[whatwg] HTML5 based Eduvid Slidecasting Demo

Odin odin.omdal at gmail.com
Sun May 22 06:49:58 PDT 2011

On Sun, May 22, 2011 at 3:03 PM, Narendra Sisodiya
<narendra at narendrasisodiya.com> wrote:
> infact I too want to do same..


> Basically you want to send slides (via websocket or comet) and sync with
> video..

Yes. My old system (early Errantia) did that, using comet.

> Here is the mistake. Sending Slides in real time has no use. Please
> do not take me wrong. Basically in almost any conference or workshop,
> presenter already has slides ready. So if something is ready then you can
> directly push it to user. send video/audio in real time.

You misunderstand :-) The slides are no problem at all, I upload them
to the server, then when done it sends a Comet PUSH to the clients
which tells them, a new slide resides at  """
http://example.com/images/test-conference/14.jpg """. So that part is
working swell.

Also, I send the time when the slide appeared. However, the syncing to
the video/audio is the impossible part, because there's no way for the
browser to know where the video is in time/place **in a live
setting**, you can very easily do this with archived video, it's just
to read currentTime.

We had a discussion about this, and that was why startOffsetTime made
it into the spec:


As you see, there are lots of buffering of video. And such, different
people will be different places in the real time video.

So I'm watching "live" and that's only one minute "off", whilst a
friend is watching via wireless internet and is a full 10 minutes off
because of his buffering etc, etc.

In order to sync slides to the video/audio in such places (when people
connect 10 minutes into the video and get a new currentTime = 0 at
that time), you need a static reference point, but as of now; all time
is still relative. Getting startOffsetTime will get us a static time
to know when we're going to show the new slides (which will also have
a  datetime  field for syncing).

> So , No need to send slides in real time.. Send all slides at once.. and
> then keep on sending the configurations
> I will also try to work on this...

Well, for some conferences this won't work, and I already have code to
do that live, so I don't need to send slides afterwards. Anyway, both
ways work. But you need to know where people are in the video in order
to sync to the slides; and that's where startOffsetTime come in.

Alternatively you might try to control the buffers/caches, but that's
not always possible. I've tried before, and can't really get it thight
enough, there's too many variables, and Icecast might not be able to
tweak itself to make a really good low latency low buffering live

Beste helsing,
Odin Hørthe Omdal <odin.omdal at gmail.com>

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