Where is the variance of Macroblock is calculated?

Hi everyone,
I was looking at the structure of MACROBLOCK struct and realized that there
is var16 field. That would be good for me because I want to make use of
this variable in the future. Unfortunaly, it seems like this field is never
calculated through the encoding process, am I right? If I want to use this
field, where should I put the code to calculate the variance of the
Macroblock? Do I need to calculate it after Motion Estimation had completed?

-- 
Best wishes,

Tran Xuan Chien
University of Information Technology
Phone: (+84) 1692 468 154