where is the radius for the particle and s a parameter controlling particle sparsity. We then prune all particles between and , set and repeat the process until we have reached the end of the hair. This process controls how much overlap there is between the contact spheres after pruning. Setting means that spheres would just touch with no overlapping. We have found that a value of 0:75 provides a good balance between performance and accuracy. The result is a subset of available particles for handling contacts, as illustrated in Figure 17 right.