http://hn.vernccvbvyi5qhfzyqengccj7lkove6bjot2xhh5kajhwvidqafczrad.onion/stories/38505530
In general, each pixel gets blurred into the function h(x,y) = f(a,b)g(x-a,y-b). Now sum up all the blurred pixels, so you get the final image (f*g)(x,y) = integral_(a,b) f(a,b)g(x-a,y-b). Same thing can be done in the time domain instead of a spatial domain, or you can write it in vector form, so (f*g)(x) = integral_(r) f(r)g(x-r).