[Vorbis] How do I seek to seek to specific samples using libvorbis?

[Madison Link]

Thanks.

 

If decoding from the last packet on the previous page works, I may have to
do that. But I was hoping to avoid it. Yes, I may be seeking a lot. A packet
spanning multiple pages shouldn't be a problem for me, because I'm only
adding index points that represent the beginning of a packet. But does this
actually happen for Vorbis? (Aside from large header packets) The large
blocks I've encountered seem to be 1024 samples per channel. Even with
8-channel audio, uncompressed floating-point PCM sound, such a packet would
only take up 129 segments on the segment table.

 

Madison

 

 

From: Jordan Verner [mailto:jv_erner at hotmail.com] 

Sent: Tuesday, May 19, 2015 12:20 PM

To: Madison Link

Subject: RE: [Vorbis] How do I seek to seek to specific samples using
libvorbis?

 

What I did was performed a binary search in my ogg vorbis file, in order to
find the page proceeding the page containing the desired sample (you can
determine this using the granule position). Next, I used ogg_page_packets()
to determine how many packets complete on that page, skip to the final
packet by calling ogg_stream_packetout N times and just ignoring the result.
Then decode the page's final packet. This should mean that the next sample
you decode will actually be the one the granule position says it is. Then
you have to calculate an offset between the sample you're at and the one you
want, decode that many and discard them. In practice this seems to work but
leaves a lot to be desired. You could be decoding up to an entire page of
audio (slow if you seek a lot), and there's an off chance that a complex and
rare case involving a packet spanning three or more pages could still throw
you off. I think it could be possible to do it with only decoding two extra
packets if you can calculate where you'll end up based on blocksize values
and window overlapping rules, but it's not well explained. The spec leaves a
lot to be desired.

 

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