[FFmpeg-devel] [PATCH] fftools/ffmpeg_enc: Prevent duplicate A53 Closed Captions at frame rate up-conversion

[Devin Heitmueller]

Hi Artem,

On Fri, Jun 9, 2023 at 9:19 AM Artem Smorodin
<artem.smorodin at telestream.net> wrote:
>
> When up-converting frame rate, ffmpeg duplicates frames including side data. This causes duplicated characters in the output. This patch removes A53 side data from duplicated frames.
>
> Signed-off-by: Artem Smorodin <artem.smorodin at telestream.net>
> ---
>  fftools/ffmpeg_enc.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/fftools/ffmpeg_enc.c b/fftools/ffmpeg_enc.c
> index 2bf4782a9f..93d18034ca 100644
> --- a/fftools/ffmpeg_enc.c
> +++ b/fftools/ffmpeg_enc.c
> @@ -1104,6 +1104,9 @@ static void do_video_out(OutputFile *of, OutputStream *ost, AVFrame *frame)
>          else if (ret < 0)
>              exit_program(1);
>
> +        //Prevent duplicate Closed Captions at frame rate up-conversion
> +        av_frame_remove_side_data(in_picture, AV_FRAME_DATA_A53_CC);
> +
>          e->next_pts++;
>          e->vsync_frame_number++;
>      }
> --
> 2.25.1
>
> Disclaimer

Unfortunately, the patch needed is more complicated than what you're
proposed.  You can't simply drop the caption from the duplicated
frame, but rather the CC data needs to be rewritten in existing frames
with the appropriate cc_count and distribution of 608 pairs.

I've reworked the logic to behave correctly if you use the "fps" video
filter, but had forgotten that using "-r" doesn't use that filter.

Anton, is there any good reason we couldn't rework the "-r" command
line argument to invoke the fps filter (like we do with scaling)
rather than re-implementing the same functionality inside of the
ffmpeg program itself?  It always seemed strange to me that it didn't
work that way from the beginning...

Artem, in the meantime you can avoid this problem if you specify "-vf
fps" on the command line rather than specifying "-r".

Devin

-- 
Devin Heitmueller, Senior Software Engineer
LTN Global Communications
o: +1 (301) 363-1001
w: https://ltnglobal.com  e: devin.heitmueller at ltnglobal.com