ORB Keypoint response calculation

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I've successfully used the response parameter of KeyPoint object to order the keypoints in my Image Retrieval software, and I can clearly see the advantage it brings. I'm having a bit of trouble though trying to figure out the science behind it, since the documentation only says: The response, by which the strongest keypoints have been selected. Can be used for further sorting or subsampling.. I'd like to know how it actually works, because I like to understand how things operate in the backstage.

For a given image, how can I calculate the response of point marked with orange cross, where each cell represents a pixel and RGB value of grey pixels is (195,195,195)