Non-local mean filter
Non linear filter which exploits similarities between different patches of the image
$$
O(p) = \sum_{p\in S}w(p,q)I_{q} \space where
$$
$$
w(p,q) = \frac{1}{Z(p)}e^{-\frac{\Vert N_p -N_q\Vert_2^2}{h^2}}
$$
$$
Z(p)= \sum_{q \in I} e^{\frac{\Vert N_p -N_q\Vert_2^2}{h^2}}
$$
