with D = 0 on solid walls and ∇D · n = 0 on other boundaries. The eikonal equation can be tricky to solve, and produces the exact distance to the closest wall. The modified equation described below is less computationally expensive to solve. It also uses a reference length to put more emphasis on solid objects larger than the reference length and to reduce the emphasis on smaller objects. The introduction of a reference length produces a more relevant wall distance in the following instances: