Volumetric Particle Shadowing

This has been available for a while in the CUDA 2.1 SDK beta download, but since the PDF isn't on the web yet it hasn't had much exposure, so I thought I'd post something about it here. It's a very simple technique which is based on an old volume rendering trick by Joe Kniss, but it can achieve some great looking results. The basic idea is to that instead of sorting the particles in back-to-front or front-to-back order, you sort them along a vector half way between the view and light directions. You then render the particles in batches, accumulating the shadows to a 2D texture as you go. You can even add some blur to simulate scattering. Details are in the PDF (fixed link). Oh, and there's a short movie here.