DiffusionFilter
# Diffusion filter
## Diffusion equation
From Wikipedia,
> The diffusion equation is a partial differential equation which describes density dynamics in a material undergoing diffusion.
$$ \frac{\partial\phi(\mathbf{r},t)}{\partial t} = \nabla \cdot \big[ D(\phi,\mathbf{r}) \ \nabla\phi(\mathbf{r},t) \big], $$