Notice that the scale level index s varies in a slightly redundant set. Once extracted, local extrema are quadratically interpolated this is very basic especially at the lower resolution scales in practice to have accurate keypoint localization at the full resolution. Just, they are filtered to eliminate low-contrast responses or responses close to follows and the orientation s are assigned, as explained next..
The code provided in this module can be used in different ways. Alternatively, you can use one of the low level functions to run only a part of the SIFT algorithm for instance, to compute the SIFT descriptors of custom keypoints. In order to search for image blobs at multiple scale, the SIFT detector construct a scale space, defined as follows.
Consider the Gaussian kernel.