How to compute variance of image in windows?

I read a paper about Guided Image Filtering. For edge preserving:

where is defined as the variance of input image whose centre is .

How to compute it in Matlab?

Thankyou.

[KSSV]

Read about var

[Bjorn Gustavsson]

If you have the image-processing toolbox have a look at wiener2:

edit wiener2

In there you can see how the local variance is calculated.

HTH