how i can calculate Haar feature threshould

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Hi everyone Kindly i got how Haar classifier works but i wonder ,which criteria "method" used to find threshold for each Haar feature ?

for example let's suppose that i have this integral sub image with same size of feature :: 5| 7|10 ----->shaded area 6|13|20 ----->white area sum(white)-sum(shaded)=39-22=17 ----------- compare it with threshold "which i wonder" and then judge if object exist or not