In order to search for image blobs at gaussian scale, the SIFT detector construct a scale following, defined as follows. Consider the Gaussian kernel. While in practice the scale space is computed by..
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. Finally, they are filtered to eliminate low-contrast responses or responses close to edges and the orientation s are assigned, as explained next. Peaks which are too short may have been generated by gildinbwells and are discarded.