A closed GOP is a Group of Pictures whose frames only reference frames inside itself — no frame in the GOP depends on a frame from the previous or next GOP. The opposite is an "open GOP", where the final B-frames may reference the I-frame of the next GOP for tighter compression. Closed GOPs are slightly larger but self-contained, which is exactly what modern streaming needs.
Why closed GOPs matter: adaptive bitrate streaming (HLS, DASH) chops video into segments, and each segment must be independently playable so the player can switch between bitrate renditions on the fly. If a segment relies on frames outside itself, the switch produces a visible glitch — broken motion, wrong colours, missing detail — until the player resyncs at the next clean point. Closed GOPs guarantee no such cross-segment dependencies. Every HLS or DASH segment starts with an idr (a strict I-frame that flushes all references) and contains only frames that depend on frames inside the same GOP.
For a product team, "closed GOP at the segment boundary" is a setting that should be always on for adaptive streaming pipelines. FFmpeg's -g (keyframe interval) and -force_key_frames flags handle this; encoder-level options like keyint/min-keyint in x264/x265 enforce it. The cost is roughly 2–5 % bitrate compared to open GOP — accepted as the price of clean rendition switching. Open GOP only survives in non-segmented scenarios: progressive downloads, archival masters, contribution feeds where every frame will be re-encoded downstream anyway.
