[whatwg] Give guidance about RFC 4281 codecs parameter

[Ralph Giles]

On Tue, Apr 10, 2007 at 11:21:10AM -0700, Dave Singer wrote:

> ># application/ogg; disposition=moving-image; codecs="theora, vorbis"
> ># application/ogg; disposition=sound; codecs="speex"
> 
> what is the 'disposition' parameter?

The idea of a 'disposition-type' is to mark content with presentational 
information. See the Content-Disposition Header for MIME described in 
RFC 1806 for an early example.

The specific proposal Silvia mentioned is to add the content-
disposition to the media-type to inform parsers of the general
nature of the content, even if they don't recognize the specific
codecs. The allowed values for the 'disposition' label come from
the Dublin Core set. This is not part of RFC 4281, and as far as
I know hasn't been formally documented with the IETF, but we do
think it's a good idea.

This arose out of the need to discover or record "audio" vs 
"audiovisual" status for media files in the context of routing
to the proper playback application, which has been particularly 
contentious with the Ogg container since we have insisted that
such distinctions be made via metadata or file inspection instead
of defining distinguishing filename extensions has has been done
with other containers. (MooV is perhaps another example.)

In terms of user presentation, "audio" vs "video" vs "text" vs 
"still image" is the important distinction, while the 'codecs' 
parameter answers the more technical question of what playback 
capabilities are necessary. A <video/> or <audio/> markup element 
already describes this adequately, but it is a larger issue for
media handling on the web.

Charles wrote a more detailed proposal in the context of RSS
media syndication, which is where I first heard of the idea.

  http://changelog.ca/log/2005/08/21/rss-disposition-hinting-proposal

We're essentially suggesting his proposal be extended to (media)
containers in general.

 -r