[FFmpeg-devel] [PATCH v2] avformat/matroska: Write WebVTT subtitles according to MKV specs
Andreas Rheinhardt
andreas.rheinhardt at outlook.com
Tue Mar 14 05:37:17 EET 2023
Gwyneth Morgan:
> When writing WebMs, FFmpeg muxes WebVTT subtitles with the D_WEBVTT/*
> codec tags from the WebM specs [1]. However, it does the same when
> muxing MKV files, and the Matroska specifications instead use
> S_TEXT/WEBVTT tags for WebVTT subtitles [2], which FFmpeg currently
> doesn't understand. Support reading MKVs using either tag, write regular
> MKVs with S_TEXT/WEBVTT, and write WebMs with the D_WEBVTT/* tags we
> already use.
>
> [1]: https://www.webmproject.org/docs/container/
> [2]: https://matroska.org/technical/codec_specs.html#s_textwebvtt
>
> Signed-off-by: Gwyneth Morgan <gwymor at tilde.club>
> Fixes: https://trac.ffmpeg.org/ticket/5641
> ---
> v2: Rebase as later changes in ffmpeg.git conflicted with this patch.
>
> libavformat/matroska.c | 11 ++++++-----
> 1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/libavformat/matroska.c b/libavformat/matroska.c
> index 79b2d09..d9e5ff9 100644
> --- a/libavformat/matroska.c
> +++ b/libavformat/matroska.c
> @@ -60,16 +60,12 @@ const CodecTags ff_mkv_codec_tags[]={
> {"A_VORBIS" , AV_CODEC_ID_VORBIS},
> {"A_WAVPACK4" , AV_CODEC_ID_WAVPACK},
>
> - {"D_WEBVTT/SUBTITLES" , AV_CODEC_ID_WEBVTT},
> - {"D_WEBVTT/CAPTIONS" , AV_CODEC_ID_WEBVTT},
> - {"D_WEBVTT/DESCRIPTIONS", AV_CODEC_ID_WEBVTT},
> - {"D_WEBVTT/METADATA" , AV_CODEC_ID_WEBVTT},
> -
> {"S_TEXT/UTF8" , AV_CODEC_ID_SUBRIP},
> {"S_TEXT/UTF8" , AV_CODEC_ID_TEXT},
> {"S_TEXT/ASCII" , AV_CODEC_ID_TEXT},
> {"S_TEXT/ASS" , AV_CODEC_ID_ASS},
> {"S_TEXT/SSA" , AV_CODEC_ID_ASS},
> + {"S_TEXT/WEBVTT" , AV_CODEC_ID_WEBVTT},
> {"S_ASS" , AV_CODEC_ID_ASS},
> {"S_SSA" , AV_CODEC_ID_ASS},
> {"S_VOBSUB" , AV_CODEC_ID_DVD_SUBTITLE},
> @@ -78,6 +74,11 @@ const CodecTags ff_mkv_codec_tags[]={
> {"S_HDMV/TEXTST" , AV_CODEC_ID_HDMV_TEXT_SUBTITLE},
> {"S_ARIBSUB" , AV_CODEC_ID_ARIB_CAPTION},
>
> + {"D_WEBVTT/SUBTITLES" , AV_CODEC_ID_WEBVTT},
> + {"D_WEBVTT/CAPTIONS" , AV_CODEC_ID_WEBVTT},
> + {"D_WEBVTT/DESCRIPTIONS", AV_CODEC_ID_WEBVTT},
> + {"D_WEBVTT/METADATA" , AV_CODEC_ID_WEBVTT},
> +
> {"V_AV1" , AV_CODEC_ID_AV1},
> {"V_AVS2" , AV_CODEC_ID_AVS2},
> {"V_AVS3" , AV_CODEC_ID_AVS3},
The reason we write it the way we do is that webvtt is muxed differently
in Matroska than WebM. This needs to be fixed, too, before S_TEXT/WEBVTT
can be used for Matroska.
- Andreas
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