Undoubtedly, the best way to achieve the smoothest time remapping (slow-motion) is when you are shooting with a camera that actually has the ability to record motion in higher frame rates in the first place. Sometimes, however, this is simply not the case, yet it doesn’t mean you still can’t get some decent results. Optical Flow is another feature for the recently released update of Premiere Pro, that definitely can improve your workflow in those terms.
In essence, Optical Flow interpolation enables users to achieve smooth speed and framerate changes by interpolating missing frames. This workflow, albeit not perfect, can still be beneficial under certain circumstances. In this article, we’ll cover some of the best practices related to this processing method.
To choose Optical Flow when adjusting playback speed right-click on a selected clip and navigate to Speed/Duration. After dialling a certain playback speed select Optical Flow in the Time Interpolation Menu and click OK. It’s always recommended to render the selection after applying the effect by navigating to Sequence and selecting Render Into Out in the drop down menu.
As a rule of thumb, Optical Flow interpolation is ideal for modifying the speed of clips containing objects with no motion blur that are moving in front of a mostly static background that contrasts highly with the object in motion so it’s really important to keep this major consideration in mind.
Generally, the algorithms of Optical Flow interpolation calculates the motion of every pixel for each frame. However, the filter does not actually know the difference between the pixels that make up your subject and the pixels that are part of your background or other objects in the frame, especially when they are also moving.
That’s why in many of those occasions some visual artefacts and so-called wobbling in the different parts of the frame may occur. To avoid this make sure that you shoot with the highest frame-rate possible to give Optical Flow more real source frames to work with. Further, use a shorter shutter speed settings on your camera.
Less motion blur will result in smoother interpolation. Be aware that the more you slow your subject down the more frames Premiere Pro will have to interpolate. The optimal results usually can be achieved at 50% speed or higher. And, if you are wondering what is the difference between Twixtor and Optical Flow interpolation in Adobe Premiere Pro CC here is an interesting comparison between the two.
It’s worth noting that Optical Flow is assisted by Premiere Pro’s Mercury Playback Engine, but it still can’t play back all of the interpolated frames in real-time. So in order to see the best results you need to render and playback the optical flow speed changes as shown in the former video above.
Ultimately, you can also use the “Time Interpolation” feature that you will find when preparing to export your media within the Export Settings dialog. This setting allows you to change the frame rate of the exported file by leveraging Optical Flow to interpolate the missing frames and should provide improved interpolation and better final results.