Re: [FFmpeg-devel] [PATCH 5/7] avformat/matroskaenc: write a MaxBlockAdditionID element

[Anton Khirnov <anton@khirnov.net>]

Quoting James Almer (2023-03-21 18:06:35)
> A non zero value is mandatory for Matroska if the track has blocks with BlockAdditions.
> 
> Signed-off-by: James Almer <jamrial@gmail.com>
> ---
>  libavformat/matroskaenc.c | 36 ++++++++++++++++++++++++++++++++----
>  1 file changed, 32 insertions(+), 4 deletions(-)
> 
> diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
> index 46f4331a18..0687d9c32e 100644
> --- a/libavformat/matroskaenc.c
> +++ b/libavformat/matroskaenc.c
> @@ -188,6 +188,8 @@ typedef struct mkv_track {
>      int64_t         last_timestamp;
>      int64_t         duration;
>      int64_t         duration_offset;
> +    uint64_t        max_blockaddid;
> +    int64_t         blockadditionmapping_offset;
>      int             codecpriv_offset;
>      unsigned        codecpriv_size;     ///< size reserved for CodecPrivate excluding header+length field
>      int64_t         ts_offset;
> @@ -1597,12 +1599,21 @@ static int mkv_write_stereo_mode(AVFormatContext *s, EbmlWriter *writer,
>      return 0;
>  }
>  
> -static void mkv_write_dovi(AVFormatContext *s, AVIOContext *pb, AVStream *st)
> +static void mkv_write_blockadditionmapping(AVFormatContext *s, MatroskaMuxContext *mkv,
> +                                           AVIOContext *pb, mkv_track *track, AVStream *st)
>  {
>  #if CONFIG_MATROSKA_MUXER
>      AVDOVIDecoderConfigurationRecord *dovi = (AVDOVIDecoderConfigurationRecord *)
>                                               av_stream_get_side_data(st, AV_PKT_DATA_DOVI_CONF, NULL);
>  
> +    if (IS_SEEKABLE(s->pb, mkv)) {
> +        track->blockadditionmapping_offset = avio_tell(pb);
> +        // We can't know at this point if there will be a block with BlockAdditions, so
> +        // we either write the default value here, or a void element. Either of them will
> +        // be overwritten when finishing the track.

IIRC matroska uints are variable-size, so the value you write later may
have a longer representation than the dummy placeholder you're writing
here.
Or am I wrong?

-- 
Anton Khirnov
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