Re: Streaming of WebVTT

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).


> 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.

HTH.

Cheers,
Silvia.

Received on Thursday, 26 July 2012 23:00:05 UTC