P-Frame (Predictive Frame)
Definition
A frame encoded as a set of changes relative to a preceding reference frame. P-frames are smaller than I-frames because they only store differences. An unexpected burst of large P-frames or a prediction error at a join point is a signature of editing.
- Encoding basis
- Difference from a preceding reference frame
- Compared to I-frame
- Smaller, since only changes are stored
- Compression family
- Used in MPEG, H.264, H.265
- Forensic signature
- Unusual bursts or prediction errors indicate editing
Common questions
Why do editing splices tend to disrupt P-frames rather than I-frames?+
A P-frame depends on the frame before it, so when footage from two different sources is joined, the reference frame the P-frame expects no longer matches, producing a decoding error or an abnormally large P-frame where a normal one should be, which analysts use as an editing marker.
How does frame deletion detection actually use the P-frame pattern?+
Video codecs typically follow a repeating I-frame and P-frame pattern (a group of pictures), so deleting frames disrupts that regular spacing. Analysts look for irregular gaps or unexpected extra I-frames within the group of pictures structure as evidence that frames were removed.
Related terms
- I-Frame (Intra-Coded Frame)
- A video frame that is encoded entirely from its own pixel data, without reference to any other frame. I-frames mark the start...
- Bitrate Anomaly
- A departure from the expected relationship between scene complexity and compressed file size. Sudden drops, plateaux, or spikes in the per-second bitrate...
- Double Compression
- The condition in which a video has been encoded at least twice through a lossy codec. The term is also used loosely...
- Group of Pictures (GOP)
- The periodic structure of I-frames, P-frames, and B-frames in a compressed video stream. Each encoding generation imposes its own GOP structure. When...
- Hash Verification
- The process of computing a cryptographic hash (SHA-256 or equivalent) of the exhibit file and comparing it against a previously recorded value...
- Inter-Frame Residual
- The pixel-level difference between a predicted frame and the actual frame after decoding. In unmodified video, residuals at scene cuts are elevated...
- Motion Vector Consistency
- In unmodified video, motion vectors across consecutive frames track the same objects and follow smooth trajectories. A deleted or inserted frame breaks...
- Presentation Timestamp (PTS)
- A value stored in the container for each packet indicating when that frame should be displayed. In an unmodified recording, PTS values...
- Re-Recording Artefact
- A visual or statistical signature introduced by pointing a camera at a screen playing back footage. Includes moiré patterns, screen reflections, temporal...
Explained in these topics
- Video Authentication and Integrity Verification
- Detecting Frame Deletion and Insertion in VideoA frame encoded as the difference from the preceding reference frame. Deleting any frame that a P-frame references produces large, anomalous residuals in that...