Re: Streaming of WebVTT

Hi Silvia,

Le 7/27/2012 12:59 AM, Silvia Pfeiffer a écrit :
> On Thu, Jul 26, 2012 at 7:33 PM, Cyril Concolato
> <cyril.concolato@telecom-paristech.fr> wrote:
>>> No, not quite. Everything until the first empty line is regarded as
>>> the header.
>> I don't understand. The syntax (http://dev.w3.org/html5/webvtt/#syntax) says
>> that the "WEBVTT" string can optionally be followed by "either a U+0020
>> SPACE character or a U+0009 CHARACTER TABULATION (tab) character followed by
>> any number of characters that are not U+000A LINE FEED (LF) or U+000D
>> CARRIAGE RETURN (CR) characters.". Since it says "that are not LF or CR" you
>> can't have text on another line. The line starting with "WEBVTT" must be
>> followed by an empty line. This seems to be also the interpretation of
>> Anne's validator, which says "no blank line after the signature". But this
>> does not change the fact that the header, indeed, may carry interesting
>> things.
> That's the Syntax definition. In step 11 in the parser, thought it says:
> http://dev.w3.org/html5/webvtt/#parsing
>
> "Header: Collect a sequence of characters that are not U+000A LINE
> FEED (LF) characters. Let line be those characters, if any."
>
> The way I read this is that while we haven't defined in the Syntax
> that there should be a header, the parser allows putting extra
> characters beyond the identifier line and skips everything until it
> finds the two line terminators it requires (i.e. the empty line).
So although the file would be syntactically invalid, it would be 
acceptable to the parser. Urgh!

>> Right. So what about the Random Access Point problem that I mentionned in a
>> my previous email? Don't you think it should possible to rewrite any WebVTT
>> file such that any (or some) cue doesn't need information from previous cues
>> to be processed? Just like you can re-encode a video to have all (or some)
>> frames to be an I frame?
> You can already do that. "Random access" for video or audio also
> doesn't mean that you can get to any time point that you like. You can
> always only get to the resolution of the data that you're looking at.
> E.g. if you seek to a time position that lies exactly in the middle of
> two video frames, what are you going to rewrite when you re-encode it?
> Do the same for WebVTT.
I've never said that ""Random access" for video or audio [...] mean that 
you can get to any time point that you like". I'm saying that for video 
and audio you can always (at the cost of bitrate) reencode a file such 
that all frames are I-frames (or equivalent). To my understanding, this 
is not possible for WebVTT and I think that's a problem.

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/

Received on Friday, 27 July 2012 08:57:47 UTC