GOP的两种结构
MPEG的格式支持open GOP或者close GOP格式。Close GOP是指帧间的预测都是在GOP中进行的。而使用open GOP，后一个GOP会参考前一个GOP的信息。使用这种方式就大大降低了码率。
Close-GOP
By definition, closed GOPs cannot contain any frame that refers to a frame in the previous or next GOP.
closed GOP中的帧不可以参考其前后的其它GOP.
Closed GOPs created by Compressor always begin with an I-frame.
Closed GOP一般以I帧开头。
Open-GOP
Open-GOP：informal structure. 在一个GOP里面的某一帧在解码时要依赖于前一个GOP中的某一些帧，这种GOP结构叫做Open-GOP。一般码流里面含有B帧的时候才会出现Open-GOP。
Open GOPs are most efficient because they allow an extra B-frame in the GOP pattern.
Open GOPs start with a B-frame that is able to look at the last P-frame from the preceding GOP as well as the first I-frame of its own GOP.
open GOPs begin with one or more B-frames that reference the last P-frame of the previous GOP.
Open GOP以一个或多个B帧开始，参考之前GOP的P帧和当前GOP的I帧。
Open and Closed GOPs
Open GOPs generally provide slightly better compression than do closed GOPs of the same structure and size.
The illustration above shows that a closed GOP contains one more P-frame than does an open GOP of the same length. Since P-frames generally require more bits than do B-frames, the open GOP achieves slightly better compression.
同等条件下，Open GOP比closed GOP具有更高的压缩效率。
总结一句话: 判断码流是否open gop，可以通过以下的逻辑来判断:
B帧在I帧之后解码 但是在I帧之前显示。
H264中的I帧
主要有2种I帧：I（IDR帧）、I（非IDR I帧）。
I（IDR帧）
NALU_type = 0x65。
IDR帧是一种特殊的I帧，在解码IDR帧前，清空所有前后向参考缓冲区，IDR帧随后的所有帧不能参考IDR前面的任何帧。
IDR帧是视频安全的随机访问点，找到一个IDR帧可以丢弃前面所有帧数据而正确解码。
I（非IDR帧）
NALU_type = 0x61。
i帧是帧内预测帧，它的解码不依赖与任何其他帧。
区别
有些解码器没有区分这两种I帧，将所有的i帧都缺省的认为是IDR帧，这样会造成解码错误。
这两种i帧在open-GOP的结构中需要明确区分，而在close-GOP中可以视为相同。
Reference:
https://blog.csdn.net/soaringlee_fighting/article/details/79650820
https://blog.csdn.net/aokewood/article/details/6893699
http://www.360doc.com/content/12/0108/11/7923573_178053426.shtml
https://documentation.apple.com/en/compressor/usermanual/index.html#chapter=18%26section=5%26tasks=true
https://compressor-4.helpnox.com/en-us/compressor-4-user-manual/creating-mpeg-2-output-files/mpeg-2-reference-information/things-to-consider-when-choosing-a-gop-setting/open-and-closed-gops/
https://blog.csdn.net/yue_huang/article/details/73432141
