Extract untouched audio track from mkv/webm programatically

Hi,

I would like to extract an .ogg file from an .mkv file programatically
(maybe from a .webm, but I care more about the .mkv at this point). By this
I mean without mkvtoolnix/ffmpeg or such tools or libraries. (The only
third party application that I could use to achieve what I want is an EBML
parser, and that's it.)

I understand that the contents of every .mkv file is described using the
EBML format. I use an EBML parser to extract information from .mkv files
and this part works great: I can successfully navigate my way through the
EBML and find the Tracks element and the TrackEntry for the audio track
that I want (which is an ogg vorbis audio track).

Now, this is the point where I'm stuck. I've read the manual/specification
for EBML but I'm not sure if I got it right. As I understood it the actual
data (video and/or audio) are located *inside* the Cluster element, which
could contain a lot of Block/SimpleBlock elements. I can find these, *all
of them*, but I guess I don't need all of them (only the ones which belong
to the TrackEntry I'm interested in).

Given these, my first question is as follows.

How can I identify the Block/SimpleBlock elements belonging to a particular
TrackEntry? (My guess is that by a TrackUID, but I couldn't figure where
does a Block/SimpleBlock contains which TrackEntry/TrackUID it belongs to.)

But, let's say I've found all the Block/SimpleBlock elements that belong to
a single audio ogg vorbis audio track. Can I just *append* these after each
other and be done with it? (My guess is *no way*, because that would be too
simple.) So, after I found all the Block/SimpleBlock elements I need, how
can I make an ogg vorbis audio file from them?

If someone could give me heads up/some pointers on these questions that
would be nice.

I'm not looking for working code snippets or stuff like that, but if anyone
knows of a *test case* that is lurking around in some ebml/mkv/whatever
code repository which tests mkv audio extraction methods or something like
that, than a link pointing to it would be nice too.

I did checkout the libebml/libmatroska repositories, but the first didn't
seem to be interested in what I would like to achieve and the second one
didn't have any useful tests - and the code was a bit overwhelming for me,
I didn't know where to start looking. (So, if someone could tell me which
classes/methods are responsible to extract audio from an mkv in some code
repository, that would be nice too.)

Heads up in general are nice too!

So, I guess that's it... thanks for any information in advance!

Regards,
Kohányi Róbert

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