说明：  水平集是跟踪轮廓和表面运动的一种数字化方法。不直接对轮廓进行操作，而是将轮廓 设置成一个高维函数的零水平集，这个高维函数叫做水平集函数：Ψ(X,t) 。然后水平集函数 运动成为一个微分方程。在任何时候，通过从输出中提取零水平集 (( ), ) { ( , ) 0} Xt Xt Γ =Ψ = 来 得到运动的轮廓。使用水平集的主要优点是可以对任何复杂的结构进行模式化和拓扑变换， 比如暗中操作融合和分离。
(The paradigm of the level set is that it is a numeri-cal method for tracking the evolution of contours and surfaces. Instead of ma-nipulating the contour di-rectly, the contour is embed-ded as the zero level set of a higher dimensional function called the level-set function,ψ(X,t). The level-set func-tion is then evolved under the control of a differential equation. At any time, the evolving contour can be ob-tained by extracting the zero level-set Γ((X),t) ={ψ(X,t) =0}from the output. The main advantages of using level sets is that arbitrarily complex shapes can be modeled and topological changes such as merging and splitting are handled implicitly.)