I-Frame
Definition
An intra-coded frame: a self-contained image compressed without reference to any other frame, like a JPEG. I-frames are the only random-access entry points in a video stream.
- Type
- Intra-coded frame
- Compression basis
- Self-contained, compressed like a still JPEG image
- Role
- Random-access entry point in a video stream
- Contrast
- P-frames and B-frames reference other frames instead
Common questions
Why do investigators favor extracting stills from I-frames when reviewing CCTV footage?+
An I-frame carries complete, self-contained image data, so a still pulled from one is free of the interpolation artefacts and motion-compensation errors that can appear in a still reconstructed from a P-frame or B-frame.
How does I-frame spacing affect video forensic authentication?+
The interval between I-frames, known as the group of pictures length, is often characteristic of the recording device or software, so an inconsistent I-frame pattern in a file can be a sign of re-encoding or editing after the original recording.
Related terms
- Codec
- A coder/decoder algorithm that compresses and decompresses media data. H.264/AVC and H.265/HEVC are the dominant video codecs; AAC and MP3 are dominant...
- Container Format
- A file format (MP4, MKV, MOV, AVI, MXF) that multiplexes video, audio, and metadata tracks into a single file with a defined...
- GOP (Group of Pictures)
- The repeating sequence of I, P, and B frames in an inter-frame compressed video. GOP size (the distance between I-frames) is an...
- Perceptual Coding
- Lossy compression that exploits psychoacoustic or psychovisual masking to discard information the human senses are least sensitive to. MP3, AAC, and AC-3...
- Rewrapping
- Moving a compressed bitstream from one container to another without decoding it. Forensically equivalent to the original encoded stream; no new generation...