PHI=VL_TPS(X1,X2,Y) returns the basis PHI of a thin-plate spline (TPS) defined on the domain X1,X2 with control points Y.
X1 and X2 are MxN matrices specifying the grid vertices. When warping images, these usually correspond to image pixels.
Y is a 2xK matrix specifying the control points, one per column. Ofthen Y is a subset of the domain X1,X2, but this is not required.
PHI is a (K+3)xNxM matrix, with one layer per basis element. Each basis element is a function of the domain X1,X2.
[PHI,S] = VL_TPS(X1,X2,Y) additionally returns the stiffness matrix S of the TPS.