[vorbis] Copy a part of an Ogg/Vorbis file

[Michael Smith]

At 05:59 PM 3/17/01 +0100, you wrote:
>What I'd like to do is to copy a part of an Ogg/Vorbis file to a new
>file, without decoding and re-encoding (say I've got several songs
>in one long file, and I want to split them into separate files).
>Sample granularity is not required, packet granularity should do.
>
>I'm reading the Ogg documentation. I think I could write some code
>based on ogg_stream_packet{out,in}. Is this the way to go, or are
>there easier ways? Or is there perhaps already existing code for
>this purpose so I don't need to reinvent the wheel?
>

I have almost-complete code for doing this at sample granularity. 
There are a couple of small bugs remaining, though. I should get
back to it some time soon - haven't touched it for a couple of
weeks. So, you could wait for that.

Alternatively, if you want to do it now, with only packet 
granularity, then this is the basic approach (it's not quite as
easy as it looks at first):
  extract ogg pages from input file
  extract ogg packets from page
  write packets to first file until you reach the split-point
  now, start the second file
  write the last packet from the first file into the second file
    (this is needed because of the overlap-add that happens - each
     sample needs data from two packets).
  write the rest of the packets to the second file, being sure to
    calculate a correct granulepos for each packet (this is the
    deceptively hard bit)

If you don't really care about adding a few extra samples at the
start and end of the output files, you can skip most of the 
difficult bits and just give it close-enough info, but that's not 
recommended.

Michael

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