Re: Cue settings vs Styling properties

Hi Silvia,

Le 7/17/2012 5:30 PM, Silvia Pfeiffer a écrit :
> Cue settings apply to the rendering box in which the text cue is
> rendered, but the CSS styles that you apply to a<c>  span apply for
> the text inside the cue. So, while some of it might work with a global
> <c>, most of it won't. They are two different features that combine to
> provide as much flexibility as captions need.
Thank you for the quick reply. It's a pity that it's not possible. I 
think it would be a good extension. I'll explain a bit more the problem 
we are facing and why it could be a good thing to extend WebVTT or CSS.

The problem is that it seems quite hard to access randomly into a WebVTT 
stream, due to the possible overlap between cues and to the fact that 
settings apply to the whole cue. Take the following example:

00:00:00.000 -->  00:00:10.000 line:0
Text1

00:00:02.000 -->  00:00:15.000 line:1
Text2

Between time 2 and 10, the 2 lines will be displayed at different positions.
The second cue is not a random access point. If you seek at time 5s, you need to roll back to the first cue, apply it and then apply the second one. If you have multiple overlaps like that, you might end up with very few random access points with which you can start.

So if you want to stream or dash that WebVTT content, a client joining at time 5s will need to fetch data up to the previous random access point, which might be far away in the past. This might be problematic for subtitle track switching also.

It would be good to be able to pre-process the WebVTT file to ensure that there is no overlap between cues, in other words, so that all samples are random access point. This means to be able to apply cue settings to a line. So, for the above example, this would give something like:

CSS:
.style1 {
   webvtt-line: 0;
}

.style2 {
   webvtt-line: 1;
}


WEBVTT

00:00:00.000 -->  00:00:2.000
<c.style1>Text1</c>

00:00:02.000 -->  00:00:10.000
<c.style1>Text1</c>
<c.style2>Text2</c>

00:00:10.000 -->  00:00:15.000
<c.style2>Text2</c>

Do you see my point here?

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/


>
> HTH.
>
> Regards,
> Silvia.
>
> On Tue, Jul 17, 2012 at 1:35 PM, Cyril Concolato
> <cyril.concolato@telecom-paristech.fr>  wrote:
>> Hi,
>>
>> While working on the carriage of WebVTT tracks in MP4, I was wondering if it
>> is possible to replace "cue settings" by wrapping the cue in a cue class
>> span and using CSS styles. The current settings are vertical alignment, line
>> position, text position, size and horizontal alignment. It looks to me that
>> some CSS properties like vertical-align or text-align are good candidates,
>> but I'm not sure about the other ones. Any idea,
>>
>> Cyril
>>

Received on Tuesday, 17 July 2012 20:59:24 UTC