Caution: Slow Motion in Apple Final Cut Pro Tends to Hallucinate

Slow motion is as much a trick of the eye and brain, as it is a trick of technology.

All video, in all formats, is composed of a series of still images which, when played sequentially at the right speed, give the illusion of movement. Nothing in a video actually moves, it’s our brain that adds the movement.

Normal playback. (Click to see larger image.)

Thinking of video as a series of still frames explains what happens when we ask video to play in slow motion.

50% speed playback. (Click to see larger image.)

When we slow video 50%, all we are doing is simply repeating the same frame twice.

25% speed playback. (Click to see larger image.)

When we slow video 25%, we repeat the same frame four times.

NOTE: This is why slow motion playback looks best when the slowdown percentage evenly divides into 100; for example: 50%, 33%, 25%, 20%, 10%, etc. The number of repeating frames matches for every frame in the clip.

For speeds faster than about 25%, these repeating frames fool our brain into thinking the action has slowed down. But slower the 25% and we start to see the video as a series of still images.

WHAT APPLE OFFERS

In the Retiming menu, Apple offers options to minimize this “moving still frames” effect:

• Frame Blending
• Optical Flow
• Smooth Slow Motion (Machine learning)

WHAT THE OPTIONS DO

To help us see where we are going here’s a still frame of the source clip. (Frame 4 in the series at the start of this article.)

Frame Blending. Frame blending was invented a long time ago to minimize these frozen images by putting a dissolve between frames so that it looks like the image is moving, when actually we are just seeing two superimposed images.

The benefit to this, though, is that both images are whole and complete. No artifacts.

Frame blending works down to about 10% of normal speed.

Optical Flow. Slower than 10%, though, the dissolves become noticeable. So, Apple (and others) invented “Optical Flow.” This uses math to “invent” frames that morph from one original frame to the next. For example, when running at 10% speed, there would be one original frame, nine morphed frames, then the next original frame.

The problem is that, very often, this morphing creates bizarre artifacts, as shown in the image above.

With the latest release of Final Cut (version 11), Apple added machine learning to the mix in a feature called “Smooth Slow-Mo.” This is designed to improve the quality of these stills by applying AI.

The problem, as you can see from the image above – is that hallucinations exist; especially when moving elements overlap each other. Here, we have her foot in two different places, while her knee is disconnected from her leg.

While optical flow and smooth slow motion can work, in most cases I’ve found the artifacts to be too distracting to use.

WHICH TO PICK?

Here are the options I suggest:

• Slowing clips to 50% or, even, 25% of normal is not a big problem. Your eye will think things are moving slowly. Just use the normal slow motion settings.
• From 25% to 10%, I recommend frame blending. Dissolves don’t hallucinate.
• Below 10%, though, be careful. Both Optical Flow and Smooth Slow Motion hallucinate, but they hallucinate differently.

My STRONG recommendation is that if you plan to use extreme slow motion in your project, shoot at a high frame rate during production. This provides the highest image quality with the greatest flexibility in making speed changes.

• If that isn’t possible, try to keep clip speeds above 25%.
• If that isn’t possible, experiment with frame blending, optical flow and smooth slow motion to see which looks best for your project.

ANOTHER OPTION

Re:Vision FX created Twixtor to solve the problem of hallucinations in extreme slow motion. This software is long-established and highly respected. If slow motion is in your future and you can’t shoot high-frame rates, Twixtor may be the answer.

Link: https://revisionfx.com/products/twixtor/