Re: [MSE] Define how presentation duration is set & updated

Since it doesn't look like there is any disagreement on this topic, I'll
start updating the spec based on this discussion.

Aaron


On Wed, Jul 18, 2012 at 11:40 AM, Aaron Colwell <acolwell@google.com> wrote:

> Hi Mark,
>
> Comments inline...
>
> On Thu, Jul 12, 2012 at 2:06 PM, Mark Watson <watsonm@netflix.com> wrote:
>
>>
>>  On Jul 12, 2012, at 1:28 PM, Aaron Colwell wrote:
>>
>> Hi,
>>
>>  I wanted to start a discussion about Bug 17071<https://www.w3.org/Bugs/Public/show_bug.cgi?id=17071> which
>> talks about how to establish the presentation duration. Here is the gist of
>> the proposal.
>>
>>  1. Add a duration attribute to the MediaSource object
>> partial interface MediaSource {
>>   attribute unrestricted double duration
>> };
>>
>>  2. duration is NaN while MediaSource.readyState == "closed"
>>
>>  3. If duration is NaN and an initialization segment with a duration is
>> appended, then duration becomes the value contained in the initialization
>> segment.
>>
>>  4. When MediaSource.readyState == "open", duration can be set anytime
>>  by the web application with a value > 0 or positive Infinity. Using other
>> values will be ignored and trigger an exception.
>>
>>  5. If duration is set to a value less than what is buffered in the
>> MediaSource.sourceBuffers, then all data beyond the duration will be
>> removed from each SourceBuffer.
>>
>>  6. HTMLMediaElement.readyState can't transition to HAVE_METADATA until
>> duration becomes something other than NaN.
>>
>>
>>  I expect people with live content will want to have the duration remain
>> as NaN.
>>
>
> [acolwell] I believe the HTML spec says<http://dev.w3.org/html5/spec/media-elements.html#dom-media-duration>duration should be positive Infinity for live.
>
>
>>  I think all of the above should be pretty uncontroversial, but I have a
>> few questions.
>>
>>  - What should happen if a media segment is appended that starts before
>> the duration and ends past the duration? Should the duration be updated or
>> should the segment be truncated?
>>
>>
>>  I'm not sure it matters much which we choose, but perhaps it is more
>> robust to update it ? Frequently there are minor differences in the
>> durations of different tracks of a piece of content (of the order of a
>> frame or sample duration) and the duration reported in some content-wide
>> metadata may be any one of these or even some duration taken from the
>> source material.
>>
>>
> [acolwell] I agree. I think it is best to update the duration.
>
>>  - What should happen if the entire media segment is beyond the duration?
>>
>>
>>  Depends on the answer to the above question. If we're auto-updating the
>> duration in that case I think the duration should be updated in this case
>> too.
>>
>
> [acolwell] Agreed. This will update the duration as well.
>
>
>> - What should happen if a new duration is set and
>> HTMLMediaElement.currentTime is beyond that? Should this act like a seek to
>> duration?
>>
>>
>>  Seems reasonable. How does a media element deal with this in the
>> non-MediaSource case ?
>>
>
> [acolwell] I don't think this case is possible in any current
> implementation of the spec because I can't think of a way for any existing
> http resource to suddenly get truncated to before the current playback
> position. I think this only becomes possible when the application can
> actually specify a duration.
>
> I also thought of one other case:
>
> If MediaSource.endOfStream() is called and media segments don't go all the
> way to the current duration, should duration be updated to reflect the end
> of the appended data?
>
> For example, say I set the duration to 10 seconds, but only append 0-5
> seconds and then call endOfStream(). Should duration become 5 seconds since
> I can't append any more data? I think the answer should be yes, but I just
> wanted to see what people think.
>
> Another variant of this example is playing a live stream where the
> duration is positive Infinity and then the event ends. It seems like it
> would make sense to cause the duration to transition from positive Infinity
> to whatever the difference between the start time of the presentation and
> the end time of the last segment appended.
>
> Aaron
>
>