Bends" when the moving subject is next to it. I can understand why Optical Flow is doing this.. so I'm wondering if there is another route for me? Or other settings for optical flow for intelligent background detection?

Something that can detect my static background and not effect those pixels? Only applying some type of optical flow or motion estimation based on what it can detect as a moving object?

Or maybe there is some way for me to auto key the background using one static frame of the background-- then apply the optical flow to just the runner on a white/transparent background?

The idea is that we want to get a clean super-slo-mo looking shot.. w/o having to get an HD camera that can overcrank to 120fps or more :)

here is a link to the clip slowed down to 5%..

Thanks All!

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Anders Larsen
Re: Optical Flow Retiming - Other Options?
on Aug 17, 2010 at 2:03:29 pm

First of all, do you still have the 60p footage? that would fare much nicer when slowing down simply because it has twice as many frames.

It will be hard to avoid some weird blurring and bending/warping of the moving subject but you can avoid the background bending by using a frame from the begining or end of the shot as a clean plate to do a difference key.

I couldn't download the video from your link, so I did an example from two screen shots.
The trick is to put the video over the clean plate, set the blend mode to difference, and then apply a bunch of filter to get a black and white silhouette of your moving subject.
This silhouette can then be used a a mask for a copy of the video.
Try taking this example file apart and see if that helps.

When you have the moving subject isolated, slow that down, and keep the clean plate in the background, that should take care of the background bending.

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