[vorbis-dev] packet rearranging questions

[Joshua Haberman]

Hello again.  If you recall, I am the guy working on enabling Audacity
to losslessly rearrange Ogg Vorbis files for my senior thesis.  I have
more or less completed the required modifications to Audacity to support
this.  You can read more about the work this entailed by reading my
journal:

http://www.reverberate.org/computers/thesis/journal

Now my attention moves to the Ogg Vorbis code necessary to do all this
splicing.  From reading the Ogg documentation and the vcut source I have
a good understanding of the Ogg format and how I will go about
rearranging Vorbis packets.  However I have a few questions I would like
to clarify:

1.  how exactly will the overlap/add nature of Vorbis packets affect me
as I attempt to rearrange them?   Is it correct to say that for any
Vorbis packet I take from an existing stream (other than the first
packet) I must also take the packet preceding it?

2.  When I am grabbing an extra preceding packet as described above,
should I set the granulepos of this preceding packet to 0 so that this
packet itself does not produce any sample data?  Will that confuse the
decoder if this is in the middle of a stream?

3.  I notice that you can get decoders to discard some of the samples
from the end of a packet by subtracting from the granulepos the number
of samples you want discarded.  A similar way to do this for discarding
samples from the beginning of a packet (used by vcut) is to set the
granulepos to -x, where x is the number of samples you want discarded
from the beginning.  But will this work in the middle of a stream, where
the granulepos should reflect the total sample offset of the stream?

4.  Is it just a special case that it is not possible to discard one
sample from the beginning of a block using this method, since -1 implies
that no packets finish in this block?

I have a few more, but I'll hold off in hopes of getting these answered
for now.  :)

Josh
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