[vorbis-dev] Reading OGG tags, without libvorbis?

[rillian]

On Wednesday, April 10, 2002, at 02:13 AM, Steve Kemp wrote:

>   I've grabbed a copy of vorbis-tools to look at how 'vorbiscomment'
>  works - but I'm confused as to the OGG header + comment formats.
>
>   Is there a simple description of the format of the header, and the
>  fields?

I think you're better off adding using libogg to parse the bitstream; 
it's very small and can be reasonably compiled in if you don't want the 
dependency in the executable, and you'll have to write something almost 
as complex to parse things properly.

You can find information on the ogg framing layer in these two documents:

        http://xiph.org/ogg/doc/oggstream.html
        http://xiph.org/ogg/doc/framing.html

or in the corresponding files in the libogg source distribution.

The ogg layer will parse the framing and return packets. You should find 
the first substream that has a packet starting with '\001vorbis' and 
ignore any other substreams. Then, look for a packet that starts 
'\003vorbis'; it should be the second packet. This  contains the comment 
headers formatted as described in v-comment.html, starting immediately 
after the magic. This kind of simplistic parsing is all you need to read 
the metadata header.

If you want to do it wrong, you can search for the first occurrence of 
'\003vorbis' in the bitstream and start parsing there, or start parsing 
at byte 0x68. This last method will of course break on any file with a 
large comment header, and may break on future vorbis files.

Hope that's helpful,
  -r

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