Matching Binary Kepoints, HAMMING2

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When using BFMatcher match() function, does it give vector of Matches in sorted order.

And what is NORM_HAMMING2.

I just understand NORM_HAMMING for e.g when comparing 2 byte (01111111B vs 01111110B) because only last bits of 2 bytes are different. Is this right? And how about NORM_HAMMING2?

How to specify the radius using radiusMatch() for HAMMING distance?