Keyframe (I-frame)
Also known as: I-frame, intra frame, keyframe video, GOP keyframe
A keyframe, or I-frame, is a fully self-contained video frame that holds a complete picture without referencing any other frame. Keyframes anchor compression and seeking, but because they store the whole image they are the largest frames in a video.
- Self-contained full-image frame
- Anchors seeking, cutting, and each GOP
- Largest frame type; spacing affects file size
Why keyframes exist
Video codecs do not store every frame as a full image. Instead they place a complete I-frame at intervals and describe the frames in between as changes from it. Each span from one keyframe to the next is a group of pictures (GOP).
Keyframes are also where you can cleanly seek or cut, because they need no prior frame to decode. That is why scrubbing a video jumps to the nearest keyframe and why precise trimming may snap to keyframe boundaries unless the editor re-encodes.
Keyframe spacing and file size
A keyframe carries a whole picture, so it is much heavier than the in-between frames that only record motion. More frequent keyframes improve seeking and resilience but enlarge the file; spacing them farther apart saves space at the cost of coarser seeking.
Encoders also insert a keyframe automatically at hard scene changes, since predicting from a totally different shot would be inefficient.