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

[Artem Smorodin]

Hi David,

I think the reworking of the "-r" option is a breaking change.

I understand that output won't conform to the spec, but at least CC won't be broken. For example libx264->a53cc option is 1 by default, so any h264 output with CC with frame rate up-conversion will be broken.
  
The behavior that adds this patch was there before, but was removed after about 5.1.1, so I think it's safe to restore it until a better solution is implemented.

Regarding a better solution:
I don't think we should change the behavior of the "-r" option, instead I suggest moving ccfifo from libavfilter to libavutil and using it in the ffmpeg program.

Thank you
Artem Smorodin

-----Original Message-----
From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of Devin Heitmueller
Sent: Friday, June 9, 2023 6:13 PM
To: FFmpeg development discussions and patches <ffmpeg-devel at ffmpeg.org>
Cc: Anton Khirnov <anton at khirnov.net>
Subject: [External Sender] Re: [FFmpeg-devel] [PATCH] fftools/ffmpeg_enc: Prevent duplicate A53 Closed Captions at frame rate up-conversion

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
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