Eigenvectors of ATA
	Suppose (x,y) is on an edge.  What is ATA?
					derive on board
	• gradients along edge all point the same direction • gradients away from edge have small magnitude •      is an eigenvector with eigenvalue • What’s the other eigenvector of ATA? – let N be perpendicular to – N is the second eigenvector with eigenvalue 0 The eigenvectors of ATA relate to edge direction and magnitude