P-frame
A P-frame (predicted frame) is a compressed video frame that stores only the changes from an earlier frame instead of the full picture. P-frames use motion compensation to describe how parts of the image moved. Because they reference earlier frames, P-frames are much smaller than I-frames. A decoder needs the earlier reference frame before it can show a P-frame.
Why it matters
P-frames carry most of the savings in everyday video, which keeps storage and delivery costs down. They also create dependency chains: if a reference frame is lost or damaged, errors carry forward until the next I-frame. Balancing P-frames and keyframes is a trade between file size and resilience.
How it works
- The encoder takes an earlier decoded frame as the reference.
- Motion estimation finds where blocks moved, and motion compensation builds a predicted picture from the reference.
- Only the motion vectors and the small difference from the real frame are stored in the P-frame.
- Unchanged areas are marked as skipped, which costs almost nothing in still scenes like slides.
P-frame vs B-frame: what is the difference?
A P-frame predicts only from earlier frames, while a B-frame can predict from both earlier and later frames. B-frames usually compress further, but they force the encoder to send frames out of order, which adds a little delay. P-frames keep frames in display order, so they suit low-delay live and real-time video.
Why does my video show smearing or green blocks after a glitch?
Smearing after a glitch usually means a reference frame was lost or corrupted. Every P-frame after it builds on the bad picture, so the error spreads and moves with the scene. It clears up at the next I-frame, which starts fresh. Shorter keyframe intervals limit how long such errors last.
Where it's used
Online learning platforms and corporate training teams stream mostly static slides and talking heads, where P-frames keep files tiny because little changes between frames. Video call apps rely on them for low delay.
