Optical Flow Time Remapping: Tips & Tricks for Best Results
How do I apply it?
For information on where and how to apply Optical Flow interpolation with speed changes and time remapping, see this article.
What are the benefits?
Optical Flow time remapping is an exciting new feature for Premiere Pro CC 2015. It enables users to achieve smooth speed and framerate changes by interpolating missing frames. Here are some best practices and advice about what to expect when using Optical Flow interpolation with your own footage.
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.
What kind of media can I use it with?
Optical Flow can be applied to any video clip inside a Premiere Pro sequence. However, similar to Morph Cut, Optical Flow technology has some inherent limitations, which you will want to be aware of before applying it to your media. Optical Flow needs to calculate the motion of every pixel for each frame. It 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. So you may see some visual warping if visual elements that are the same color conflict with each other, or if parts of an object get occluded from one frame to the next. For example, if your subject goes in front of a pole, the algorithm may get confused and you may see some visual artifacts there.
To minimize such outcomes, you might try some of the following things:
- Shoot with the highest framerate possible to give Optical Flow more real source frames to work with. Also, a shorter shutter speed means less motion blur, resulting in smoother interpolation.
- Be aware that the more you slow your subject down (say, to 20% or less) the more frames Premiere Pro will have to interpolate. The best results often come at 50% speed or higher, because the interpolation rate in that case is 1:1 or less.
When and why to build render Preview files for Optical Flow
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. To really see the full result of Optical Flow Time Remapping, you should build render preview files. Once you apply Optical Flow, the render bar will turn red to let you know that it needs rendering for real time performance, even if you have Hardware Rendering enabled. If you park the playhead over any Optical Flow interpolated frames, the high quality frame will draw in after about a second if you’re in GPU rendering mode, and a little longer if you’re in Software Only rendering mode. This is a good way to make sure the interpolation is applied and to see what the interpolated frame actually looks like. Pressing ‘Enter’ with the timeline selected is the shortcut for ‘Render Effects In to Out’ (or ‘Render Effects in Work Area’ if the Work Area bar is enabled), so that is the quickest way to render and playback your optical flow speed changes.
Interpolation of Frame Rate Changes During Export
When preparing to export media within the Export Settings dialog, you will also notice a new pulldown dialog for “Time Interpolation” which offers the same three options you see you see within the Clip Menu > Time Interpolation flyout menu.
Note that this option is not related to the Time Interpolation setting for speed and frame-rate changes on a clip in the main app. The two settings operate independently. The setting in the export dialog allows you to change the frame rate of the exported file by leveraging Optical Flow to interpolate the missing frames for you. For example, if you have footage that is 30 fps and you want it to be 60 fps without simply repeating every frame, then you may choose to use this option on export.