[Bug 15632] New: Dealing with out of order cues in WebVTT

https://www.w3.org/Bugs/Public/show_bug.cgi?id=15632

           Summary: Dealing with out of order cues in WebVTT
           Product: TextTracks CG
           Version: unspecified
          Platform: PC
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: WebVTT
        AssignedTo: ian@hixie.ch
        ReportedBy: silviapfeiffer1@gmail.com
         QAContact: dave.null@w3.org
                CC: mike@w3.org, public-texttracks@w3.org


The WebVTT syntax specification says about the start time of a cue:
"The time represented by this WebVTT timestamp must be greater than or equal to
the start time offsets of all previous cues in the file."

This is an authoring requirement. 

However, when we go to the parsing section of the spec, there is no step in the
parser that makes sure that cues that are out of time order are ignored. This
means that an implemented parser will pick up such cues and enter them into the
list of cues to be used at the time that they are relevant.

Instead, I propose that we should enter a check into the parser and drop late
cues onto the floor. I believe this is the correct thing to do, because the cue
is out of order.

I am particularly concerned about this for consistency with two situations:
encapsulation into media files and live streaming text.

In the encapsulation case, if a cue is encapsulated into a media file and it
comes too late, then the demuxer and decoder will only come across this cue at
a time where it's too late to present it and therefore will have to drop it on
the floor. Similar reasoning applies to the live streaming case.

I therefore suggest to include such a requirement before step 38 of the
parser at http://dev.w3.org/html5/webvtt/#parsing .

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Received on Friday, 20 January 2012 03:11:40 UTC