[FFmpeg-devel] headers for gapless playback

[Tomas Härdin]

tor 2019-11-21 klockan 23:35 +0100 skrev Marton Balint:
> 
> On Thu, 21 Nov 2019, Tomas Härdin wrote:
> 
> > tis 2019-11-19 klockan 19:16 +0000 skrev John Kaplan:
> > > Hi Guys,
> > > 
> > > We just did some research on gapless playback across multiple client
> > > devices, and could not find support for adding gapless metadata
> > > headers in ffmpeg. Our interest is specifically in AAC/MP4, and we
> > > ended up shifting to the fdk-aac package, which exposes options for
> > > adding moov/edts/elst headers (which are the ones we used) as well as
> > > iTunSMPB.
> > > 
> > > So, I wanted to open a discussion with the team on ffmpeg support for
> > > gapless metadata.
> > > I’m not sure if you guys have discussed before, so here are a few
> > > opening questions to get me up to speed…
> > > 
> > > 
> > >   1.  Does ffmpeg support addition of gapless metadata in any form
> > > currently, and we missed the doc as to how to specify it?
> > >   2.  If not, have you discussed previously? If so, what were the
> > > results? (I.e. rejected as evil, put on roadmap but in future,
> > > currently in development, etc.)
> > 
> > Something very similar to this is being discussed in 
> > https://trac.ffmpeg.org/ticket/8366
> > 
> > >   3.  If support is still in the future, would you be open to me
> > > contributing a patch that implements it?
> > 
> > Yes please
> > 
> > I think opening a generic ticket for this would be a good move. Having
> > the ability to trim essence would be very useful. Then we could also
> > remove that godawful hack in mov.c as well, hopefully. There's more
> > than a few formats that need some "warm-up" data or that leave a few
> > frames of garbage at the end of the file, like all block transform
> > audio codecs
> 
> For audio we have AV_PKT_DATA_SKIP_SAMPLES packet side data

This is not enough for sample-level cutting as needed for gapless
playback

> for video AV_PKT_FLAG_DISCARD packet flag. What else is needed?

Why is this on the packets? It's not the demuxer's job to decide what
should and should not be cut.

/Tomas