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

[James Almer]

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.