Watch the points move according to the music, and see the algorithm in action! The visual effect is that the points move in different directions corresponding to the sound components, effectively splitting the screen in different music zones. When a bass is played, the point moves in one direction, and in another for the trebles. With some training, one can “see” the instruments.
For the curious, here are more details. XMMS decomposes music in frequencies and send them to the plugin. For each frequency, a direction is associated computed on a logarithmic scale (and in an attempt to "linearize" the angle with the music notes). Then, a vector sum of all the directions defines a unique move for each audio canal.
Thus, the hope is that different note successions lead to different zones on the screen. If XMMS plays a bass or a percussion instrument, the point will move in a direction. And so on for the other instruments.
Trajectories is alpha quality software. This means it may compile, or not, on your own computer, that the features are motly implemented, but that bugs may remain. Actually, it works quite well on my box, and as it pleases me likes this, I never spent more time on it. So, after more than two years and no evolution, I release it in its current state. Play with it as much as you want, and don't hesitate to contact me if you do something interesting with it :)
You can download the code here.