Vorbis dev - Jun 2009 - Splitting Ogg Vorbis file

Hi there,

in fact, my purpose is to write tool which will allow one to split ogg
file with vorbis audio *and* theora video codecs, but at first I'd
like to understand how the splitting of Ogg Vorbis stream works. So I
looked at existing tools that split Ogg Vorbis audio files, i.e.
`vcut' utility in vorbis-tools package, and Ogg plugin from
`libmp3splt', but I found that engine code is pretty the same. The one
of the core functions of `vcut' source starts with the comment:

/* Write the first stream to output file until we get to the appropriate
?* cut point.
?*
?* We need to do the following:
?* ? - Adjust the end of the stream to note the new end of stream.
?* ? - Change the final granulepos to be the cutpoint value, so that we
don't
?* ? ? decode the extra data past this.
?* ? - Save the final two packets in the stream to temporary buffers.
?* ? ? These two packets then become the first two packets in the 2nd stream
?* ? ? (we need two packets because of the overlap-add nature of vorbis).
? ? ?...
?*/

So my questions are: what does "overlap-add nature of vorbis" means?
Why should I copy even two last packets from first fragment of Vorbis
file to the second one? Why I can't finish first fragment with n-th
packet, and just start next fragment with new (n + 1)th packet? Say, I
don't want to break any vorbis packet, ?don't want to cut vorbis file
at *exactly* e.g. 5000 samples, and breaking stream at nearest
packet's boundary is okay for me.

Much thanks in advance!

--
WBR,
Timur

> So my questions are: what does "overlap-add nature of vorbis"
means?
Decoding a Vorbis packet makes use of the two previous packets
(that's also known as 'preroll'). Thus, if you're missing those
packets,
you'll get different data out (though I don't know *how* different
that output would be).
You'd need this even if you want to cut at a packet granularity.

2009/6/29  <xiphmont at xiph.org>:
> Hmmm... perhaps vcut.c is wrong? :-)
Don't know, really. That's what I'd actually like to clarify,
what's
the right way to split ogg stream :)

--
WBR,
Timur