Closed GOP (group-of-pictures). (Warning: video jargon ahead. You can likely ignore this.) In closed-GOP video, all B frames reference other frames in the same GOP. Closed GOP always begins with an IDR (Instantaneous Decoder Refresh) frame. This means that every GOP can be played independently, without reference to another GOP. Standard input must be closed-GOP, which means that open-GOP video will be treated as non-standard and will be normalized to standard.