What is a Group of Pictures?
A group of pictures, or GOP, is a sequence of coded video frames arranged according to a codec’s prediction structure. It typically includes an intra frame and related predicted frames.
How Groups of Pictures work
A GOP defines the dependency pattern among successive coded pictures, including independently coded pictures and frames predicted from earlier or later references. Display order may differ from coded order when bidirectionally predicted pictures are present. Closed GOPs prevent references from crossing their boundary, whereas open GOPs permit some cross-boundary prediction. Encoders coordinate this structure with segment duration, scene changes, random access, and rendition switching.
Key facts
- 1I-frames and IDR frames are not interchangeable terms: an I-frame is intra-coded, while an IDR picture additionally prevents later pictures from referring to pictures before the refresh point.
- 2B-frames can improve compression by using references on both temporal sides, but they may require frame reordering and buffering, which matters for real-time encoders and low-latency playback.
- 3Adaptive renditions generally need compatible random-access points at matching timestamps; otherwise a player switching variants may lack the reference pictures required to decode cleanly.
When Groups of Pictures matter
Developers tune GOP length to balance compression efficiency against seeking, error recovery, and live latency. Long GOPs save bitrate but can delay random access and amplify damage from lost data.