Re: Streaming of WebVTT

Le 7/25/2012 4:40 PM, Glenn Maynard a écrit :
> On Wed, Jul 25, 2012 at 9:18 AM, Cyril Concolato 
> <cyril.concolato@telecom-paristech.fr 
> <mailto:cyril.concolato@telecom-paristech.fr>> wrote:
>
>     I'm trying to integrate WebVTT content in streaming context (MP4,
>     RTP, DASH, MPEG-2 ...) in general. Aside from the problem of
>     random access points that I mentionned in my previous email [1],
>     there is a problem with the fact that the "WEBVTT" string has to
>     be at the beginning of the file. Consider the following case, I
>     have a live event and I want client to connect to an
>     HTTP-delivered WebVTT resource, I'd like to update that resource
>     from time to time (removing old cues, adjusting times and adding
>     new cues) and enable both clients already connected to get the
>     update and for new clients to get the new content. That same file
>     has to have the "WEBVTT" string for new clients, but cannot be
>     used as is for the already connected client (I'll have to strip
>     off the "WEBVTT" string). What is the use of that string? Is it
>     really needed given that many resources on the web don't use magic
>     numbers (HTML, XML, SRT, SVG, ...)?
>
>
> You'll need the ability to deal with "header" data anyway.  There have 
> been proposals for adding headers for metadata, default cue settings, 
> inline CSS, and so on.  It may be a while if any of them are 
> implemented, but please try to avoid designing a system that expects 
> WebVTT to be a headerless format.
Thanks for your comment. I wasn't aware of these proposals. Would you 
have a link to them?
Of course, if the header is meaningful, I agree it makes sense to keep 
it. In all the technologies I mentioned, there is a way to carry headers 
(stsd, sdp, initialization segment, descriptors ...). I was just 
wondering if there was a real need to have it for WebVTT. If I'm 
correct, according to the current syntax in WebVTT, the header can only 
be one line of text. Is that correct?

Cyril

-- 
Cyril Concolato
Maître de Conférences/Associate Professor
Groupe Multimedia/Multimedia Group
Telecom ParisTech
46 rue Barrault
75 013 Paris, France
http://concolato.wp.mines-telecom.fr/