Hence a DoG image between scales formula_15 and formula_16 is just the difference of the Gaussian-blurred images at scales formula_15 and formula_16. For scale space extrema detection in the SIFT algorithm, the image is first convolved with Gaussian-blurs at different scales. The convolved images are grouped by octave (an octave corresponds to doubling the value of formula_19), and the value of formula_20 is selected so that we obtain a fixed number of convolved images per octave. Then the Difference-of-Gaussian images are taken from adjacent Gaussian-blurred images per octave.