Group of Pictures (GOP)
Definition
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 a video is re-encoded, the second encoder's I-frames may fall at positions that reveal the prior generation's GOP period.
- Full name
- Group of Pictures
- Contains
- I-frames, P-frames, and B-frames
- Forensic relevance
- Re-encoding leaves a detectable second GOP structure
- Field
- Video and multimedia forensics
Common questions
Why does GOP structure matter for detecting a re-encoded video?+
Each encoding pass fixes I-frames at regular intervals set by that encoder's configuration. When a video is compressed a second time, the new I-frame positions rarely align with the original ones, and that misalignment leaves a periodic artefact examiners can measure to infer prior compression history.
Can GOP analysis alone prove a video has been tampered with?+
No. GOP structure indicates that a file has passed through more than one encoding generation, which can happen through legitimate re-saving, platform transcoding, or format conversion, so it does not on its own point to malicious editing. It is one indicator among several, not standalone proof of tampering.
Related terms
- DCT Coefficient Histogram
- A histogram of the discrete cosine transform coefficients across macroblocks or coding units in a frame or clip. After a single encode,...
- Double Compression
- The condition in which a video has been encoded at least twice through a lossy codec. The term is also used loosely...
- 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...
- 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...
- Intra-Refresh Artefact
- The elevated count of intra-coded macroblocks or CTUs at frame positions that correspond to the I-frame interval of a previous encoding generation....
- Macroblock (MB) / Coding Tree Unit (CTU)
- The basic spatial coding unit: 16x16 pixels in H.264 (macroblock) and up to 64x64 pixels in H.265 (CTU). Each unit is classified...
- 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...
- P-Frame (Predictive Frame)
- A frame encoded as a set of changes relative to a preceding reference frame. P-frames are smaller than I-frames because they only...
- 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...
- Quantisation Parameter (QP)
- A per-block or per-slice value that controls the coarseness of the DCT coefficient rounding step in H.264 and H.265. Higher QP means...
Explained in these topics
- Compression History Estimation in Video Streams
- Detecting Frame Deletion and Insertion in VideoThe repeating unit of inter-frame compressed video. A GOP begins with an I-frame (a complete image) followed by a fixed or near-fixed number of P-frames and B-...