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Two unexpected uses for Motion Interpolation

3 minute read

Gary HangoMotion Interpolation

If you’re a sports cinematographer and don’t have access to a high frame rate film or video camera, you’ve probably experimented with motion interpolation to create slow motion from video captured at a normal speed. Gary Hango looks at some new uses.

Some NLEs come with this effect built in, or 3rd party plugins can be installed with each giving you varying degrees of usable slo-mo.

Also, most FX/compositing applications now have this ability in their arsenal of effects.  But what if we’re not looking to create slow motion video?  Are there any practical uses for motion interpolation?  The answer is yes.  We’ll take a look at a few examples below.

Ruined footage

You’ve just come back from documenting that once in a life-time event using a tape based camera and upon capturing the footage to your computer you discover that the premium tape you purchased was not so premium.  You get one of those dreaded tape drop-outs at the most inopportune time.  Footage ruined, day ruined, reputation ruined?  You could try duplicating a few frames to replace the missing ones but if there’s a lot of movement, you will notice the discontinuity.  So what do you do?  Motion interpolation might save you.  Just tell the motion interpolation software to re-create the missing frame(s) using the existing good frames on either side of the glitch.  The software compares the locations of objects in the good frames and “interpolates” where those objects would be in the missing frames and re-creates them.  If you look at a still image of the newly created frames you might see some artifacts where the interpolation isn’t perfect, but as long as they aren’t too glaring, when played back at 1/24, 1/30, etc. of a second, they will most likely never be noticed.  The following video illustrates this technique on the most difficult of subject matter for motion interpolation, fast moving objects, water, and panning.

This application of motion interpolation can also be used where, for some reason, you need to delete several good frames but must maintain a continuous shot (flash bulb, object in frame, quick camera bump, etc.).

Quality conversion

Another practical use is converting frame rates.  There are simple ways to convert frame rates such as frame skipping, frame duplicating or adding pulldown flags to the video stream.  All these methods introduce a discontinuity in the motion of objects or the camera.  What you will see is a jerkiness to objects and pans.  Motion interpolation can be used to smooth out the motion of objects and camera moves when you convert from one frame rate to another.
Let’s say you have a 720p 60fps project and have a mix of 60fps footage and 30fps footage.  You could just duplicate every frame of the 30fps footage but it will stand out when compared to the 60fps footage.  Using motion interpolation you can create one new frame between the 30fps frames, thus making smooth 60fps footage.  This is exactly what some of the new HDTV’s can do.  Instead of just duplicating frames and displaying them at a higher frame rate, on the fly, they create one, two or three new frames between the existing frames using motion interpolation and display all these at the higher rate.  This results in a smooth, almost hyper video look to any footage, including films shot at 24fps.

Film look

And speaking of 24fps, this speed has been deemed one of the earmarks of what makes footage have that “film look”.  Whether you agree with this or not, many film makers want to release their work using this frame rate.  But what if your camcorder only shoots in 60i, 60p or 30p, or you need to use footage shot in these frame rates in a 24fps project?  With 60i or 60p footage you can do frame skipping by using only 2 of every 5 frames.  But, again, we have to deal with the jerkiness of skipping an odd number of frames.  So how do we get smooth 24fps video from 30p, 60p or 60i footage (60i footage should to be deinterlaced to 60p or 30p before being interpolated)?  We motion interpolate to a higher frame rate that’s evenly divisible by both frame rates.  For this situation, that magic number is 120fps, evenly divisible by 30, 60 and 24.  So we end up with 120fps footage and then select every 5th frame to get smooth 24fps video.  The following video compares the results of frame skipping and motion interpolating 60i footage to produce 24fps footage.

As you’ve now seen, there really are practical uses of motion interpolation.  And you thought it was just a gimmick, whose only use was as a poor man’s high speed camera or a selling point for HDTVs.

Tags: Post & VFX