Replaced the previous KD-tree (for caching points) with a BVH-tree (thanks to Andre 'jaguarandi' Pinto for help here!). The bvh is quite a bit faster and doesn't suffer some of the artifacts that were apparent with the kd-tree. I've also added a choice of falloff types: Standard, Smooth, and Sharp. Standard gives a harder edge, easier to see individual particles, and when used with a larger radius, Smooth and Sharp falloffs make a much cloudier appearance possible. See the image below (note the settings and render times too) http://mke3.net/blender/devel/rendering/volumetrics/pointdensity_bvh.jpg