说明：  我们使用了一种变分公式来纠正水平集函数与符号距离函数的偏差,从而实现了水平集函数在演化过程中不需要再次初始化的目标。
(we use a variational formulation that forces the level set function to be close to a signed distance function, so realize the level set function without re-initialization during the evolution, and the level set function do not must be signed distance function .)