[FFmpeg-devel] [PATCH] avformat/movenc: allow Apple Lossless inside mp4

[Jan Ekström]

On Fri, Nov 20, 2020 at 10:53 PM Jan Ekström <jeebjp at gmail.com> wrote:
>
> On Fri, Nov 20, 2020 at 7:02 PM Leo Izen <leo.izen at gmail.com> wrote:
> >
> > MP4 already supports Apple Lossless, and it can be put inside by using
> > muxer=ipod, but it's not tagged as supported in mp4. The mp4ra lists
> > alac as a valid fourcc inside mp4, so it should be supported by spec.
> >
> > See: https://mp4ra.org/#/codecs
>
> I would just note in the paragraph that the identifier has been
> registered to ISO and thus towards ISOBMFF (which we usually call
> "mp4") at the MP4 registration authority.
>
> This can be changed when applying the patch, so no second revision
> needed for this change.
>
> > ---
> >  libavformat/movenc.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/libavformat/movenc.c b/libavformat/movenc.c
> > index fea8a86192..18fa3f9b5e 100644
> > --- a/libavformat/movenc.c
> > +++ b/libavformat/movenc.c
> > @@ -7128,6 +7128,7 @@ static const AVCodecTag codec_mp4_tags[] = {
> >      { AV_CODEC_ID_VP9,             MKTAG('v', 'p', '0', '9') },
> >      { AV_CODEC_ID_AV1,             MKTAG('a', 'v', '0', '1') },
> >      { AV_CODEC_ID_AAC,             MKTAG('m', 'p', '4', 'a') },
> > +    { AV_CODEC_ID_ALAC,            MKTAG('a', 'l', 'a', 'c') },
> >      { AV_CODEC_ID_MP4ALS,          MKTAG('m', 'p', '4', 'a') },
> >      { AV_CODEC_ID_MP3,             MKTAG('m', 'p', '4', 'a') },
> >      { AV_CODEC_ID_MP2,             MKTAG('m', 'p', '4', 'a') },
> > --
> > 2.29.2
> >
>
> Not sure of the location in the list (this matches the ipod muxer's
> ordering, while additional formats seem to have just been added into
> their list/group as they popped up - that said since this is a static
> symbol I would expect even if the addition was in the middle, it
> should be a-OK), but after taking a quick look at the ALAC code paths
> this seems to cause writing of boxes that seem to make sense (audio
> description plus whatever is in the extradata).
>
> I will attempt to check tomorrow if there's a spec for this to
> double-check against, but preliminarily this looks good to me.

Verified against the spec
(https://github.com/macosforge/alac/blob/master/ALACMagicCookieDescription.txt#L177).

As we already had the non-legacy (mode != MOV) support in place, the
output - as expected - matches the specification (checked with
L-SMASH's boxdumper).

Thus, the change itself is LGTM.

Jan