[FFmpeg-trac] #11055(avcodec:reopened): OGOP (Open GOP) in certain conditions may cause missing frames of decoding

[FFmpeg]

#11055: OGOP (Open GOP) in certain conditions may cause missing frames of decoding
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             Reporter:  markfilipak  |                    Owner:  (none)
                 Type:  defect       |                   Status:  reopened
             Priority:  normal       |                Component:  avcodec
              Version:  git-master   |               Resolution:
             Keywords:  everything   |               Blocked By:
  OGOP                               |
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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Comment (by markfilipak):

 Replying to [comment:203 Balling]:
 > "pdr0 & Balling at
 > trac.ffmpeg.org hint that key frames are specific I-frames"
 >
 > Keyframes are only when you clean start from it. In Open GOP by
 definition you can have a situation where some after I frames or its
 slices you can have dependence on frames before that I frame ...

 Then it can't be an I-frame. I've seen some people 'talk' about SI-frames.
 Show me a specification on it. Show me an MPEG tag for it.

 Thank you for the link to the bitmovin article. It seemed like hogwash to
 me. It seemed totally conceptual, notional, not practical, not specific,
 not real.

 I'll give my latest take on IDR frames: They don't exist, per se. What I
 think an IDR frame is, is an ordinary I-frame, open or closed GOP, that
 follows a closed GOP.


 {{{
  closed GOP _____ _________ An IDR frame
 DTS order ..B B P I P B B P..
             ^^^^^
    open GOP _____ _________ Not an IDR frame
 DTS order   ..B B I P B B P..
               ^^^
               The difference is here
 }}}


 Now, in MPEG-2 TSes, there may be an MPEG tag that forces the previous GOP
 to _become_ a closed GOP, but I doubt it because that would require all
 decoders to buffer all B-frames up to the next P- or I-frame and to then
 throw them away if the next I-frame said "IDR"! That defeats the purpose
 of open GOP, and, besides that, what's on the screen in the meantime?
 Nothing? A picture repeat? No, no, no.
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/11055#comment:204>
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