Interframe compression
Also known as: inter-frame compression, P-frame, B-frame, temporal compression
Interframe compression shrinks video by storing only what changes between frames instead of a full picture each time. After a keyframe, later frames (P-frames and B-frames) record motion and differences, which is how video files stay far smaller than a folder of separate images.
- Stores differences between frames, not full images
- P-frames and B-frames predict from keyframes
- Low-motion footage compresses the most
Storing change, not whole frames
Most consecutive video frames are nearly identical, so encoding each one in full would waste space. Interframe (temporal) compression keeps a full keyframe, then describes the frames that follow as differences from earlier or later frames.
These predicted frames come in two kinds: P-frames reference a previous frame, and B-frames reference both previous and upcoming frames for even tighter compression. Together they let video carry smooth motion in a fraction of the space.
Why it matters for storage
Interframe compression is the main reason a minute of video is dramatically smaller than a minute of full-resolution stills would be. Footage with little movement — a talking head, a static scene — compresses especially well because frame-to-frame differences are tiny.
High-motion or noisy footage compresses less, since more pixels change every frame. That, along with bitrate and resolution, is why some clips stay large even after a transcode.