[FFmpeg-devel] [RFC]separation of multiple outputs' encoding

[Tao Zhang]

Marton Balint <cus at passwd.hu> 于2020年5月15日周五 上午2:33写道：
>
>
>
> On Thu, 14 May 2020, Tao Zhang wrote:
>
> > Hi,
> > FFmpeg supports multiple outputs created out of the same input in the
> > same process like
> > ffmpeg -i input -filter_complex '[0:v]yadif,split=3[out1][out2][out3]' \
> >        -map '[out1]' -s 1280x720 -acodec … -vcodec … output1 \
> >        -map '[out2]' -s 640x480  -acodec … -vcodec … output2 \
> >        -map '[out3]' -s 320x240  -acodec … -vcodec … output3
> > In ffmpeg.c, multiple outputs are processed sequentially like
> > for (i = 0; i < nb_output_streams; i++)
> >    encoding one frame;
> >
> > As below wiki noted, the slowest encoder will slow down the whole
> > process. Some encoders (like libx264) perform their encoding "threaded
> > and in the background", but x264_encoder_encode still cost some time.
> > And it is noticeable when multiple x264_encoder_encodes run in the same thread.
> > https://trac.ffmpeg.org/wiki/Creating%20multiple%20outputs
> >
> > For API users, they can use separate thread for multiple encoding in
> > their own code. But is there any way to rescue command line users?
>
> I am not sure I understand what you want. Processing will still be limited
> to the slowest encoder, because input processing will still be driven by
> the slowest encoder, even if the encoders run in separate threads.
>
> Buffering encoder input frames is not an option, because input frames are
> uncompressed, therefore huge. So if you want the faster x264 encoder to
> finish faster, you have to drive it from a different input, ultimately you
> should run 3 separate encode processes and accept that decoding and yadif
> processing happens in all 3 cases separately causing some overhead.
Currently FFmpeg works like below:
main thread:
encoding frame 1 for output 1; encoding frame 1 for output 2; encoding
frame 1 for output 3; encoding frame 2 for output 1; encoding frame 2
for output 2; encoding frame 2 for output 3;...

What I want to do is
thread 1:
encoding frame 1 for output 1; encoding frame 2 for output 1;...
thread 2:
encoding frame 1 for output 2; encoding frame 2 for output 2;...
thread 3:
encoding frame 1 for output 3; encoding frame 2 for output 3;...

As the input frame is ref counted, so the buffering memory is not
huge? Also we can set the max buffering frame limit.

Forgive me for not using multiple separate processes, because mostly
inputs are network streaming and bandwidth is expensive, a few inputs
are capture card working as exclusive device.
>
> Regards,
> Marton
> _______________________________________________
> ffmpeg-devel mailing list
> ffmpeg-devel at ffmpeg.org
> https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
>
> To unsubscribe, visit link above, or email
> ffmpeg-devel-request at ffmpeg.org with subject "unsubscribe".