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1、Level set methodFrom Wikipedia, the free encyclopediaJump to: navigation, search The level set method (sometimes abbreviated as LSM) is a numerical technique for tracking interfaces and shapes. The advantage of the level set method is that one can perform numerical computations involving curves and
2、surfaces on a fixed Cartesian grid without having to parameterize these objects (this is called the Eulerian approach).1 Also, the level set method makes it very easy to follow shapes that change topology, for example when a shape splits in two, develops holes, or the reverse of these operations. Al
3、l these make the level set method a great tool for modeling time-varying objects, like inflation of an airbag, or a drop of oil floating in water.Contentshide 1 Level set method 2 The level set equation 3 History 4 See also 5 References 6 External links edit Level set methodAn illustration of the le
4、vel set methodA very simple, yet powerful way to understand the level set method is by first studying the accompanying illustration before proceeding towards a more technical definition, which then becomes quite accessible. The figure on the right illustrates several important ideas about the level
5、set method. In the upper-left corner we see a shape; that is, a bounded region with a well-behaved boundary. Below it, the red surface is the graph of a level set function determining this shape, and the flat blue region represents the x y plane. The boundary of the shape is then the zero level set
6、of , while the shape itself is the set of points in the plane for which is positive (interior of the shape) or zero (at the boundary).In the top row we see the shape changing its topology by splitting in two. It would be quite hard to describe this transformation numerically by parameterizing the bo
7、undary of the shape and following its evolution. One would need an algorithm able to detect the moment the shape splits in two, and then construct parameterizations for the two newly obtained curves. On the other hand, if we look at the bottom row, we see that the level set function merely got trans
8、lated downward. We see that it is much easier to work with a shape through its level set function than with the shape directly, where we would need to watch out for all the possible deformations the shape might undergo.Thus, in two dimensions, the level set method amounts to representing a closed cu
9、rve (such as the shape in our example) using an auxiliary function , called the level set function. is represented as the zero level set of byand the level set method manipulates implicitly, through the function . is assumed to take positive values inside the region delimited by the curve and negati
10、ve values outside.23edit The level set equationIf the curve moves in the normal direction with a speed v, then the level set function satisfies the level set equationHere, is the Euclidean norm (denoted customarily by single bars in PDEs), and t is time. This is a partial differential equation, in p
11、articular a Hamilton-Jacobi equation, and can be solved numerically, for example by using finite differences on a Cartesian grid.23The numerical solution of the level set equation, however, requires sophisticated techniques. Simple finite difference methods fail quickly. Upwinding methods, such as t
12、he Godunov method, fare better; however the level set method does not guarantee the conservation of the volume and the shape of the level set in an advection field that does conserve the shape and size, for example uniform or rotational velocity field. Instead, the shape of the level set may get sev
13、erely distorted and the level set may vanish over several time steps. For this reason, high-order finite difference schemes are generally required, such as high-order essentially non-oscillatory (ENO) schemes, and even then, the feasibility of long-time simulations is questionable. Further sophistic
14、ated methods to deal with this difficulty have been developed, e.g., combinations of the level set method with tracing marker particles advected by the velocity field.4edit HistoryThe level set method was developed in the 1980s by the American mathematicians Stanley Osher and James Sethian. It has b
15、ecome popular in many disciplines, such as image processing, computer graphics, computational geometry, optimization, and computational fluid dynamics.A number of level set data structures have been developed to facilitate the use of the level set method in computer applications.edit See also Volume
16、 of fluid method Image segmentation Immersed Boundary Method LSM/J Level set method for drawing dynamical plane LSM/M Level set method for drawing parameter plane edit References1. Osher, S.; Sethian, J. A. (1988), Fronts propagating with curvature-dependent speed: Algorithms based on Hamilton-Jacobi formulations, J. Comput. Phys. 79: 1249. 2. a b Osher, Stanley J
