Magnitude Squared Coherence C or C++ implemenation

I have two questions about the mscohere function in matlab. The first is, is it a requirement for length(N*M) of the image you are trying to find be equal to the length of a column in the image you are looking in. For example if y is the image I am trying to find, and x is the image I am searching

x = zeros(10,10);
y = zeros(5,2);
a = mscohere(x,y);

This works fine, but if I do something like

x = zeros(10,10);
y = zeros(6,2);
a = mscohere(x,y);

Matlab throws an error. Is this just because the way it is implemented in matlab, and does this also explain why for a 2D signal, or an image, the vector i get back is one dimensional because the function operates column wise?

My second question is, does anyone know of any robust libraries that implement magnitude squared coherence for images in C or C++?