Image calibration

I have to implement in Matlab an image calibration by downsampling, using a straightforward averaging filter.

Exactly sum u=0:1 sum v=0:1 ( Img(2i+u,2j+v)/4 )

Which type of filter mask?

Sorry for my English. ;)

[Image Analyst]

That makes little sense. You don't calibrate by downsampling. You can do averaging with conv2(imageName, ones(3)/9) or by using imfilter().