  We introduce a particle-based simulation method for granular material in
interactive frame rates. We divide the simulation into two decoupled steps. In
the first step, a relatively small number of particles is accurately simulated
with a constraint-based method. Here, all collisions and the resulting friction
between the particles are taken into account. In the second step, the small
number of particles is significantly increased by an efficient sampling
algorithm without creating additional artifacts. The method is particularly
robust and allows relatively large time steps, which makes it well suited for
real-time applications. With our method, up to 500k particles can be computed
in interactive frame rates on consumer CPUs without relying on GPU support for
massive parallel computing. This makes it well suited for applications where a
lot of GPU power is already needed for render tasks.
