In this video, I'm going to show you how
to use optical flow when creating slow motion from regular speed footage. There are a variety of reasons
you may play back footage in slow motion. It could be to emphasize
action or elongate a clip in your edit. We're going to demonstrate and
talk about the limitations of optical flow. Remember, doing something for real on
production is always going to be better than faking it or replicating it in post. I have four clips in my timeline that will
help demonstrate the various aspects of optical flow. To play footage slower, you must reduce its frame rate. For example, this footage is shot at 24 frames per second. Speed controls can be accessed in your inspector. I'll enable ripple timeline to allow
resolve to manage the other clips in my timeline as I make speed changes. If I play this clip back at 25% speed,
it will now play back at 6 frames per second. That's why each frame is held
longer and my playback appears to stutter. Optical flow is a form of frame interpolation. This means it can generate the
missing frames required for smooth playback. Optical flow can be enabled
on a per clip basis in the inspector. You'll find retime process options at the bottom. When retime process is set to project
settings, this clip will use the default retime process configured in your project settings. The stuttering effect that we're currently
experiencing in playback is because the project settings by default is set to nearest. Instead of jumping between frames,
frame blend cross-dissolves between frames. But it's not suitable in this instance. What we want is optical flow. Once enabled, you can note the difference immediately. Beforehand, the motion stuttered. Now it appears to play smoothly. But this process has added some artifacts to the image. Optical flow works by
attempting to predict motion in an image. It uses the information it
does have to draw in the missing frames. But depending on the motion in the
image, well, some is easier to predict than others. Optical flow excels with linear,
predictable motion, like the background. However, the hiker's movement here is far more erratic. When zoomed in, you can see smearing. Changing the motion estimation to
enhanced better can help to reduce these artifacts, but it's difficult to eliminate them completely. Despite this, the results are more than
acceptable, especially when viewed in context. Onto our next example. This garden is beautiful,
but I need the shot to last longer for my edit, so I'll slow
it to 25% and enable optical flow. It does a great job with the
foliage, but as the butterfly's motion is harder to guess, motion
artifacts are visible under closer scrutiny. The next shot of the talent smiling works perfectly at 25%. As the motion in the scene is slight
and smooth, optical flow yields perfect results. However, this shot of the same talent dancing
produces a great deal of artifacts when slowed down. The tutu causes particular difficulty for optical flow,
as its shape and texture changes almost randomly as she dances. Optical flow is so distracting in this
image, I wouldn't even use this in my edit. Let's check out one final example. Let's slow down this hiking
shot to 25% and enable optical flow. As before, it does an excellent job with the
environment as its motion is predictable and easier to compute. The hiker and safety rope does
exhibit some minor motion artifacts, but again, when viewed in
context, I think these are acceptable results. I would use short sections of this in an edit. Optical flow is a great tool to
squeeze just a little bit more out of your rushes. Results will vary depending on your footage. If you are experiencing many
artifacts, it might be better just to play that clip at normal
speed or swap it out for another shot. If you enjoyed this video on optical flow,
make sure you give it a like and subscribe for more. [MUSIC]
