… for Apple Final Cut Pro X

Tip #549: What Is Optical Flow?

Larry Jordan – LarryJordan.com

Use Optical Flow for clips slower than 10%.

Optical Flow is a way to generate artificial frames to smooth extremely slow motion video. The ideal way to create extreme slomo is to shoot at a high frame rate, then slow it down. But, if you are editing after production is complete and no high-frame rate video was shot, you need to go to Plan B.

Optical Flow is Plan B.

When slowing a clip, you’ll get the best results by picking a speed percentage which divides evenly into 200. For example, 50, 33, 25, 20, 10, 5 and so on.

There are three choices for image quality:

• Normal. Use this for speeds of 50% or faster, including fast motion/timelapse.
• Frame Blending. Use this for speeds between 10 and 50%. This quickly dissoves between each slowed frame.
• Optical Flow. This creates frames, what animators call “tweens” for very slow motion. Use this for speeds slower than 10%.

The problem is that optical flow often doesn’t work. By that I mean it generates strange artifacts, especially between foreground and background.

Over the years, I’ve found very few clips where optical flow works reliably. I tend to prefer frame blending with speeds at 20% or faster.

For extreme slow motion, the best option – and most reliable – is to shoot a high frame rate.

EXTRA CREDIT

To apply Optical Flow, slow a clip using the Retime menu, then choose Optical Flow from Video Quality.