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[sdw] WebVMT: Investigate optional endTime in HTML5 TextTrackCue interface (#1106)

From: Rob Smith via GitHub <sysbot+gh@w3.org>
Date: Wed, 12 Dec 2018 16:35:13 +0000
To: public-sdwig@w3.org
Message-ID: <issues.opened-390308361-1544632510-sysbot+gh@w3.org>
rjksmith has just created a new issue for https://github.com/w3c/sdw:

== WebVMT: Investigate optional endTime in HTML5 TextTrackCue interface ==
WebVMT supports cues without end times for streamed media use cases, where the media end time may be unknown, e.g. during a live broadcast.

Map annotation which displays persistently is a common use case, but this is not currently addressed by [`TextTrackCue`](https://www.w3.org/TR/html51/semantics-embedded-content.html#texttrackcue-texttrackcue) (or [`DataCue`](https://www.w3.org/TR/html51/semantics-embedded-content.html#datacue-datacue)) interface in HTML5 which requires an [`endTime`](https://www.w3.org/TR/html51/semantics-embedded-content.html#dom-texttrackcue-endtime) attribute.

There are two ways in which this could be addressed:

   1. Define a special value of `TextTrackCue.endTime` to signify that the time is unknown.
   1. Add a `noEndTime` flag to indicate that the cue persists indefinitely.

Currently, negative `endTime` values are allowed and signify cue times that occur before the start, according the note in [`TextTrackCue`](https://www.w3.org/TR/html51/semantics-embedded-content.html#cue). One way of encoding an unknown time would be to set the `endTime` before the `startTime` for a cue, i.e. a negative cue duration. The HTML5 specification does not explicitly preclude this and it requires no change to the interface, but this may have unintended consequences for existing implementations.

Adding a boolean `noEndTime` flag provides a much clearer solution, but requires a change to the existing HTML5 [`TextTrackCue`](https://www.w3.org/TR/html51/semantics-embedded-content.html#cue) definition to include it.

I intend to investigate this issue as part of the planned [Web Incubator CG](https://github.com/WICG) activity, [proposed at TPAC](https://github.com/w3c/strategy/issues/113#issuecomment-432971265) in Lyon.

Please view or discuss this issue at https://github.com/w3c/sdw/issues/1106 using your GitHub account
Received on Wednesday, 12 December 2018 16:35:14 UTC

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