[opus] [ogg-dev] comment writing in opus / liboggz

[Mark Harris]

Ian Malone wrote:
>>> Another thought, could vorbiscomment do with having Opus comment support
>>> added?
>>
>>
>> Or make an opuscomment for opus-tools. I'm not sure which is the better
>> approach, though the latter certainly seems like it would be less surprising
>> for users.
>
> It'd certainly make sense for it to be included with other Opus tools,
> vorbiscomment is a bit of an oddity within vorbis-tools since (hazily
> recalled) it supports other stream types than just Vorbis (and a
> general comment editor is useful, though it sounds like liboggz has
> that covered again).


What might be most useful is some kind of opusedit or opusutil utility
that can make simple changes to an Opus file.  Although the initial
version might support only adding, removing, or changing comments,
having such a utility could make it easier to support other simple
changes in the future, such as:

 * Add/remove/change attached pictures (album art)
 * Adjust output gain in header
 * Change original sample rate in header
 * Change channel mapping / extract channels
 * Sample-accurate trimming by adjusting pre-skip and end trimming and
removing whole frames
 * Adjust timestamps (granule positions), e.g. to start at time 0
 * Adjust Ogg page durations, to reduce either overhead or streaming delay
 * Repair some broken files
 * Remove any CBR packet padding
 * Add/remove comment/picture padding

Several of these may require re-writing (but not re-encoding) the
whole file, for example if there is not enough padding space for
additional comments or attached pictures.  Even if multiple operations
are performed, the file should be re-written at most once.

Thoughts?

 - Mark