Porting the hlsdemux work in issue 698155 to 1.x

[Sebastian Dröge]

On So, 2013-12-01 at 19:52 +0100, Andoni Morales wrote:
> 2013/11/29 Arnaud Vrac <rawoul at gmail.com>
> 
> > On 29 nov. 2013, at 10:57, Duncan Palmer <dpalmer at digisoft.tv> wrote:
> >
> > > Hi all,
> > >
> > > I'd like to make use of the hlsdemux work described in
> > > https://bugzilla.gnome.org/show_bug.cgi?id=698155 , and I'm running
> > > gstreamer 1.x. So, I've spent some time nosing around the changes
> > > introduced by 698155 with a view to figuring out what would be
> > > required to get this stuff rebased onto the 1.x HEAD. I'm hoping
> > > Andoni (the author), and maybe a few other people who may be familiar
> > > with this can comment on what I think needs to be done, as I'd say
> > > there's a good chance I may have missed a few things.
> > >
> > > From what I can see, the hlsdemux work was branched from the
> > > sdk-0.10.23-hls branch in Andoni's github repository about a year ago.
> > > After work on the hlsdemux branch finished, it was rebased onto some
> > > other branch. Since this time, much (but not all) of the hlsdemux work
> > > has been merged into the sdk-0.10.23-hls branch, and there have been
> > > additional bug fixes applied to this branch.
> > >
> > > Andoni, could you tell me what the story with the sdk-0.10.23-hls
> > > branch is, and whether it would be sensible to pull any bug fixes from
> > > this branch?
> > >
> > > I think as a starting point, Andoni's hlsdemux branch would need to be
> > ported
> > > to 1.x, and all 1.x changes then merged in (if they still make sense),
> > perhaps
> > > along with bug fixes from Andoni's sdk-0.10.23-hls branch. This would be
> > quite
> > > a manual process. There are some complications;
> > > - Andoni's hlsdemux branch and the 1.x hlsdemux implementation both
> > support
> > >  decryption, but the implementations are conflicting. Also, the 1.x
> > >  implementation relies on gnutls whilst the hlsdemux branch relies on
> > openssl.
> > >  I'd probably be inclined to stick with gnutls, to avoid the openssl
> > license
> > >  complications.
> > > - I think there are conflicting changes in m3u8.c
> > > - Refactoring of uridownloader into it's own library on 1.x. Should be
> > >  straightforward to deal with.
> > >
> > > The hlsdemux branch creates hlsdemux as a bin, encapsulating a
> > > typefind element and a tsdemux, plus some elements for subtitle
> > > handling (haven't looked into it). I gather from the bug report that
> > > there is some disagreement over whether this is the right way to go.
> > > Could anyone enlighten me on the details?
> > >
> > > From what I can see, these are the reasons for the hlsdemux being a bin.
> > Have I
> > > missed any?
> > > - To fiddle with the timestamps on SEGMENT events after a 'catch-up'
> > seek to
> > >  the head of a live window.
> > > - To ensure that no SEGMENT events produced by tsdemux make it
> > downstream (does
> > >  tsdemux actually produce it's own SEGMENT events - I haven't looked
> > >  properly).
> > > - To facilitate the implementation of gst_hls_demux_drain_streams(); on a
> > >  discontinuity, hlsdemux waits for the tsdemux to drain (and filters out
> > the
> > >  EOS event it sends), before sending a SEGMENT event.
> > >
> >
> > Hi Duncan,
> >
> > While some of the patches would be welcome to be ported to gstreamer 1.x,
> > I think the choices Andoni made to support multiple renditions are not best
> > suited to the gstreamer architecture. Ideally I think all the possible
> > tracks should exposed as pads, instead of having to change a demux property
> > to switch audio or subtitle tracks. The issue with the proper design is
> > that you need a stream selection API, which has been talked about some time
> > before. Without this API, all streams would be downloaded at the same time
> > when used in a decodebin environment.
> >
> 
> I see it in a completely different way, having to expose all the pads is a
> hack to workaround the lack of a proper stream selection API. If a stream
> has Video,  Audio and Text, the element should only expose 3 pads and the
> stream selection API should provide a way to select which alternative
> stream you want to activate (like a different bitrate or a different
> language) , similar to what decodebin does with properties and what the
> element does right now, making it easier to port it to this future API.
> 
> Exposing all pads for the audio alternatives, means you also have to expose
> pads for each video bitrate (or for each encoding type like it might happen
> in DASH when you have not only several languages, but also several encoding
> options like AAC or AC3). The other issue with exposing one pad for each
> stream configuration is that you could potentially connect to any of the
> pads and expect to get data, which is not the case as you have cached
> content, and  you could also try to download the same video stream for
> different bitrates and audio languages at the same time,  making it quite
> hard to manage internally, which is by the way a use case that's not
> described in the spec and shouldn't even be considered for this element.

Yes, we should really implement a proper stream selection interface
instead of such solutions. This will also be needed for other adaptive
streaming solutions like DASH and Smooth Streaming, but is also useful
for e.g. DVD playback.

The idea would then be that playbin/uridecodebin/decodebin can proxy
their properties (e.g. current-audio) and other parts of the interface
to the demuxers and sources that implement this new interface, and also
aggregate all the streams that these elements provide in their own
interface.

There should be a bug about this already in Bugzilla.

-- 
Sebastian Dröge <sebastian at centricular.com>
Centricular Ltd - http://www.centricular.com
Expertise, Straight from the Source
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