[ogg-dev] video chapters and subtitles in ogg containers

Hans J. Koch hjk at linutronix.de
Fri Nov 14 03:11:50 PST 2008

On Fri, Nov 14, 2008 at 10:48:46AM +0100, ogg.k.ogg.k at googlemail.com wrote:
> Hi,
> (odd, I did get this reply for Silvia, but not the original post)

Hmm, it was properly CCed to the list.

> > There is CMML and kate support in vlc, and kate in mplayer though I am
> > not sure how it is displayed on-screen. Subtitles may display, but
> > chapter markers, I am not so sure about.
> Would you mind expanding on what chapters are, and what you'd expect
> to be able to do with them ?

Chapters are a list of timepoints stored in the metadata. They are an
information for player software that is usually used to allow the user
to jump to certain significant points within a stream. This probably
only makes sense for media that has a start point (t=00:00:00.000) and a
duration, not for continous streams like web radio.
Player software might need to take additional measures, e.g. round the
time up or down to the nearest keyframe.

An important factor is the ability to seek an arbitrary point within a
stream. Depending on the codec used, this might be a more or less costly
process. Some media (like DVD) slightly modify their stream encoding to
simplify seeking.

Chapters like this are mainly used on video DVDs, but are also defined for
SVCD and OGM containers. My personal use is watching movies from SDcard
or harddisk. I'm the type of guy who often wants to watch only some
parts of a movie, and chapters greatly simplify finding a certain scene
(or skip one that is too exciting...).

> > There is an old python script for cmml at
> > http://svn.annodex.net/scripts/trunk/subtitles/script-subrip2cmml.py .
> > I'm not sure something like this exists for kate.
> > But it should not be too hard to write one.
> kateenc (a tool bundled with libkate) can convert a srt file to a Kate stream,
> which can then be multiplexed along with a Theora video (and other Kate,
> Vorbis, etc streams).

That's an interesting hint. I'll have a look at that.


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