On 9/9/2025 9:38 AM, Jerome Martinez via ffmpeg-devel wrote:
> diff --git a/libavformat/matroskaenc.c b/libavformat/matroskaenc.c
> index 8142d9125e..5339b6fd33 100644
> --- a/libavformat/matroskaenc.c
> +++ b/libavformat/matroskaenc.c
> @@ -1706,16 +1706,16 @@ static void mkv_write_blockadditionmapping(AVFormatContext *s, const MatroskaMux
>      const AVDOVIDecoderConfigurationRecord *dovi;
>      const AVPacketSideData *sd;
>  
> -    if (IS_SEEKABLE(s->pb, mkv)) {
> +    if (IS_SEEKABLE(s->pb, mkv) && par->codec_type == AVMEDIA_TYPE_VIDEO) {
>          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.
> -        put_ebml_uint(pb, MATROSKA_ID_TRACKMAXBLKADDID, 0);
>          if (par->codec_type == AVMEDIA_TYPE_VIDEO) {

This is now redundant. Did you intend this entire code to not be called 
for audio and subtitles?

>              // Similarly, reserve space for an eventual
>              // HDR10+ ITU T.35 metadata BlockAdditionMapping.
> -            put_ebml_void(pb, 3 /* BlockAdditionMapping */
> +            put_ebml_void(pb, 4 /* MaxBlockAdditionID */
> +                            + 3 /* BlockAdditionMapping */
>                              + 4 /* BlockAddIDValue */
>                              + 4 /* BlockAddIDType */);
>          }
> @@ -3294,7 +3294,7 @@ after_cues:
>              for (unsigned i = 0; i < s->nb_streams; i++) {
>                  const mkv_track *track = &mkv->tracks[i];
>  
> -                if (!track->max_blockaddid)
> +                if (!track->max_blockaddid || !track->blockadditionmapping_offset)
>                      continue;
>  
>                  // We reserved a single byte to write this value.