Re: WebVTT, Regions and live streams.

Hi Bill,

Le 30/09/2015 19:14, May, Bill a écrit :
> Hi, Silvia,
>
> Thanks for your replies.
>
> The questions that I’m raising are not for VOD, but for live streams, where you do not know the end times of the cue, nor do you know ahead of time if a clear is needed.
I tackled the problem when we designed the carriage in MP4, and I 
believe the same solution can be applied even if not packaged in MP4. 
The solution is to generate cues such that all cues valid at for given 
time range are replaced by the new cues becoming active. In other words, 
there are no-overlapping cues, no cues spanning segments. In MP4 
terminology, this means that all samples are RAP. You can read [1] for 
further explanation. In your case (i.e. non MP4 packaged), you probably 
don't need to apply that approach to each sample, only to each segment, 
i.e. to have non-overlapping segments.

This has advantages that each sample/segment is random accessible, 
because you can seek at any time in the stream. One drawback of this is 
that each sample/segment is bigger because you need to repeat all cues 
that were overlapping with the current ones.

This might also create problems for instance when two consecutive 
samples will have almost exactly the same content (all cues similar 
except for one deleted and one new cue) and you may see flickering. In 
practice, I haven't seen that problem but I haven't tested thoroughly. 
If this is a problem, you can always detect in the player that two cues 
are exactly the same (content, settings, ...) and extend in JavaScript 
the lifetime of a cue. I could probably extend my demo [2] to make a POC 
if needed.

[1] https://concolato.wp.mines-telecom.fr/2012/09/12/webvtt-streaming/
[2] http://biblio.telecom-paristech.fr/cgi-bin/download.cgi?id=13223

-- 
Cyril Concolato
Multimedia Group / Telecom ParisTech
http://concolato.wp.mines-telecom.fr/
@cconcolato

Received on Friday, 9 October 2015 13:07:42 UTC