addcue event and on-receive event processing

[chrisn]

The DASH-IF event processing model describes two dispatch modes for timed metadata events:

• on-receive: provide the event/timed metadata sample data to the Application as soon as it is detected by DASH Player

• on-start: provide the event/timed metadata sample data to the App at the start time of Event message or at the presentation time of timed metadata sample

As a proposed solution to this, the explainer describes a new addcue event, which is intended to support the on-receive mode.

Consequences for existing text track support in HTML of introducing this event, include:

• addcue events would be fired for each cue in a text track loaded via an HTML <track> element
• addcue events would be fired each time an application calls textTrack.addCue()

This could be large number of events. Is this a concern? Are there any other concerns with doing this?

[chrisn]

Discussed on the 2020-09-21 call. Some implementations don't support surfacing in-band cues ahead of time, and so such an addcue event could only be raised at the same time as cuechange, and having two events per cue in this way is not desirable.

An idea raised during the call is to have a TextTrack early dispatch hint that the web app could set which would - in implementations that support early dispatch - cause the cue events to be fired ahead of time. The hint could be some number of seconds.

However, web apps may want early visibility only for some cue types, not others. The DataCue proposal uses a single metadata TextTrack for cues of all types, and so the hint would affect all cues on that track. This makes it hard for the web app to know how to respond to data cue events in the case where normal (rather than early) processing is expected.

Also, it is awkward if we try to repurpose the time marches on steps to implement early dispatch for a TextTrack. Early dispatch shouldn't be coupled to media playback.

Shaka Player and DASH.js both support emsg boxes. Shaka Player supports on-receive mode, and DASH.js has both on-receive and on-start.

Thoughts welcome!