Specific example of function for background fit

I am baffled by the function that is given in ImageRemoveBackground.

FN(x,y) = ...

I can guess that, when N = 0, the coefficient wave has one coefficient that is a constant offset. Could someone provide the specific examples for increasing polynomials from N = 1 to N = 3. My guess for N = 2 with ic having six values is this

        img_BGx = ic[0] + ic[1]*x + ic[2]*y + ic[3]*x^2 + ic[4]*x*y + ic[5]*y^2

But this does not in any way reproduce what I used as the image.

The top is the source. I chose the gray region inside. It is removed properly. The bottom is generated by the coefficient wave function above.
Never mind. I figured out my problem. I had pixelated the source by a factor of 2 before I fit the background and forgot to allow for this in the x,y values of the equation.

J. J. Weimer
Chemistry / Chemical & Materials Engineering, UAH