Hi Charles, Dave,
On 4/10/07, Charles Iliya Krempeaux <[EMAIL PROTECTED]> wrote:
Hello,
On 4/9/07, Dave Singer <[EMAIL PROTECTED]> wrote:
> * Theora video and Vorbis audio in Ogg container. (application/ogg; .ogg)
> * Dirac video and Vorbis audio in Ogg container. (application/ogg; .ogg)
> * Theora video and Vorbis audio in Matroska container. (video/x-matroska;
.mkv)
> * Dirac video and Vorbis audio in Matroska container. (video/x-matroska;
.mkv)
>
>
> My understanding is that the Ogg container is 'specific' to these codecs,
and therefore the codecs parameter is not needed. But I am not an Ogg or
Matroska expert; perhaps they could chime in?
No. The containers are independent of the codecs put inside.
However, whether software, that supports Ogg or Matroska containers will
actually be able to "play" other codecs is another issue altogether.
Yes, the codecs are independent of the containers.
Xiph is using multiple MIME types to deal with this situation.
* application/ogg (http://www.rfc-archive.org/getrfc.php?rfc=3534)
provides for the container format, basically with any codecs included:
this could e.g. be Theora/Vorbis inside Ogg or Dirac/Vorbis inside
Ogg, but is not restricted to this.
Xiph has defined a Skeleton track for Ogg files through which the
MIME type of the contained codecs is specified at a known location in
the header of the Ogg file. Thus, a Ogg Theora/Vorbis File with a
skeleton track will have a MIME type of application/ogg and contain
two tracks with video/x-theora and audio/x-vorbis as the MIME types.
Xiph encourages everyone to use the Skeleton track in all future
created Ogg files. Skeleton is built into a standard Ogg container
format (as defined in RFC 3533
http://www.rfc-archive.org/getrfc.php?rfc=3533) and thus Ogg files
with Skeleton are compatible with Ogg files without Skeleton.
* audio/x-vorbis is used to specify the vorbis codec. This is of
particular importance in rtp/rtsp streaming applications.
* video/x-theora is particularly used to specify the theora codec.
The community has also been seen to use these specifications:
# audio/x-vorbis+ogg for Ogg Vorbis files
# video/x-theora+ogg for Ogg Theora/Vorbis files
though Xiph is not supportive of these.
Recent discussion at Xiph around http://www.ietf.org/rfc/rfc4281
suggests the use of the following parameters:
# application/ogg; codecs="theora, vorbis" for Ogg Theora/Vorbis files
# application/ogg; codecs="theora, speex" for Ogg Theora/Speex files
# application/ogg; codecs="vorbis" for Ogg Vorbis files
and also use the disposition parameter:
# application/ogg; disposition=moving-image; codecs="theora, vorbis"
# application/ogg; disposition=sound; codecs="speex"
Skeleton and the use of these MIME parameters should make things clear
for the application developers.
We did have the discussion over profiles of these codecs; I
understand profiles
are not used, but I am still unclear as to which of the following is true:
a) features are never added to the bitstreams, so any release of the decoder
will decode bitstreams made by any release of the encoder (modulo bugs);
b) the decoder release needed is signalled somehow in the bitstream
('need at
least the April 2005 release or later to decode this file')
c) neither of the above are true, it's left to the users to stay up
to date, and if
they don't, then, well, that's their problem.
In reply to this: Xiph has very carefully created and frozen (1st June
2004) a definition of the theora codec bitstream format, which should
be persistent for a while to come. So, while it is possible that there
may be new features added, it is not currently a consideration. There
are still multiple rounds of software optimisation possible to theora
encoders before they will require a change to the bitstream format.
For the case of a bitstream format change, there is version
information in the header of a theora bitstream. Major and minor
version numbers are being used similarly to the way that *nix library
version numbers work - anything with a minor change is backwards
compatible, but a major change may not be. So, if a major version
number change was to occur with theora, a new MIME type would probably
be required, e.g. video/x-theora2.
I hope this answers the question?
Best Regards,
Silvia.
