Regarding ways to encode XMP metadata in Ogg format files (principally Vorbis)...
Conrad Parker wrote: > On 25/03/07, Terry Hancock <[EMAIL PROTECTED]> wrote: >> stored in GIF files). Is that going to be a problem with Ogg, or can we >> just create a named field "XMP" (say), and fill it with XMP data, IOW: >> >> * "XMP" metadata named field (Is that a good choice of name?). > > Sounds fine by me. > >> * XMP data stored in contiguous, uncompressed UTF-8 encoded block of >> text (Adobe's scanner spec calls for checking UTF-8, UTF-16-LE, and >> UTF-16-BE, and possibly UTF-32-LE, UTF-32-BE (which I haven't supported >> -- apparently Python 2.4's codecs module doesn't include them)). >> >> * What's the limit on the size of keyword text fields in an Ogg file? If >> it has no built-in limit (really I mean if it's about 60+ kilobytes, as >> it would be if it were addressed by a short int), then we should have no >> problem. > > Up to (2^32 - 1) comments may be stored in a Vorbis comment packet. > Each comment vector consists of a "name=value" field of maximum total > length (2^32 - 1) bytes > > http://www.kfish.org/sounds/textual_data.txt > > So, all we need to do is agree on a field name ("XMP" obviously, > unless it should also contain a version number or anything like that). > The comment value string for VorbisComment is arbitrary UTF-8. > >> Basically, this would just be following the PNG model. >> >> I haven't read the spec for Ogg Vorbis, so I don't know what happens to >> data in a metadata field. However I did just hexdump my sample Ogg file, >> and it looks like the text fields are in clear, so that's good news. >> >> I'm not really planning on writing Theora support, but ISTM that it's >> probably a short step from Vorbis support (same container format? >> perhaps they don't even need separate plugins?). > > Yes, exactly the same format for Theora, Speex etc. When I wrote my reply, I assumed this would be the most natural way, but I may've been a little naive. I noticed some description at xiph.org of putting such data in a separate "XML stream" alongside the data in an Ogg container format. Advantages cited included being able to tag combined audio/video streams, and not "abusing" the fields (from Vorbis documentation page): """ The comment field is meant to be used much like someone jotting a quick note on the bottom of a CDR. It should be a little information to remember the disc by and explain it to others; a short, to-the-point text note that need not only be a couple words, but isn't going to be more than a short paragraph. """ So maybe this is an undesireable solution (even though it is attractively simple to implement)? Or would that be prudish of us? :-) How hard is it to create/read one of these "XML streams" in an Ogg file, and can a binding like pyogg do that for me so I don't have to think about it? Cheers, Terry -- Terry Hancock ([EMAIL PROTECTED]) Anansi Spaceworks http://www.AnansiSpaceworks.com _______________________________________________ cc-devel mailing list [email protected] http://lists.ibiblio.org/mailman/listinfo/cc-devel
