[whatwg] Video feedback

[Mark Watson]

On Jun 9, 2011, at 4:32 PM, Eric Carlson wrote:

> 
> On Jun 9, 2011, at 12:02 AM, Silvia Pfeiffer wrote:
> 
>> On Thu, Jun 9, 2011 at 4:34 PM, Simon Pieters <simonp at opera.com> wrote:
>>> On Thu, 09 Jun 2011 03:47:49 +0200, Silvia Pfeiffer
>>> <silviapfeiffer1 at gmail.com> wrote:
>>> 
>>>>> For commercial video providers, the tracks in a live stream change all
>>>>> the time; this is not limited to audio and video tracks but would include
>>>>> text tracks as well.
>>>> 
>>>> OK, all this indicates to me that we probably want a "metadatachanged"
>>>> event to indicate there has been a change and that JS may need to
>>>> check some of its assumptions.
>>> 
>>> We already have durationchange. Duration is metadata. If we want to support
>>> changes to width/height, and the script is interested in when that happens,
>>> maybe there should be a dimensionchange event (but what's the use case for
>>> changing width/height mid-stream?). Does the spec support changes to text
>>> tracks mid-stream?
>> 
>> It's not about what the spec supports, but what real-world streams provide.
>> 
>> I don't think it makes sense to put an event on every single type of
>> metadata that can change. Most of the time, when you have a stream
>> change, many variables will change together, so a single event is a
>> lot less events to raise. It's an event that signifies that the media
>> framework has reset the video/audio decoding pipeline and loaded a
>> whole bunch of new stuff. You should imagine it as a concatenation of
>> different media resources. And yes, they can have different track
>> constitution and different audio sampling rate (which the audio API
>> will care about) etc etc.
>> 
>  In addition, it is possible for a stream to lose or gain an audio track. In this case the dimensions won't change but a script may want to react to the change in audioTracks. 

The TrackList object has an onchanged event, which I assumed would fire when any of the information in the TrackList changes (e.g. tracks added or removed). But actually the spec doesn't state when this event fires (as far as I could tell - unless it is implied by some general definition of events called onchanged).

Should there be some clarification here ?

> 
>  I agree with Silvia, a more generic "metadata changed" event makes more sense. 

Yes, and it should support the case in which text tracks are added/removed too.

Also, as Eric (C) pointed out, one of the things which can change is which of several available versions of the content is being rendered (for adaptive bitrate cases). This doesn't necessarily change any of the metadata currently exposed on the video element, but nevertheless it's information that the application may need. It would be nice to expose some kind of identifier for the currently rendered stream and have an event when this changes. I think that a stream-format-supplied identifier would be sufficient.

...Mark

> 
> eric
> 
>