[FFmpeg-devel] [PATCH 2/3] ffmpeg.c: refine picking default video stream

[Anton Khirnov]

Quoting James Almer (2020-10-14 19:26:51)
> On 10/14/2020 2:09 PM, Michael Niedermayer wrote:
> > On Wed, Oct 14, 2020 at 10:53:10AM +0200, Anton Khirnov wrote:
> >> Use a floating-point score value to take into account bitrate, when
> >> multiple streams with the same resolution are present.
> >>
> >> Stop accessing private AVStream.codec_info_nb_frames field, as the
> >> sample in question
> >> http://samples.ffmpeg.org/ffmpeg-bugs/trac/ticket2639/Thailand-Wave.wmv
> >> is now handled by the above change.
> >> ---
> >>  fftools/ffmpeg_opt.c | 23 ++++++++++++++++++-----
> >>  1 file changed, 18 insertions(+), 5 deletions(-)
> > 
> > Breaks:
> > -i tickets/4496/08_lect_01.rm  file.avi
> > 
> > as refernce here is what ffmpeg shows about the streams:
> >     Stream #0:0: Audio: sipr (sipr / 0x72706973), 8000 Hz, mono, flt, 4 kb/s
> >     Stream #0:1: Audio: sipr (sipr / 0x72706973), 8000 Hz, mono, flt, 8 kb/s
> >     Stream #0:2: Audio: cook (cook / 0x6B6F6F63), 22050 Hz, mono, fltp, 4 kb/s
> >     Stream #0:3: Audio: sipr (sipr / 0x72706973), 8000 Hz, mono, flt, 4 kb/s
> >     Stream #0:4: Video: rv20 (RV20 / 0x30325652), yuv420p, 320x240, 252 kb/s, 15 fps, 15 tbr, 1k tbn, 1k tbc
> >     Stream #0:5: Video: rv20 (RV20 / 0x30325652), yuv420p, 320x240, 252 kb/s, 15 fps, 15 tbr, 1k tbn, 1k tbc
> >     Stream #0:6: Video: rv20 (RV20 / 0x30325652), yuv420p, 320x240, 252 kb/s, 15 fps, 15 tbr, 1k tbn, 1k tbc
> >     Stream #0:7: Video: rv20 (RV20 / 0x30325652), yuv420p, 320x240, 252 kb/s, 15 fps, 15 tbr, 1k tbn, 1k tbc
> >     Stream #0:8: Video: rv20 (RV20 / 0x30325652), yuv420p, 320x240, 252 kb/s, 15 fps, 15 tbr, 1k tbn, 1k tbc
> >     Stream #0:9: Video: rv20 (RV20 / 0x30325652), yuv420p, 320x240, 252 kb/s, 15 fps, 15 tbr, 1k tbn, 1k tbc
> >     Stream #0:10: Video: rv20 (RV20 / 0x30325652), yuv420p, 320x240, 252 kb/s, 15 fps, 15 tbr, 1k tbn, 1k tbc
> >     Stream #0:11: Video: rv20 (RV20 / 0x30325652), yuv420p, 320x240, 252 kb/s, 15 fps, 15 tbr, 1k tbn, 1k tbc
> >     Stream #0:12: Video: rv20 (RV20 / 0x30325652), yuv420p, 320x240, 252 kb/s, 15 fps, 15 tbr, 1k tbn, 1k tbc
> >     Stream #0:13: Video: rv20 (RV20 / 0x30325652), yuv420p, 320x240, 252 kb/s, 15 fps, 15 tbr, 1k tbn, 1k tbc
> > 
> > sample should be here according to the ticket:
> > https://samples.ffmpeg.org/ffmpeg-bugs/trac/ticket2152/
> > 
> > I dont think you can reliably detect non empty streams from the header alone
> > in every case.
> > 
> > thx
> 
> This whole chunk of code is apparently done because
> av_find_best_stream() can't be used here (The list of input streams is
> in a custom struct InputStream instead of an AVFormatContext).
> codec_info_nb_frames is evidently needed to detect non-empty streams, so
> either making it or a replacement public or hand crafting a dummy
> AVFormatContext with the input streams then using av_find_best_stream()
> are two options for this.
> 
> I personally think the latter option is a good solution.
> av_find_best_stream() does not seem to care about its contents beyond
> nb_streams and the streams list.

That seems immensely hacky to me tbh.

I am also not very comfortable with making codec_info_nb_frames
officially public, as that hardcodes internal workings of
find_stream_info() into the public API, making it harder to change
later.

More generally, I do not think it is reasonable to demand that the
automatic mapping code in ffmpeg.c magically choose the best stream for
all pathological cases. Files with empty streams are IMO pathological,
and the user can always override the mapping manually.

-- 
Anton Khirnov