[theora] oggcat or other ways to combine theora videos (with or without re-encoding)?

Gregory Maxwell gmaxwell at gmail.com
Tue Sep 15 08:04:29 PDT 2009

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On Tue, Sep 15, 2009 at 10:48 AM, Orton AKINCI aka .-_-.
<ortonak at yahoo.com> wrote:
> hi yorn, i have reported the issue (not a bug possibly) i have e-mailed to
> this list about "packet number for stream <2> not matching:"  to the support
> request of sourceforge bug tracker...

> my question for now, is there a safe conversion method to theora, so that i
> can use these theora files to cat safely with oggcat? i never use
> oggz-chop.  i am using the latest builds of ffmpeg2theora on windows... or
> is it about the input files to be converted to theora? if so what is the
> safe input file format to convert to theora(if there is any or that is the
> case:)).
> as i know, a part of the problem is about vorbis, that cannot be cut at a
> desired precise point... is there a way to cut the video(video) according to
> where vorbis(audio) let to be cut, so that there should be no sync problems,
> 'cos theora can be cut any precise point? no?

Typically Theora can only be cut at keyframes unless you're willing to
tolerate garbage in any output that doesn't consist of the cut parts
back together exactly as they came.  You can have full,
frame-granularit, lossless editing with Theora by running with
keyint=1. This results in an enormous loss of compression efficiency.
The same is true of pretty much all video formats.

Vorbis can be cut with sample granularity but if you are going to
paste the cut vorbis streams back together without creating a chained
stream (which many things will not play) the cuts will need to have
vorbis-frame granularity.  Vorbis frames will not align with video
frame times.  This shouldn't be a problem for simple excerpting where
the granpos can be preserved, but it would be a problem for retaining
sync with chainless recombination.

You can't reliably concatenate streams from different sources without
chaining, because they may have different setup headers.

Chained files will not reliably play in all players.

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