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

Le 21/10/2015 15:39, Philip Jägenstedt a écrit :
> In the DASH/MP4/VTT software stack, is WebVTT the input or the output, 
> and is it a file or a stream? AFAICT, the only issue would be with a 
> WebVTT input stream (using the syntax in the spec, not any other 
> framing) with STYLE blocks at the end, but since streaming standalone 
> WebVTT doesn't exist yet I'm uncertain if that's really what you mean.
These are the good questions. It is currently possible to have a 
never-ending WebVTT file being produced live, delivered over HTTP (e.g. 
using chunked transfer encoding). Such WebVTT 'stream' cannot easily be 
consumed by a browser today because the Streams API is not there yet, 
but it will be available in the future. Other (non-browser) WebVTT 
implementations can already use that today. This might require careful 
creation of cues to insure that each point is a random access, but 
that's possible today. Several services can be done based on that: think 
of a WebRadio with subtitling. Regarding MP4 packagine, an 
implementation could consume such stream and produces MP4 segments on 
the fly, if needed.

For those implementations, if a new untimed style header would arrive in 
the input WebVTT stream and if such style would be defined to have 
effects on the whole 'file', i.e. including to cues prior in the 'file', 
then playing the live stream versus recording the stream and then 
playing a file would not have the same result. That would be 
problematic. That's why I think that styles should either be in the 
header (with semantics that they are valid for the whole file and 
without the ability to be in between cues) or as a timed block with a 
well defined time validity (like cues), or as settings of a cue. For the 
last two options, it really looks like WebVTT would become a multiplex 
of two types of timed data (cue and styles), I'm not sure we should go 
in this direction and if a separate style file/stream wouldn't be better.

Cyril

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

Received on Thursday, 22 October 2015 08:47:54 UTC