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This was actually a false question ;o)

SIFT is already L2 normalized using the cited normalization method. But for optimization purpose, SIFT signature is converted to unsigned char format (loose precision but gain computational efficacy for the next processing stages). To do so, each descriptor bin has been multiplied by 512.f (check constant value : SIFT_INT_DESCR_FCTR within opencv/modules/nonfree/src/sift.cpp).

Have a nice coding and experimentation !