Algorithm-generated frames inserted between originals — creates slow-motion, higher frame rates, or fluid motion repair. Quality depends on scene complexity and optical flow accuracy.
You're shooting an action scene at 24fps but suddenly need 60fps for a smooth slow-motion effect in the edit — or your camera movement is jerky because intermediate frames are missing. This is where interpolation comes in: a mathematical process that calculates missing frames between two existing images. The computer analyzes the motion vectors between Frame A and Frame B, estimates pixel displacement (motion estimation), and generates plausible intermediate frames. Applied correctly, it turns choppy footage into fluid motion — done incorrectly, it looks like a ghost floating through your scene.
In a practical workflow, this usually happens in your NLE or in dedicated VFX tools like After Effects, Nuke, or specialized frame rate converters. For slow-motion production, you often work with optical flow or block-matching algorithms — both attempt to track motion with pixel-level accuracy. Optical flow is more elegant but computationally intensive; block-matching is more robust for complex scenes but can produce artifacts when objects overlap or move out of frame. The problem: interpolation doesn't *invent* information — it guesses. With fast movements, occlusion, or camera pans, ghosting effects or blur artifacts occur that are immediately visible.
The critical limit is usually around 1.5x to 2x the original frame rate. If you double 24fps to 48fps, it often works acceptably. If you try to upscale 24fps to 120fps, every light and every edge turns into a mess. That's why ambitious slow-motion scenes are shot with true high-speed cameras (60fps+) — then no interpolation is needed, just the correct playback speed. Some DoPs also use interpolation for subtle stabilization: slight jitters from handheld shots disappear when the motion flow is smoothed. This is a gray area between technique and fakery, but completely normal in modern cinema.
Remember: Interpolation is craftsmanship, not magic. It works best with simple, predictable movements (camera pan, steady walk). For chaotic scenes (crowds, rain, fire) or extreme zooms, manual frame blending or true high-frame-rate recording is the more honest solution. And always expect to pull your hair out — rendering takes time.