[vorbis] ov_pcm_seek() is very slow...

[Michael Smith]

At 11:56 AM 3/29/01 +0100, you wrote:
>
>I realise the it's hard to do a seek in a variable bitrate
>file but I didn't think it was this bad. It seems to vary
>a lot with the file but the total time to do an ov_open()
>and ov_pcm_seek() can add upp to a second or more. By this
>time my DirectX buffers have wrapped around and are looping.

This is a known problem (not exactly a bug - it's doing what it's designed
to do, but it is a problem).

Libvorbisfile was written primarily as example code - it's intended to be
correct, but it isn't all that fast. Seeking and opening are the only
places where it's particularly bad (i.e. much worse than it should/could
be), that I know of.

There's no fundamental reason for seek to be that slow. However,
sample-precise seeking is never going to be super-fast.

vorbisfile provides another function - ov_pcm_seek_page(), which seeks to
(I think) the page which contains the offset you specify. At 128 kbps, a
page will typically be about a quarter of a second - probably too much for
your purposes. 

It would also be possible (but vorbisfile doesn't do this currently) to
provide a packet-precision seek function. That would be sufficiently
accurate for you, I think, and would be easy to add to vorbisfile.
Otherwise (as happens currently), it actually decodes part of a packet just
to throw it away (unless your seek point is at the start of the packet). 

It's actually fairly easy to make vorbisfile (libvorbis itself has no
particular speed problem with seeking) much faster for seeking - there's a
patch floating around somewhere for that, and it'll get committed sometime
soon, probably (it's not perfect, though, so it hasn't yet been added).

The alternative, if you need a solution _now_, is to not use vorbisfile
(i.e. just use libvorbis/libogg directly). This is fairly easy for decoding
straight through, but seeking is quite difficult to get right. 

Michael

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