[ogg-dev] Ogg/Kate preliminary documentation

Silvia Pfeiffer silviapfeiffer1 at gmail.com
Thu Feb 7 11:10:52 PST 2008

On Feb 8, 2008 12:28 AM, ogg.k.ogg.k at googlemail.com <
ogg.k.ogg.k at googlemail.com> wrote:

> Hi,
> I recognize the main name behind CMML here :)
> Does the redesigning of CMML allow overlapping clips ?
> This is the main reason of my current ramblings about seeking.

> While karaoke was one of the initial goals behind kate, it is just
> a way the format can be used with (in fact, the format itself does
> not refer to karaoke at all, but styles and motions).
> At the moment, it is a fairly versatile text based presentation/video
> composition that can be superimposed onto a video (or not).
> This includes overlapping events, and animations, which I think
> CMML doesn't do (and which I didn't see it do, due to it being
> metadata about other data, and thus not "changing" in itself). But
> it seems to be changing goals, based on docs Conrad sent me
> links to.
> About requirements from Ogg itself, I don't think there is anything
> really - I've just listened to Ralph's talk about seeking in Ogg, and
> what I'm getting from it is "well, though, a multiple seek is required",
> so that's just the way it is. Otherwise, everything sits just fine within
> Ogg.
> You're quite right about CSS too. It's awfully complicated (but then
> I originally thought the same about CMML including HTML bits),
> and I made a simpler (and of course much less versatile) style setup.
> I wonder if this is a mistake.
> I didn't know you could time CSS changes however, unless you just
> refer to replacing the text by another text with the same actual visible
> string but css markers placed differently ?
> Anyway, seeking is the only bit that's really left now, it all works fine
> (from the encoder/decoder lib point of view anyway), and I didn't hit
> any showstoppers from ogg, hence no extra requirements. What did
> you have in mind when you referred to those issues that came up when
> you designed CMML ?
> As a bit of background, so you understand what I'm talking about, one
> of the things I can do with kate is to have (eg):
> - a (TV station ?) logo in a corner
> - a scrolling transcript of what is being said
> - a digital time in another corner
> - a channel number in a third
> - and whatever else you'd need
> all at the same time on screen, optionally moving around.
> It's not really showable now because you need a patched version of xine
> to actually view a video with the kate bitstream rendered on top of it,
> but
> you get the idea.
> Unless the changes to CMML are geared towards allowing this kind of
> things too ?

Some of the things you talk about were not solved at the CMML level, but
rather through using different Ogg
logical bitstreams.

For example:
* the logo in a corner would be a logical bitstream of a timed image track -
such as Ogg MNG
* the transcript would be in a CMML logical bitstream
* the digital time could be either part of that CMML logical bitstream or a
separate CMML logical bitstrem; if it was part of the same CMML track, it
would be positioned through CSS
* same for channel number
* overlapping timed text pieces would be coming in through differnt logical
bistreams or the CSS (there may be a timing extension necessary to CSS to do
so - if you have found a better way of doing this, I'll be keen to see)

The advantage of having things in different logical bitstreams is that you
can create addressing schemes can refer to just a subset of logical
bitstreams if you e.g. only want some part of the composition delivered to
you from a server. For example,
http://example.org/video.ogx?track=video,audio,transcript will avoid giving
you the digital time,logo, and channel number tracks for the above example.
The CMML design has always focused on trying to keep things in components
that can easily be added or taken away.

I'm very keen to seeing your specifications and seeing kate at work - it may
well be that you have found some better solutions to some of the problems
that we attack differently with CMML and thus we should think about picking
the best designs. Really wanting to see it working - post your specs and the
patched vlc version here if you can!

BTW: on the kate wiki page, Annodex is mentioned - what annodex is is simly
a Ogg file with skeleton and a CMML track in addition to other digital
media. It's a term that we used to specify the particular multiplexed file
with which we wanted to work, but it hasn't really much meaning in itself

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