Optical Flow. Looking for better understanding of how FCPX uses it.

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Level 1

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Optical Flow. Looking for better understanding of how FCPX uses it.

Hi, I've used Optical Flow for years, but I've recently been paying attention to the Analysis files FCPX generates and it's huge file size. It's prompted two questions:

When slowing down 60fps footage (shot at 1/120 shutter) to 24fps timeline, I understand 40% is the true amount of slow motion that even "automatic speed" in retiming will give you. But when slowing down that same footage between 99% and 41%, I've noticed that unless I apply Optical Flow, the camera movement is very choppy when fully rendered in the end. I understand the need to use optical flow when slowing down beyond the available frames in footage, e.g. if I slowed that same footage to 20%, or if I slowed 24fps footage in a 24fps timeline to anything. But in this case, shouldn't that 60fps footage produce smooth slow motion within the 99% to 41% range without Optical Flow?
The other question is, After FCPX generates optical flow analysis files, is the app now reading that clip completely from the analysis folder, and no longer my external drive where the original media is kept? Or is it only reading the extra bits of data is created to make the retiming smooth, in which case, it's reading from both locations simultaneously?

Thank you for any insight!

MacBook Pro (M2 Max, 2023)

Posted on Jul 25, 2024 12:08 PM

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Answer 1

Top-ranking reply

BenB

Level 6

13,438 points

Jul 25, 2024 7:42 PM in response to ReynaldoMar

When your clip is set to Automatic Speed, all original frames are used, which then makes is slow motion. Any other percentage of slowing down, Optical Flow creates fake frames based on the data of the original frames, to fill in the empty gaps. FCP reads from both the original media and the analysis file. The analysis file is a database file, and has no media inside of it.

When you use any other method besides Optical Flow, FCP spreads out the original frames and there are empty gaps between them. So it will by default repeat the same frame until the next original frame is encountered. With Frame Blending it is doing a sort of cross fade between original frames. Thus you can go from very jittery to jittery with some slight blur.

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Answer 2

ReynaldoMar Author

Level 1

12 points

Jul 29, 2024 9:55 AM in response to ReynaldoMar

Thank you Ben. Your response gave me better understanding of how frame blending works and I've found that it's a great option for when I slow down my 60fps shots just a little without needing to wait for Optical Flow analysis and also sparing my storage space. What made me question the need for Optical Flow when slowing down between 99%-41% in a 24fps timeline is because the footage has more than enough original frames when retiming, so I wondered why would FCP need to to create fake frames to make it smooth.

Also, thanks for explaining that the analysis file is a database file and so it reads from both places during playback. My reason for that question was because I was trying to decide if it would be better to set the library cache where those optical flow analysis files are stored on the same external drive as my original media, or on the local drive where FCP is installed. The goal being to optimize performance.

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Answer 3

terryb

Level 4

3,536 points

Jul 30, 2024 9:25 AM in response to BenB

A while back I recall Mark at Ripple saying he keeps his cache on the internal SSD because it was faster than the external drives. That said, I did not notice any significant difference when testing it stored on the internal SSD in my NAS environment, so like you Ben, I always store the cache adjacent to the library on the external drive. Maybe if the external drives are slow it would make a difference.

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Answer 4

BenB

Level 6

13,438 points

Jul 30, 2024 9:17 AM in response to ReynaldoMar

I've always kept the cache on the same drive as my libraries. I've never had issues. Some folks get picky and use different drives, but I question if that really achieves anything.

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Optical Flow. Looking for better understanding of how FCPX uses it.

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