[whatwg] Timestamp from video source in order to sync (e.g. expose OGG timestamp to javascript)

Robert O'Callahan robert at ocallahan.org
Sun May 23 18:03:15 PDT 2010

On Tue, May 18, 2010 at 9:46 PM, Silvia Pfeiffer
<silviapfeiffer1 at gmail.com>wrote:

> To be honest, it doesn't make much sense to display the "wrong" time
> in a player. If a video stream starts at 10:30am and goes for 30 min,
> then a person joining the stream 10 min in should see a time of 10min
> - or better even 10:40am - which is in sync with what others see that
> joined at the start. It would be rather confusing if the same position
> in a video would be linked by one person as "at offset 10min" while
> another would say "at offset 0min". And since the W3C Media Fragments
> WG is defining temporal addressing, such diverging pointers will even
> end up in a URL and how should that be interpreted then?

Here's how I think it should work:
-- currentTime (and related times, such as times in TimeRanges) range from 0
to 'duration'
-- media resources are allowed to have a non-zero "initial playback time".
This is what currentTime should be set to on media load. We could create a
new DOM attribute to expose this.
-- media resources are allowed to have a "real time offset". This is an
optional date+time (in UTC) that corresponds to currentTime=0, exposed as a
DOM attribute. Players would be encouraged to use this to display real
times, when it's present.

This would be similar in power to what the spec already has. In your example
you could either let currentTime=0 be the start of the stream that the
user's loading, and use the "real time offset" to get the correct time
displayed, or you could let 0 be the real "start", and set the initial
playback time to match where the user joined. However, I think describing
things the way I just did is simpler and avoids weirdness like the "start
time" changing dynamically. It also preserves the invariant that currentTime
ranges from 0 to 'duration', which I think players will come to depend on if
the cases where it's not true are rare.

"He was pierced for our transgressions, he was crushed for our iniquities;
the punishment that brought us peace was upon him, and by his wounds we are
healed. We all, like sheep, have gone astray, each of us has turned to his
own way; and the LORD has laid on him the iniquity of us all." [Isaiah
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