Squeezebox Visualizer Plugin Architecture

Overview:

Squeezebox supports a versatile and powerful visualizer architecture which
allows many kinds of effects to be synchronized to the audio playback. 

			[graphic]

Audio destined for the squeezebox is read in through a visualizer process
and piped to Squeezebox Server. As the data is fed through the visualizer, it is
made available to plugins which generate their effects based on either PCM
or spectral data extracted from the stream. 

The visualizer engine does the work of communicating with squeezebox and 
maintaining synchronization. As the stream passes through the process, it 
is spliced out into a queue, where each of the audio chunks is held 
until the client reaches the corresponding point in the stream. At that 
time, the chunk is released from the queue into the visualizer plugin, 
where it can be processed in order to generate the effect.

Control

All plugins must provide a minimal set of functions for startup, shutdown,
and control purposes. The functions that a plugin can provide are as
follows:

init	

	Optional - called when the visualizer engine first starts up.  
Initialize any permanent data tables and such here.

instance_alloc	

	Visualizers must be designed to work for multiple clients. Create
your per-client state here, and return a client-specific opaque pointer 
which will be given to you with each chunk of audio data.

instance_destroy

	Shut down and free any client specific data. Before it calls this,
the visualizer engine will take care of releasing any pending frames.

shutdown
	Optional - this is only for if you need to do stuff when the
visualizer process goes away due to slimserver shutdown.

feed_pcm
	Optional - Accept an array of samples. Usually you will have
either this OR feed_spec... you could have neither or both of them, but
usually you would want just one.

feed_spec
	Optional - Accept an array of spectral magnitudes

set_graphics_area
	Recommended for plugins that generate slimproto graphics. Allows
the server to restrict the display area used by your plugin in order to
allow for other things to be displayed during navigation. You can choose
to use less than the display area given here, but using more is probably
not nice.