On Tue, 07 Sep 2010 02:46:29 +0200, Gregory Maxwell <gmaxw...@gmail.com>
wrote:
On Mon, Sep 6, 2010 at 3:19 PM, Aryeh Gregor <simetrical+...@gmail.com>
wrote:
On Mon, Sep 6, 2010 at 4:14 AM, Philip Jägenstedt <phil...@opera.com>
wrote:
The Ogg page begins with the 4 bytes "OggS", which is what Opera
(GStreamer)
checks for. For additional safety, one could also check for the
trailing
version indicator, which ought to be a NULL byte for current Ogg. [1]
[2]
"OggS\0" as the first five bytes seems safe to check for. It's rather
short, I guess because it's repeated on every page, but five bytes is
long enough that it should occur by random only negligibly often, in
either text or binary files.
Um... If you do that you will fail to capture on files that most other
ogg reading tools will happily capture on. Common software will read
forward until it hits OggS then it will check the page CRC (in total,
9 bytes of capture). For example, here is a file which begins with a
kilobyte of \0: http://myrandomnode.dyndns.org:8080/~gmaxwell/test.ogg
Everything I had handy played it.
This could fail to capture on a live stream that didn't ensure new
listeners began at a page boundary. I don't know if any of these
exist.
I don't know if breaking these cases would matter much but herein lies
the danger of sniffing— everyone thinks they're an expert but no one
really has a handle on the implications.
Your test file is too short, perhaps it was truncated? I made my own one
by adding 1024 NULL bytes to the beginning of
http://v2v.cc/~j/theora_testsuite/320x240.ogg
That file doesn't play in Totem, because it (GStreamer) relies on
sniffing. It also won't play in Opera for this reason, but I haven't seen
any bug reports about failure to play similar files since Opera introduced
support for Ogg. It does play in Firefox, but not in Chrome. Just like
with WebM, I think browsers should not support files that begin with
arbitrary amounts of garbage, as it requires reading the whole file before
failing.
The file doesn't play in VLC or MPlayer, but does play in xine.
--
Philip Jägenstedt
Core Developer
Opera Software
