Support for MPD Anchors

[LloydW93]

Dash.js already has support for "#s=n", where n is a start offset in seconds. However, ISO23009-1 Appendix C specifies four MPD anchors:

Key	Value	Semantics
t	Time or time range in the same format as defined in Media Fragments URI 1.0	Time since the beginning of the period indicated by the period parameter. If the t parameter is not present, its default value is 0.
period	String	Period@id. If period parameter is not present, the default value is the ID of the Period with the earliest PeriodStart.
as	string	Value of a single AdaptationSet@id
track	string	Value of a single AdaptationSet@group

Dash.js should add support for at least #t, as this is something that current works if you pass an media url to a native media tag in a browser (https://www.w3.org/TR/media-frags/).

[davemevans]

Note that the examples in Appendix C are incorrect and were fixed in Corrigendum 1.

[LloydW93]

#1390 handles #t, which is the one we're interested in our use case. Does anyone have a desire to use the others? If not I think they may be best left on the backlog until someone wants them.

[davemevans]

As discussed at the F2F, #1390 only supports value specified in seconds for t. That's probably OK, but worth noting somewhere that it does not fully support Time or time range in the same format as defined in Media Fragments URI 1.0.

[wilaw]

Media Fragments URI 1.0 is a set of broad requirements, including spatial descriptors, many of which are edge case requirements. In support of @LloydW93's proposal, I would think we can cherry pick a subset of this to support at first for dash.js, specifically:

t=10,20 # => results in the time interval [10,20)
t=,20 # => results in the time interval [0,20)
t=10 # => results in the time interval [10,end)

Additionally, I think supporting the utc parameter as defined by Amd.3 of ISO/IEC 23009-1:2014 would be most useful in practice:

Start a live stream at Wed, 21 Jan 2015 20:04:05 GMT
   my.mpd#utc=1421870645
A live stream range from Wed, 21 Jan 2015 20:04:05 GMT to Wed, 21 Jan 2015 23:44:33 GMT
   my.mpd#utc=1421870645,1421883873
If mpd@type=dynamic, play the stream from the latest available segment to Wed, 21 Jan 2015 23:44:33 GMT
   my.mpd#utc=now,1421883873
If mpd@type=static, play the stream from the earliest available segment to Wed, 21 Jan 2015 23:44:33 GMT
   my.mpd#utc=,1421883873

We can add support for other features of MediaFragments and Amd 3:2014 if and when people come forth and say they need them.

[LloydW93]

The case of stopping at specific times isn't yet implemented. This should be relatively simple:

• A check in timeupdate to stop playback (and fake an ended event at that time)
• A check in the scheduled to consider the stream complete if the requested fragment starts beyond the requested range end time
• BOLA's consideration of buffer in this case?

However, what UX should be exposed in this case, at least in the reference player? Is the entire available period exposed in the seek bar (and in the DVR Info by dash.js for live streams), or only the requested range? I'm thinking dash.js should always expose the entirely available range and leave that decision to the client, but that then means we need to ensure that seeking out of the initially provided range is handled.

[bwidtmann]

the changes are currently not part of development branch. is this intented?

[LloydW93]

We currently only support #t= and #s=, and then only without ranges. We do generally have a desire to implement these further though, as it will be incredibly useful for MPD chaining.

[bwidtmann]

@LloydW93 I only see support of #s in development branch in PlaybackController.js in getStreamStartTime

[dsparacio]

So looks like it was reverted by dave and I do not see the changes in the PR in dev either so must of not been included after the revert. @LloydW93 I faintly remember a discussion around this but do not have a clear idea if we made a mistake and did not re-add the code lines.

[bwidtmann]

as @LloydW93 already mentioned: at least the support of #t should be implemented, which should be a 1 liner.

[fvalleeHbbTV]

FYI I'm interested in #period support !

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