Wirebook › Glossary › Keyframe
What is a keyframe in a video?
In one sentence: A complete picture saved every so often inside a video, with the pictures in between stored only as the changes since.
A little more
A video is a fast flipbook, but most of its pictures are never stored whole. Every so often the codec saves one complete picture, a keyframe. The pictures in between are stored as changes: this patch moved left a bit, that corner got brighter. Because most of a scene stays the same from one moment to the next, those in-between pictures are tiny, which is a big part of why video files are so much smaller than a pile of photos.
Keyframes cost the most space, so encoders usually place one every second or two, and at every scene cut. They're also where a player can start: when you skip ahead in a video, it jumps to the nearest keyframe and rebuilds from there, and when a stream glitches, the picture often smears until the next keyframe arrives and wipes the slate clean. A GIF has no in-between trick like this, which is one reason it ends up so big.
See it in action
- BonusWhat happens when you squish a video too much?Squish a real (tiny) picture the way phones squish video: watch thin whiskers crumble first, then 8x8 blocks appear, then a splat.10 min · interactive
- BonusWhy does a video call go blurry when the connection is slow?Dial a pretend video call's bandwidth thinner, spend a tiny 10-bit budget on eyes vs mouth vs wallpaper to still read a facial expression, then reveal the clear face.7 min · interactive
- BonusWhy does a video go blurry, then sharp?You press play and the picture is smudgy for a second, then it snaps sharp.8 min · interactive
Do it for real
Free grown-up tools and guides on Tooltern, our sister site. They work in your browser and nothing is uploaded.