Image segmentation

E(R)=∑s∈R(ys−μR)2

with

(ys−μR)2=ys2−2ysmyR+μR2

=>

∑s∈R(ys−μR)2=∑s∈Rys2−2∑s∈RysmyR+∑s∈RμR2=area_square_sum−2⋅mean⋅area+mean2⋅area