My first suggestion is if you are using R2014a, then imfilter is supported for code generation so you may be able to avoid declaring it as an extrinsic function:
If you need to use it as an extrinsic function for other reasons please read on.
When using the output of an extrinsic function, as you are doing with imfilter, it may often be necessary to preinitialize the variable to which it is assigned. Doing so is what tells the code generation software about the type, size and complexity of the value returned. If foo is extrinsic, then you could use:
x = zeros(3,4);
x = foo(y);
to call foo extrinsically and assign the output as a 3-by-4 array. See:
http://www.mathworks.com/help/simulink/ug/calling-matlab-functions.html#bq1h2z9-47 http://www.mathworks.com/matlabcentral/answers/122167-subscripting-into-an-mxarray-is-not-supported
for more details.
So you would just need to assign the values of the calls to imfilter to be the correct size, type etc. From the documentation for imfilter, the output is the same size and type as the input. So you should be able to try something like:
mean_I = zeros(size(I),'like',I);
mean_I = imfilter(I,ones(win_var)/win_var/win_var,'same','symmetric','conv');
mean2_I = zeros(size(I),'like',I);
mean2_I = imfilter(I.^2,ones(win_var)/win_var/win_var,'same','symmetric','conv')
