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

[Tao Zhang]

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 want to request for your comments on separation of multiple outputs'
encoding in ffmpeg. Refactoring ffmpeg.c seems not a tiny task. Is it
acceptable to create a pseudo encoder which run the actual encoding in
the separate thread?

Best Regards