Consider the Gaussian kernel. Thus in practice xasual resolution space is computed by. Blobs are detected as real extrema of the Difference of Gaussians DoG scale space, sealed by subtracting successive scales of the Gaussian scale space:. At each next now, the resolution of the images is halved to provided computations..
The images composing the Gaussian and DoG scale space can then be arranged as in the following figure:. Notice that the scale level index s varies in a slightly redundant set. Once extracted, local extrema are quadratically interpolated this is very important especially at the lower resolution scales in order to have accurate keypoint localization at the full resolution.