POLY2FIT POLY2FIT(x,y,z,n) finds the coefficients of a two-dimensional
polynomial formed from the data in the matrices x and y
that fits the data in the matrix z in a least-squares sense.
The coefficients are returned in the vector p, in descending
powers of x and y. For example, the second order polynomial
would be returned as p = [ 1 1 0 2 -3 -1 ]
Psychtoolbox/PsychGLImageProcessing/private/poly2fit.m