Keyframe Interpolation

keyframe interpolation

Editing software isn’t just about cutting shots together; there are also myriad effects and other tools to finish your show. How you use any of these can affect your viewer’s impressions.

Most effects tools aren’t just static controls. They can be changed over time via keyframes. For example, if you add a blur effect to a clip, you can animate how much blurring is applied over time by setting a starting keyframe (let’s call it frame A) and an ending keyframe (call it frame B).

Do you already use keyframes? Great! But I want to talk about keyframe interpolation, that is, how the path from A to B is calculated.

For example, if you move a graphic from off-screen (A) to on-screen (B), should that movement be linear? If so, if you animate that movement over 30 frames, each frame will show the same amount of movement. That approach works, but it may be jarring at the final keyframe—the graphic just stops. Think of riding in a car. When someone steps quickly on the brake to stop, it’s a bit jarring. But if the brake pedal is slowly depressed, you’ll come to a smooth stop.

keyframe interpolation
In Blackmagic Design’s DaVinci Resolve, the interpolation options appear as four buttons when you open the curve editor in the timeline.

“Ease-in” interpolation slows the motion of your graphic when it approaches keyframe B, eventually coming to a smooth stop. Think of “ease in” as meaning “as you approach the keyframe.”

Ease out smooths as you move from one keyframe to the next. Do you step on the gas quickly or slowly?

keyframe interpolation
In Adobe’s Premiere Pro, the interpolation options Ease In and Ease Out appear when you right-click on the keyframe.

Once you start experimenting with keyframe interpolation, you may notice that most controls available to you use keyframes. For instance, if you use an opacity control to animate the appearance of a graphic, you’ll see keyframes. But since there’s no motion involved in, say, opacity why do you care?

Here’s why: If you intend to bring in a bright graphic over a dark scene, you might want it to appear gradually. If you spend two-thirds of the time going from 0% opacity to 30% and then one-third of the time going from 30% to 100% opacity, the result might be less jarring than a simple dissolve.

Keyframe interpolation gives you more control over how things appear on screen.

 Michael Guncheon is an accomplished editor who has cut a wide range of projects, including music videos for Prince, a documentary on Genesis, and numerous commercials and corporate pieces. A partner at HDMG, a Minneapolis video production and post-production company, Guncheon has written several books on DSLRs and is the author of the Kodak Digital Photo Guide. He has presented his talk on shooting with HDSLRs at Twin Cities Public Television, WGBH in Boston, PBS in New York, the Hollywood Post Alliance and at the annual SMPTE conference in Hollywood.