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

[Gyan Doshi]

On 14-10-2020 02:23 pm, 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(-)
>
> diff --git a/fftools/ffmpeg_opt.c b/fftools/ffmpeg_opt.c
> index a0d1b06f2d..afef23919c 100644
> --- a/fftools/ffmpeg_opt.c
> +++ b/fftools/ffmpeg_opt.c
> @@ -19,6 +19,7 @@
>    * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
>    */
>   
> +#include <float.h>
>   #include <stdint.h>
>   
>   #include "ffmpeg.h"
> @@ -2208,15 +2209,27 @@ static int open_output_file(OptionsContext *o, const char *filename)
>           char *subtitle_codec_name = NULL;
>           /* pick the "best" stream of each type */
>   
> -        /* video: highest resolution */
> +        /* video */
>           if (!o->video_disable && av_guess_codec(oc->oformat, NULL, filename, NULL, AVMEDIA_TYPE_VIDEO) != AV_CODEC_ID_NONE) {
> -            int best_score = 0, idx = -1;
> +            double best_score = 0.0;
> +            int idx = -1;
>               int qcr = avformat_query_codec(oc->oformat, oc->oformat->video_codec, 0);
>               for (i = 0; i < nb_input_streams; i++) {
> -                int score;
> +                double score;
>                   ist = input_streams[i];
> -                score = ist->st->codecpar->width * ist->st->codecpar->height + 100000000*!!ist->st->codec_info_nb_frames
> -                           + 5000000*!!(ist->st->disposition & AV_DISPOSITION_DEFAULT);
> +
> +                /* base score is just the area in pixels */
> +                score = (double)ist->st->codecpar->width * ist->st->codecpar->height;
> +                /* add a fractional part favoring higher bitrate among same-area streams */
> +                if (ist->st->codecpar->bit_rate) {
> +                    const double bitrate_max = 100e6; // cap at 100Mb/s
> +                    const double bitrate = FFMIN(ist->st->codecpar->bit_rate, bitrate_max - 1.0);
> +                    score += bitrate / bitrate_max;
> +                }

> +                /* default streams get max score */
> +                if (ist->st->disposition & AV_DISPOSITION_DEFAULT)
> +                    score = DBL_MAX;

This is actually a mistake that shouldn't ever been committed.

A default disposition only has relevance among companion video streams 
from the same input. It does and should not apply to inter-input 
comparisons.
Only a handful of demuxers set disposition. Default audio selection is 
already broken with respect to its documented behaviour for, I believe, 
a couple of years now.

The saner way is to select best stream from each input and then select 
from that shortlist without considering disposition.


> +
>                   if (ist->user_set_discard == AVDISCARD_ALL)
>                       continue;
>                   if((qcr!=MKTAG('A', 'P', 'I', 'C')) && (ist->st->disposition & AV_DISPOSITION_ATTACHED_PIC))

Regards,
Gyan