参考文章:Introduction to Spectral Graph Theory and Graph Clustering 作者:Chengming Jiang,ECS 231 Spring 2016 University of California, Davis 本文的目的是进行计算机图像分割:
图1 图像分割
一、预备知识
关于图(G)、度矩阵(D)、邻接矩阵(A)皆在上一篇理解中交代过,现补充一些新的定义: 1、权重矩阵 A weighted graph is a pair G=(V,W) where
V={vi} is a set of vertices and ∣V∣=n;
W∈Rn×n is called weight matrix with wij={wij≥00if i̸=ji=j W是权重矩阵,V是顶点,它们构成对G=(V,W),即是权重图G。 The underlying graph of G is G^=(V,E) with E={{vi,vj}∣wij>0}
If wij∈{0,1},W=A, the adjacency matrix of G^
Since wii=0, there is no self-loops in G^ W是对A的一个扩展,当wij∈{0,1},W即是A。定义W后,需要重新定义顶点的度(degree of a vertex)和度矩阵(degree matrix): d(vi)=j=1∑nwijdegree of vi Let d(vi)=diD=D(G)=diag(d(v1),⋯,d(vn))=diag(d1,⋯,dn) 2、A的体积(Volume) 对于V的一个子集A(A⊆V),定义A的体积(Volume): vol(A)=vi∈A∑d(vi)=vi∈A∑j=1∑nwij 即A中所有顶点的度和,若A中所有顶点都是孤立的(isolated),则vol(A)=0,举例如下:
图2 vol(A)的计算方法 3、顶点集间的连接(links) Given two subsets of vertices A,B⊆V, we define the linkslinks(A,B) by links(A,B)=∑vi∈A,vj∈Bwij Remarks:
A and B are not necessarily distinct;
Since W is symmetric, links(A,B)=links(B,A)
vol(A)=links(A,V) 有了连接(links)定义,就可以定义分割(cut),它的定义如下: cut(A)=links(A,V−A) 在连接(links)基础上,还可以定义一个量assoc,如下: assoc(A)=links(A,A) 即A中顶点自己的连接。cut是A和外部的links,assoc是A与内部的links。因此有:cut(A)+assoc(A)=vol(A) 4、Graph Laplacian 对于权重图 G=(V,W),the (graph) Laplacian L of G is defined by L=D−W Laplacian具有以下的属性:
xTLx=21∑i,j=1nwij(xi−xj)2 for ∀x∈Rn,这是一个二次型
L≥0 if wij≥0 for all i,j;
L⋅1=0
If the underlying graph of G is connected, then 0=λ1≤λ2≤λ3⋯≤λn
If the underlying graph of G is connected, then the dimension of the nullspace of L is 1.
for any i and j, the edges between (Ai,Aj) have low weight and the edges within Ai have high weight. 要使分割后各子集之间的edges的权重最小,对于2-way分割有: cut(A)=links(A,Aˉ)=vi∈A,vj∈Aˉ∑wij, where Aˉ=V−A 分割问题转化成了优化问题:mincut(A)=minvi∈A,vj∈Aˉ∑wij