Re: Inband styling (was Re: Evidence of 'Wide Review' needed for VTT)

> On Oct 22, 2015, at 13:36 , Philip Jägenstedt <philipj@opera.com> wrote:
> 
> 
> Do you have a pointer to such a never-ending WebVTT file deployed on
> the public web? I honestly didn't think they would exist yet.

Apple’s HLS supports VTT streams natively, and one can tune in at any point, and they can go on ‘indefinitely’.  (I think DASH people are also able to do the same.)

> 
> To be pendantic, the reason that never-ending WebVTT files don't work
> in browsers isn't because of the Streams API, but because the media
> element's readyState cannot reach HAVE_FUTURE_DATA until the text
> tracks are ready:
> https://html.spec.whatwg.org/multipage/embedded-content.html#the-text-tracks-are-ready
> 
> This is what the spec bug is about, some mechanism to unblock
> readyState before text track parsing has finished:
> https://www.w3.org/Bugs/Public/show_bug.cgi?id=18029
> 
> Anyway, letting the parser discard style blocks after any cues until
> we've figured out the live streaming issues is OK with me. However,
> let's spell out the implications of keeping this restriction for live
> streams: If you don't know all of the style up front, your only
> recourse is to add a new text track at the point where new style is
> needed. This will involve scripts, at which point handling multiple
> WebVTT tracks will compare unfavorably with just using a WebSocket
> connection to deliver cues and style using a custom syntax.

I agree it’s worth thinking about a good solution.

> On Oct 22, 2015, at 15:51 , Nigel Megitt <nigel.megitt@bbc.co.uk> wrote:
> 
> It would also be possible to take the same approach with VTT as we have
> taken with TTML, which is that you have a sequence of independent
> documents each of which contains the styling etc needed to display itself,
> for whatever time period applies. 

That gets us back to the problem I cited earlier — the size of this segment determines the minimum streaming segment size and hence delay; it’s not very flexible. Also, finding ‘clean’ boundaries can be hard (places to break into independent documents).

David Singer
Manager, Software Standards, Apple Inc.

Received on Friday, 23 October 2015 15:01:36 UTC