Re: [FFmpeg-devel] Flushing while decoding , but need already decoded frames

[Andrey Turkin <andrey.turkin@gmail.com>]

Have to say, this issue has been a long grievance of mine. There is no
reason to delay frames when the decoder is set up to ignore B frames
as there is no reordering to be done; ideally this should be
zero-delay case (packet goes in, frame goes out) yet the most common
decoders delay frames anyway, as if to decode B frames. Moreover, with
the "new" send/receive API I think there is no reason to delay frames
at all - a single send_packet could decode and queue multiple frames
to be received, so it makes sense to send frames as soon as possible -
yet that is not the case as well.

пт, 24 мая 2024 г. в 13:17, Andreas Rheinhardt <andreas.rheinhardt@outlook.com>:
>
> Michael Henrik Bodenhoff via ffmpeg-devel:
> > Hi ,
> >
> > my team recently had to abandon switching to using FFmpeg from specific decoder implementations (NvDEC, Intel Media SDK , IPP and quite a few codec specific decoders) because of big performance issues because of the way FFmpeg works….. or at least we think it is (we’re FFmpeg noobs 😃  )
> >
> > It's actually an issue we also had with Intel Media SDK,  leading us to pay Intel to extend Media SDK to do what we needed.
> >
> > Our product is a video surveillance system, and that means we have to decode a LOT of video streams simultaneously.
> >
> > For motion detection we want to only decode keyframes, and skip P and B frames , and that works fine with FFmpeg most of the time, except for when the video stream contains B frames.
> > Without B-Frames it’s really simple (simplified pseudocode) :
> >
> > while(true)
> > {
> >   receiveStreamCompleteFrame();
> >   If(KeyFrame)
> >   {
> >     avcodec_send_packet();
> >     if(avcodec_receive_frame()==0)
> >     {
> >        // do motion detection
> >     }
> >   }
> > }
> >
> > But! with B Frames FFmpeg doesn’t return keyframes when they are decoded, they are kept, and we can’t seem to flush them out. avcodec_flush_buffers allow us to continue to next keyframe, but it doesn’t seem to give us the keyframe we just gave to FFmpeg with avcodec_send_packet.
> >
> > while(true)
> > {
> >   receiveStreamCompleteFrame();
> >   If(KeyFrame)
> >   {
> >     avcodec_send_packet();
> >     if(avcodec_receive_frame()==0)
> >     {
> >        // do motion detection
> >     }
> >     Else
> >     {
> >       avcodec_flush_buffers();
> >       if(avcodec_receive_frame()==0)
> >       {
> >          // do motion detection
> >       }
> >     }
> >   }
> > }
> >
> > Calling avcodec_receive_frame after calling avcodec_flush_buffer results in -11 and no frame
> >
> > is there anyway around this ? And if not, could FFmpeg be made to have this functionality ?
> >
> > I tried contacting one of the FFmpeg consultants from https://ffmpeg.org/consulting.html but never got a response
> >
>
> Send your packet with the keyframe, send a NULL packet (to signal EOF),
> then the internally stored frames should be output by
> avcodec_receive_frame(). Then flush the decoder (to be able to send new
> packets to it).
>
> - Andreas
>
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