Talking about the GUI

I'm wondering how to handle the "live" paramaters of each plugin in the 
system. For example changing the volume of the mixer plugin, the font 
used by the subtitle plugin, or the decoding quality of an MPEG4 stream.

I think it should be possible to have a way to change that from a 
hardware controller (like a MIDI keyboard) as well as from some 
plugin-specific GUI or the general settings of the host.

I've thought about this when I was working with Transors
http://matroska.org/technical/transor/index.html

I came to the (intermediate) conclusion that this could be handled the 
same way in the 3 cases with an XML/EBML communication. This would allow 
the plugin not to care about how the settings are presented and make is 
completely UI independant.

I understand that a plugin will be compiled for a specific platform. So 
it should be able to display a UI the way it wants. But I don't imagine 
a plugin developper making a UI each of these systems : Windows, Aqua, 
GTK, Qt, MPlayer, BeOS, etc. The same plugin should be recompilable on 
most platforms and just work... IMO a codec designer should not have to 
care about how to display a fancy property page. And the host 
application may want to deal with the UI itself (more consitent UI 
design) and not done by the plugin.

What do you think of that ? IMO that's the biggest missing feature in 
existing projects for the moment.

That would require a sort of data<->UI interpreter. One for each of the 
systems mentioned, but reusable between projects/hosts.