How can i get the brisk's scale?

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Hello,

I'm trying to extract the scale from brisk's descriptors, but i don't know exactly how. I've already get the keypoints and the descriptors from my image.

I saw in another posts that I can get the size from the descripor wich is proportional to the scale in this way: size = scale * magnifier) but i need the scale value.

How can i get the scale value? Or the magnifier value?

Can anyone help me?

Thanks in advance,