Re: TextTrack questions

On Thu, Jan 16, 2014 at 12:09 PM, Brendan Long <self@brendanlong.com> wrote:

>
> On 01/16/2014 11:39 AM, Aaron Colwell wrote:
>
>
>
>> Keeping track of cues by ID would be nice, but WebVTT cues don't
>> necessarily have ID's (and as far as I know, CEA-708 cues don't either).
>>
>
>  I was assuming that the UA would create some sort of private ID based on
> the contents of the cue and/or any other signals available to the format
> parser like a sample index. This private ID would then be used to keep
> track of whether an existing cue, modified or not, is the result of a
> previous add by the file format parser.
>
>
> But the UA doesn't necessarily have any information besides the start
> time, end time and text. WebVTT cues can have ID's, but it's not required,
> and Matroska/WebM doesn't seem to have a Block ID:
>
> http://www.matroska.org/technical/specs/index.html#block_structure
>
> If we're deciding the ID just based on the start time, end time and text,
> then we might as well just compare them.
>

The file offset of the cue information in the souce file could be used to
create the unique ID. Just using the start time, end time, and text would
not necessarily be unique if there are duplicate cues in the file. Does
WebVTT or CEA-708 have rules to prevent 2 cues from having the exact same
timestamps and text? Obviously such duplicate cues would be rare, but I'm
just trying to understand if they are possible and making it clear what the
UA behavior should be if they are allowed in the underlying formats.

Aaron