[vorbis-dev] some more granulepos questions

[Conrad Parker]

On Sat, Nov 15, 2003 at 07:09:57PM -0800, Joshua Haberman wrote:
> I've made a lot of progress on my lossless Vorbis editing project.  I
> have a few more questions about granulepos issues I don't quite
> understand.
> 
> granulepos is a property of Ogg pages, yet it is a field in the
> ogg_packet struct.  When reading packets from an Ogg stream, the
> granulepos is set to -1 for all packets except the last packet in a
> page.  From this I infer that
> 
> - for encoding, the granulepos needs to be correctly set for every
> packet submitted to the stream because any packet could be the last on a
> page.  If it is the last on a page, its granulepos is used for that
> page's granulepos.
> 
> - for decoding, the stream cannot determine a granulepos for each packet
> since it doesn't know anything about the codec, so it uses -1 to
> indicate that it doesn't know.  For the last packet on a page, it uses
> the page's granulepos.
> 
> Q1.  Is this accurate?

yes, that info is lost at the ogg level

for packets that don't end a page, you may be able to reconstruct the
granulepos using codec-specific knowledge, hence the other magic in
vcut (which I'm not too familiar with :)

Conrad.
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