[FFmpeg-devel] [PATCH] avformat/mov: Store trak > udta metadata on each stream

[Martin Storsjö]

Hi Rémi,

On Wed, 15 Jan 2025, Rémi Bernon wrote:

> Some files keep extra metadata such as 'name' fields within udta, and
> it is useful for Wine to access them with the "export_all" option so
> they can then be exposed to Windows applications.
>
> Signed-off-by: Rémi Bernon <rbernon at codeweavers.com>
> ---
> libavformat/mov.c | 10 ++++++++--
> 1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/libavformat/mov.c b/libavformat/mov.c
> index c016ce8e41..f067ca4905 100644
> --- a/libavformat/mov.c
> +++ b/libavformat/mov.c
> @@ -354,6 +354,12 @@ static int mov_read_udta_string(MOVContext *c, AVIOContext *pb, MOVAtom atom)
>     int (*parse)(MOVContext*, AVIOContext*, unsigned, const char*) = NULL;
>     int raw = 0;
>     int num = 0;
> +    AVDictionary **metadata;
> +
> +    if (c->trak_index >= 0 && c->trak_index < c->fc->nb_streams)
> +        metadata = &c->fc->streams[c->trak_index]->metadata;
> +    else
> +        metadata = &c->fc->metadata;
>
>     switch (atom.type) {
>     case MKTAG( '@','P','R','M'): key = "premiere_version"; raw = 1; break;
> @@ -572,10 +578,10 @@ retry:
>             str[str_size] = 0;
>         }
>         c->fc->event_flags |= AVFMT_EVENT_FLAG_METADATA_UPDATED;
> -        av_dict_set(&c->fc->metadata, key, str, 0);
> +        av_dict_set(metadata, key, str, 0);
>         if (*language && strcmp(language, "und")) {
>             snprintf(key2, sizeof(key2), "%s-%s", key, language);
> -            av_dict_set(&c->fc->metadata, key2, str, 0);
> +            av_dict_set(metadata, key2, str, 0);
>         }
>         if (!strcmp(key, "encoder")) {
>             int major, minor, micro;
> -- 
> 2.45.2

So instead of storing metadata on the demuxer level, it is stored on the 
stream level, to avoid clobbering metadata if multiple streams provide 
metadata with the same name.

I guess that sounds reasonable. However, wouldn't this be a notable change 
for consumers that currently expect to see such metadata on the demuxer 
level?

I guess we don't have any firm guarantees about such things, and if the 
metadata specifically is for a track, it is the more correct thing to do 
anyway.

I don't have all the usual cases around mov/mp4 metadata fresh in mind 
though. CCing James to hear his opinion on this.

// Martin