advanced rendering for AutoCAD
They aren't discrete particles-- (or a true density for that matter. ) What appears to be particlulate is actually just remaining noise in the path tracer. This one ran for a half hour-- it will get smoother (slowly).
What you're actually specifying with the algorithm I'm playing with is a probability per unit distance that a ray entering the medium will scatter, absorb, or pass through (neither scatter nor absorb.)