说明：  利用有限差分法 (Finite Differential Method, FDM)将场域离散为许多小网格，应用差分原理，将求解连续函数的泊松方程问题转换为求解网格节点上的差分方程组的问题。
(The use of finite difference method (Finite Differential Method, FDM) will field for many small discrete grid, the application of differential principle, the continuous function for solving Poisson' s equation for solving the problem is converted to grid nodes on the issue of differential equations.)