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

[Marton Balint]

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.

Regards,
Marton