Volume rendering dark matter simulations using cell projection and order-independent transparency

Dark matter simulations, performed using N-body methods with a finite set of tracer particles to discretize the initially uniform distribution of mass, are an invaluable method for exploring the formation of the universe. Definining a tetrahedral mesh in phase space-with the tracer particles at initialization serving as vertices-yields a more accurate density field. At later timesteps, the mesh self-intersects to an enormous degree, making pre-sorting impossible. Kaehler et al [2012] visualize the mesh using cell projection, but their method requires order-independent compositing, which limits its flexibility. Our work renders the mesh using state of the art order-independent transparency (OIT) techniques to composite fragments in correct depth order. This also allows us to render variables other than density, such as velocity. We implement a number of OIT optimizations to handle the high depth complexity (on the order of 10⁷ depth layers for 2×10⁹ particles) of the data. Our performance measurements show near-interactive framerates for our hybrid renderer despite the large number of depth layers.