Re: Resolving TextTrackCue issues

On Sat, Aug 31, 2013 at 9:26 AM, Silvia Pfeiffer
<silviapfeiffer1@gmail.com> wrote:
> Hi all,
>
> Recent changes to the TextTrackCue interface had led to a fork with
> the WHATWG spec [1] when resolving bug 21851 [2].
>
> This caused extensive discussion on blink-dev [3] when an intent to
> implement was proposed.
>
> In the W3C WG we recognize the need for a generic cue interface type
> with a constructor and a text attribute. It allows browsers to expose
> cues in text tracks of video or audio files for which browsers don't
> intend to implement parsers. It also allows JavaScript developers to
> create time-synchronized data for media elements in any format they
> require.
>
> The discussion on blink-dev exposed that the currently specified
> solution of bug 21851 [2] in the HTML5 spec is flawed in several ways:
>
> (1) TextTrackCue objects that are not fully abstract create hard to
> debug issues of backwards compatibility due to existing code that
> assumes "new TextTrackCue()" constructs a cue with VTT semantics;
> (2) in order to transition old TextTrackCue interface usage to "new
> VTTCue()", it is better to remove the existing TextTrackCue
> constructor causing hard failure (easily recognizable) instead of soft
> failure (more difficult to recognize);
> (3) the abstract TextTrackCue interface of the WHATWG is desirable for
> extensibility to non-text-based cue interfaces of the future;
> (4) the interface fork between the WHATWG and W3C spec should be removed.
>
> An alternative resolution to bug 21851 [2] has previously been
> proposed and discussed: create a new interface that has the text
> attribute and the constructor and inherits from the abstract
> interface.
>
> This will result in the following interfaces:
>
> interface TextTrackCue : EventTarget {
>   readonly attribute TextTrack? track;
>
>            attribute DOMString id;
>            attribute double startTime;
>            attribute double endTime;
>            attribute boolean pauseOnExit;
>
>            attribute EventHandler onenter;
>            attribute EventHandler onexit;
> };
>
> [Constructor(double startTime, double endTime, DOMString text)]
> interface GenericCue : TextTrackCue {
>            attribute DOMString text;
> };
>
> Whether VTTCue will inherit from GenericCue or from TextTrackCue will
> be resolved in the TextTrack CG once this change has been applied to
> the HTML5 spec.
>
> It is my understanding that this proposed change resolves all the
> above listed issues. I will therefore apply these changes next week
> unless there are any further concerns.
>
> Regards,
> Silvia (as HTML spec editor).
>
> [1] https://www.w3.org/Bugs/Public/show_bug.cgi?id=22903
> [2] https://www.w3.org/Bugs/Public/show_bug.cgi?id=21851
> [3] https://groups.google.com/a/chromium.org/d/msg/blink-dev/-VHGnuNNUxM/Yibbv2TgDoYJ
>

Am I correct to assume that GenericCue will never be rendered? If so,
a format initially exposed using GenericCue can likely never be
"upgraded" to a fully functional interface, since scripts will have
come to assume that those formats aren't rendered by the browser. That
seems pretty bad to me, and a good reason for browsers to not stop
half way in supporting a format.

In the blink-dev thread Silvia made reference to TextTrackCueGeneric,
[1] so it would be nice to hear from Apple (CC:ed) about whether or
not GenericCue would be a suitable replacement. AFAICT they don't seem
very similar, as TextTrackCueGeneric has rendering information
attached (e.g. setFontSize) and presumably can be rendered.

Are there any browser vendors who support this change and want to
expose any in-principle-renderable format using GenericCue?

Also CC Hixie, since removing the spec fork was one of the goals.

Philip

[1] https://github.com/WebKit/webkit/blob/master/Source/WebCore/html/track/TextTrackCueGeneric.h

Received on Monday, 2 September 2013 08:21:05 UTC