ogg dev - Sep 2007 - The use for an XML based metadata format

On 11/09/2007, Ian Malone <ibmalone@gmail.com>
wrote:
>
> Embedding in Ogg is the simple bit; the only point of contention
> being whether you use a magic number to label it as metadata or
> just package XML and let the parser sort it out.  (With a bit
> more experience under my belt I'm persuaded a magic number might
> be worthwhile, otherwise there'll be someone who hardcodes their
> app to expect XML to be a metadata stream).
Agreed, having a magic identifier at the beginning of the stream is
useful where you only need to decide how to deal with a track (logical
bitstream), for example in the demux stage of a player. It is also
useful in a server that does stream recomposition or selection without
decode (eg. serving a temporal subset of the stream, or selecting
language tracks).

For CMML, we first tried just using raw XML from byte 0 but later opted for
using a binary header with magic identifier in the first (bos) packet
for these reasons.

cheers,

Conrad.

On Tue, Sep 11, 2007 at 09:33:43AM +0900, Conrad Parker wrote:
> For CMML, we first tried just using raw XML from byte 0 but later opted for
> using a binary header with magic identifier in the first (bos) packet
> for these reasons.
Can you give a little more on why you made this decision? I see it goes 
back a long way, but couldn't find a discussion about it.

I always thought it would be better to start with raw XML, since it's 
fairly recognizable (especially if you require an initial <!DOCTYPE...> 
or <?xml...?>) and let the XML parser sort it out. With the prepended 
magic, you need a mapping spec for every variety of XML you want to 
embed in Ogg.

Of course, you could just define '<?xml' as the magic for a generic
xml
stream...

 -r

On 13/09/2007, Ralph Giles <giles@xiph.org> wrote:
> On Tue, Sep 11, 2007 at 09:33:43AM +0900, Conrad Parker wrote:
>
> > For CMML, we first tried just using raw XML from byte 0 but later
opted for
> > using a binary header with magic identifier in the first (bos) packet
> > for these reasons.
>
> Can you give a little more on why you made this decision? I see it goes
> back a long way, but couldn't find a discussion about it.
>
> I always thought it would be better to start with raw XML, since it's
> fairly recognizable (especially if you require an initial
<!DOCTYPE...>
> or <?xml...?>) and let the XML parser sort it out. With the prepended
> magic, you need a mapping spec for every variety of XML you want to
> embed in Ogg.
>
> Of course, you could just define '<?xml' as the magic for a
generic xml
> stream...
Hi,

some reasons from memory:

 * XML is often generated by libraries, making it difficult for
applications to control exact byte positions

 * applications which need to be able to seek on a stream need to be
able to find information about the granulepos->time mapping for a
bitstream. Requiring an XML parser just to get this information is a
bit of overkill, compared to simply reading a few bytes from a known
location (for something like a stream cutter which has no intention of
decoding the various bitstreams).

Although this mapping is one of the things that may be given by Ogg
Skeleton, we wanted CMML's Ogg mapping to be independent of that (ie.
either CMML, or Skeleton, or both, can be used in Ogg).

* in the demux stage of players, it should be as easy as possible to
identify a track and route it to the correct decoder. Having a magic
identifier, with a binary versioning field, makes this task somewhat
easier.

cheers,

Conrad.