SURF Descriptor Length

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Hi everybody, i'm using the SURF-Algorithm for some object recognition tasks. In this paper they tested the algorithm with a descriptor length of 36 and the results were good. I've tried to change the OpenCV-Source and rebuild it to test it too, but it doesnt really work. My question is if i just have to change the surf.cpp file and rebuild it or are there other tasks i have to do before rebuilding?? I'm using VS2010.