[Webkit-unassigned] [Bug 110479] [Meta] Implement support for TTML

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=110479





--- Comment #14 from Silvia Pfeiffer <silviapf at chromium.org>  2013-02-21 20:15:32 PST ---
(In reply to comment #13)
> > > Nothing here requires populating cues. That is, the set of cues could be exposed or not via the DOM.

They don't go into the DOM, but into the shadow DOM. They are just represented as objects.


> > Are you suggesting to parse a <track> element into a HTMLTrackElement, but not actually giving it a TextTrack object?
> 
> No. I'm saying that it isn't necessary to populate the cue list.

OK... but a TextTrack object without cues is basically useless to the JS developer: it doesn't fire oncuechange events and it doesn't give access to the parsed cues. Might as well not create a TextTrack object.

> > Are you further suggesting to "just render" the text on top of the video without adding it to the shadow DOM?
> 
> I'm not exactly sure what you are referring to as shadow DOM here, but if you are referring to TextTrack, then there is no reason not to provide a TextTrack object. But individual cues aren't needed in TextTrack to  perform formatting of TTML content.

The way it's currently specified, rendering relies on having TextTrackCues.

> > How is this rendering supposed to work? Are you intending to render pixels into the video before the video is displayed?
> 
> I'm assuming there is a reasonable implementation specific way to overlay a graphics layer over a video layer in WK's display pipeline. Since WebVTT is displaying over video, then clearly there is some functionality there to support this.

Try opening up an example video with <track> in Chromium or Chrome and open your inspector. Activate the "Shadow DOM" functionality of inspector and look at how captions are rendered. They are not rendered into a graphics layer but into the Shadow DOM.


> > The way in which <track> is specified in HTML5 is that it introduces content into the browser that a JS developer can manipulate. Therefore it populates all of the objects. This is intentional and not a bug.
> 
> Sure. But there is nothing in the HTML5 spec that says that cues (or what might be considered a "cue") *must* be exposed and can't be implicitly presented or otherwise processed without exposing actual cue objects. At least there is nothing I'm aware of that mandates this. Please correct me if I've missed it.

I think you've missed it. Try implementing TTML support and you'll certainly come across it.

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