Centroid

Notes:

Calculate the centroid and slopes for all apertures:
The structure of the return on the function is a row vector of slopes:
[s_x(0,0),.,s_x(0,N),.,s_x(M,0),.,s_x(M,N),s_y(0,0),.,s_y(0,N),.,s_y(M,0),.,s_y(M,N)]

Centroid calculation using thresholding method.

you should make clear why you can sum the variances directly, and why there are no cross-terms for 'ph' and 'r'