Extracting SIFT/SURF descriptor from pre-cropped patches

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Hi,

I have a set of 100K 64x64 gray patches (that are already aligned, meaning they all have the same orientation) and I would like to extract a SIFT descriptor from each one.

It is clear to me all I need to do is to define a vector with one keypoint kp such that: kp.x=32, kp.y=32.

However, I don't know how to set the kp.size parameter. From going over SIFT's code, it looks as it's doing some non-trivial calculations with that parameter instead of just assuming that it's the size of the patch.

Question 1: what should be the kp.size parameter when extracting SIFT descriptors from patches of size 64x64?

Question 2: what should be the kp.size parameter when extracting SURF descriptors from patches of size 64x64?

Thanks in advance,

Gil.