Some more potentially interesting resources.
The first has a PS paper and source code of a density-based particle sorting
mechanism (uses kd-trees):
http://www.sns.ias.edu/~eisenste/hop/hop.html
And a brief description of kd-trees:
http://web.mit.edu/napoli/www/6.838/kd.html
--
Margus Ramst
Personal e-mail: mar### [at] peakeduee
TAG (Team Assistance Group) e-mail: mar### [at] tagpovrayorg
edu