I am performing illumination correction using regression methods, let's say polynomial method. If we use 3rd order polynomial in 2D we will have 10 coefficients to estimate. I manually chose these coefficient values and created an illumination profile as shown below using 'once' square in MATLAB.
My question is, ignoring polynomial series, how to create similar kind of bias patterns in 2D(similar smooth but not the exact same)? I mean what are the different ways I can create such smooth surfaces?