How to Get the Best Slow Motion Results with Optical Flow


Get higher sluggish movement when filming at 24fps through the use of optical circulate. Take a look at our tutorial to see some optical circulate examples in actual movies.

Usually, if you wish to movie in sluggish movement, you need to use a better body fee than the mission default. For instance, in case your mission body fee is 24fps, you’ll have to movie at a body fee of 48 or 60 to realize a slight diploma of sluggish movement. However immediately, we’ll discover how one can nonetheless get sluggish movement when filming at 24fps with optical circulate.

Trendy software program will get us sluggish movement pictures by way of body interpolation. It permits us to take a 24fps shot, apply a time remapping technique, and have the footage seem as if it was shot at a better body fee, then decreased to 24fps.

Body interpolation permits for the slowing impact.

Nevertheless, as with most digital manipulation types, whether or not saving highlights, eradicating background hiss, or body interpolation, there’s solely thus far you may push one thing earlier than the media breaks down.  

Body interpolation is nothing new. It’s been round for some time and it’s typically used to revive archival footage that wasn’t shot at immediately’s customary body fee. It’s additionally utilized in timelines to match media at totally different body charges. Extra not too long ago, these options have been added on to NLEs, versus being particular person software program or plug-ins, permitting filmmakers to make their 24fps footage fluidly slower than ordinary. Nevertheless, optical circulate is only one of some body interpolation strategies accessible. Let’s have a look.

Body Interpolation Strategies

1. Nearest

Nearest is the default technique. Your NLE will duplicate the adjoining frames to make the footage playback slower. Nevertheless, with duplicated frames, the looks of the slow-motion might be staggered.

2. Body Mix

Body mix is analogous in that it duplicates frames, however it mix the duplicated and authentic frames with cross dissolves to easy out the jitteriness.

3. Optical Move

Optical circulate, which is what we’re immediately, makes use of movement estimation to generate new frames from the supply. Primarily, the software program interpolates the lacking body by analyzing and evaluating pixel knowledge from the encircling frames, creating what would seem in-between.

Frame Manipulation
Three body interpolation strategies.

In most circumstances, when you intend on your shot to be displayed in sluggish movement, it’s greatest to movie at a better body fee. However that’s not all the time potential. Maybe your digital camera doesn’t shoot with a excessive FPS, or possibly it does however solely at 1080p. Or maybe you need your shot to enter slow-motion from a 24fps sequence, however your digital camera can’t do pace ramping. Due to this fact, you could movie at 24fps 4K.

Analyzing Optical Move

First, we’ll have our mannequin stroll in direction of the digital camera with a comparatively static background, and we’re going to lower the pace by 25 %. At this level, we must always click on pause and take a actuality verify — utilizing optical circulate isn’t going to show 24fps footage and make it seem as if it was shot with a Phantom digital camera. At most, you’re going to get a mild lower of sluggish movement, and relying as regards to your video, you might introduce artifacts and ghosting into your picture.

The outcomes, seen within the video, are respectable, with little-to-no ghosting artifacts.

Change in Speed
The slight change in pace made little distinction within the outcome.

However, what if we introduce some easy cross-frame motion? We are able to immediately see that ghastly ghosting.

Cross-Frame Movement
Introducing just a little cross-frame motion.

Nevertheless, as famous within the video, the ghosting is minuscule after I really transfer with the topic. Why are we getting much less ghosting with a shifting shot when a static picture ought to, in principle, be higher because of much less change of total pixel data getting into the scene?

Properly, optical circulate interpolation works quite a bit higher with media that comprises little-to-no movement blur and with topics shifting in entrance of a largely static background. The issue is at 24fps; there’s all the time movement blur with a topic shifting throughout the body — static background or not. Nevertheless, after I transfer with the topic and preserve them heart, it considerably negates that. Utilizing that foundation, if we now have the mannequin do the identical factor, however on a background that strikes sporadically (in our case, the ocean waves), will this make the interpolation even worse? The video instance proves it makes no distinction.

Is there a motive for this? On this explicit occasion, it’s costume.

Optical circulate wants to investigate the movement of each pixel for every body. It doesn’t know what the background is or what the foreground is, and it could actually battle to separate the 2. Because of this, you may even see some warping if visible parts are the identical shade. For instance, if I apply optical circulate to a set of waves, we are able to see it doesn’t work that properly because of the similarity in distinction and shade.  

Poor Use of Optical Flow
An instance of poor use of optical circulate.

Solely when there’s tonal separation can the information to be appropriately analyzed. And even when filming a topic with robust distinction and a shade distinction in opposition to the background, if it strikes quick by way of the body, it’s nonetheless going to trigger ghosting. 

I discover optical circulate to be essentially the most helpful for slow-moving panorama pictures, as there are fewer foreground parts, fewer pixel adjustments shifting throughout the display, and you may typically cut back the clip greater than if the topic was centered on display. 

When to Use Optical Move

  • If the topic will stay comparatively nonetheless with a static background
  • If the topic strikes, attempt to transfer with them
  • If the visible has robust tonal distinction
  • For extensive pictures, make certain the realm has an array of various colours and distinction.
  • Restrict the pace lower to 10–50 %, however for minimal ghosting, lower much less.

And bear in mind, utilizing optical circulate isn’t an acceptable alternative for a better body fee, however it’s an superior device to create the phantasm subtly.

Get extra filmmaking ideas and methods with our newest video tutorials.

Cowl picture by Annette Shaff


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