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[Bug 24161] New: <track> short duration cues fail to reach the application

From: <bugzilla@jessica.w3.org>
Date: Mon, 23 Dec 2013 16:35:09 +0000
To: public-html-admin@w3.org
Message-ID: <bug-24161-2495@http.www.w3.org/Bugs/Public/>
https://www.w3.org/Bugs/Public/show_bug.cgi?id=24161

            Bug ID: 24161
           Summary: <track> short duration cues fail to reach the
                    application
           Product: HTML WG
           Version: unspecified
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: HTML5 spec
          Assignee: dave.null@w3.org
          Reporter: david.lewis4@bbc.co.uk
        QA Contact: public-html-bugzilla@w3.org
                CC: mike@w3.org, public-html-admin@w3.org,
                    public-html-wg-issue-tracking@w3.org

Applications will typically access cues in a track by attaching a cuechange
event handler which operates on the list of cues found in activeCues. This
works well for most cues e.g. subtitles where the duration is likely to be at
least a few seconds.
Cues with a duration of less than 250ms may never appear in the activeCues list
due to the way the time marches on algorithm (described in 4.7.10.8) operates.

The algorithm is typically fired by the timeupdate event which in most browsers
is every 250ms. Cues which start and end between two iterations will be "missed
cues" and never appear in the activeCues list.

To ensure all cues from a track would be fired, an application would have to do
one of the following:
Attach an enter/exit event to every cue with duration <250ms
Inspect the full list of cues when cuechange is fired to check for missed cues.
Use cues with a duration of >250ms.

The problem has been highlighted by liaison from the HbbTV Association in
relation to MPEG DASH stream events received by hybrid TV devices. Because of
the media format and platform limitations the work-arounds listed above would
be difficult to implement. Please see this mailing list post for further
details:
http://lists.w3.org/Archives/Public/public-inbandtracks/2013Dec/0005.html

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Received on Monday, 23 December 2013 16:35:20 UTC

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