Open gop and Closed gop of H264

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本文详细解释了视频编码中GOP（Group of Pictures）的概念，包括闭合GOP和开放GOP的不同编码顺序及其对解码过程的影响。重点阐述了B帧在不同GOP结构中的位置变化以及IDR帧如何防止使用开放GOP风格的B帧。

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In temporal order :

Code:

closed gop : PbbbPbbbPI
open   gop : PbbbPbbbbI

In coding order :

Code:

closed gop : PPbbbPbbbI
open   gop : PPbbbIbbbb

Note how, in coding order (ie, in the bitstream), the b frames temporally before the I frame are actually after the I frame. That's why the gop is called open. If you start decoding at the I, you won't be able to decode the b (you'll have to skip them), but it's no big deal because they are temporally before the I. IDR prevents you to use bframes in the open gop fashion because it flushes the reference buffer (thus, were you to use an IDR in an open gop situation, the bframes wouldn't not be able to reference the preceding P : they wouldn't be bframes anymore).