• How can SIFT features extend over the borders of the image? 

    I don’t understand how the arrows can represent the radius of the region used to compute the descriptor if it is larger than the image. Including blank space from outside the image to compute the descriptor would make it unstable and unreliable for matching so I don’t understand why the arrow would ever be larger than the image. I’m going to clear this up for myself as it seems to be common across all implementations of SIFT that I have seen.

    Image from p.15: David G. Lowe, “Distinctive image features from scale-invariant keypoints,”International Journal of Computer Vision, 60, 2 (2004), pp. 91-110. [PDF]

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