2D filtering
		A 2D image f[i,j] can be filtered by a 2D kernel h[u,v] to produce an output image g[i,j]: This is called a cross-correlation operation and written: h is called the “filter,”  “kernel,”  or “mask.” The above allows negative filter indices.  When you implement need to use:  h[u+k,v+k] instead of h[u,v]