\begin{equation} H_x=\frac{1}{3}\times{ \left[ \begin{array}{ccc} 1 & 0 & -1\\ 1 & 0 & -1\\ 1 & 0 & -1 \end{array} \right ]}, H_y=\frac{1}{3}\times{ \left[ \begin{array}{ccc} 1 & 1 & 1\\ 0& 0 & 0\\ 1 & 1 & 1 \end{array} \right ]} \end{equation}
