[ This article was first published in the September, 2008, issue of
Larry’s Final Cut Pro Newsletter. Click here to subscribe. ]
Karen Irvine, from Blind Eye Productions, and Jeff Cipin, from Toronto, both sent in separate questions similar to this:
Some time ago you demonstrated how to create very smooth slow motion using Motion.
Unfortunately, I can’t find my notes and I seem to recall it was something different that just using the “Set Speed” function in Motion 3.
Larry replies: Karen and Jeff, thanks for writing.
This process is called Optical Flow, and while it has its problems, when it works, it works great. There are a number of ways to do this, here is an easy method:
1. Edit the clip you want to slow down to the Final Cut Timeline. (While Optical Flow can be used to speed up a clip, its primary benefit is in slowing things WAAAY down.)
2. Select the clip, go to Modify > Speed, and enter the speed you want the clip to run. Numbers smaller than 100% slow the clip down; numbers greater than 100% speed the clip up. In this case, I’ve slowed it down to 10% of normal speed.
3. Control-click on the speed-changed clip and select Send to > Motion project.
4. In the resulting dialog box, give this effect a name (I used “slomo clip 01”) and leave both checkboxes checked. This will start Motion, load the clip into Motion, and replace the original clip in the FCP timeline with the new, improved Motion clip.
5. When Motion starts, the selected clip will appear in the Canvas. Click once on the image to select the clip.
6. Go to Inspector > Properties, then twirl down the Timing tab. The second pop-up from the bottom will say “Blending.” Change it to Optical Flow.
7. This starts a lengthy calculation where Motion tracks the movement of every pixel in every frame. Depending upon the speed of your computer, I’ve found that it takes about 10 seconds to calculate each frame in SD, and about a minute to calculate each frame in HD. Your timings will vary. You can tell that Motion is calculating by the small spinning gear in the low-center of the Canvas.
8. To see how long it will take to calculate the shot, click once on the spinning gear to view the Background Task List dialog box.
9. When the calculations are done, save the Motion project and switch back to Final Cut Pro. Your clip is instantaneously updated with the new version. Simply render the shot (as Motion always saves at the highest possible quality) and you are done.
There are also a couple of notes to improve your success in using Optical Flow.
First, cameras that are locked down always do better than hand-held shots. Shots with rapid pans or tilts, even when on a tripod, are also a problem. The key is to have ONLY the object you want to slow down be moving in the frame. The more the background moves, the harder it is for the software to figure out what to slow down.
Second, objects which have a high contrast level with the background create a better looking effect. Things start looking really funky when the foreground and background match too closely.
Still, when this effect works, the results can be stunning.
UPDATE – Sept. 2
Jonathan Tyrell writes:
One thing you may want to warn your readers about, especially the more casual Motion users, is the size of the file generated by Motion to create the Optical Flow effect. In my experience they get big pretty quickly, even with DV25, and the default cache location is your system drive! A quick trip to the Cache tab in the Motion Preferences will give you an opportunity to choose a more appropriate location.
Larry replies: Thanks for mentioning this. Optical Flow cache files (which are created during analysis) require about 2.7 MB per frame for DV and 16 MB per frame for 1080 HD. Make sure to change the default location of your Motion cache files from your boot disk to a large second drive in Motion > Preferences > Cache.
Also, Optical Flow analyzes the entire clip, not just the portion you have marked with an In and an Out. Keep your clips short, or create separate self-contained QuickTime movies.
